CA2262395C - Loop data transmission system - Google Patents

Abstract

A data transmission system according to the present invention comprises transmission devices connected to a loop transmission path, wherein each of the transmission devices, if the destination address in a received frame is its own address, will get the frame and, if the source address in the received frame is its own address, will remove the frame from the transmission path.

Description

TITLE OF THE INVENTION
LOOP DATA TRANSMISSION SYSTEM
BACKGROUND OF THE INVENTION
This invention relates to a loop data transmission system suitable for data transmission over a long transmission path including transmission delay elements, such as a railroad control system.
With the recent advance in computer communication technology, various data transmission network systems have been developed. One of them is a loop data transmission system where a center that controls trains and a plurality of railroad stations are connected in a loop.
FIG. 1 shows the configuration of a loop data transmission system for controlling trains.
In the data transmission system, a center transmission device 51 acting as a primary station and transmission devices 52 acting as secondary stations installed in individual railroad stations constitute a loop network via transmission paths 53. A plurality of computers #1, #2 are connected to the center transmission device 51. A personal computer 54 and a station controller 55 are connected to the transmission device 52 at each station.
In such a train control data transmission system, since the distance between transmission devices is as long as several kilometers, a modem complying with the V.32 standard or the like, whose transmission speed is relatively low (hundreds of bits/second to tens of kilobits/second), is used. The system employs a one-to-N data transmission scheme where any secondary cannot transmit data unless the primary gives permission to transmit data. As the loop data transmission scheme, for example, the SDLC (Synchronous Data Link Control) procedure proposed by IBM is used.
The SDLC data transmission scheme uses a data frame as shown in FIG. 2. In FIG. 2, a flag is frame synchronous sensing data, DA is destination address, a control section is information used for transmission control, an information section is application information, and FCS (Frame Check Sequence) is frame error sensing data.
The SDLC procedure is characterized in that there are one primary station and a plurality of secondary stations in the system and, as shown in FIG. 3, the transmission device 52 acting as a secondary station in each station can transmit a data frame only when it has received a polling signal (POLL) as a transmission permission signal from the center transmission device 51 acting as the primary station. Therefore, for example, when the transmission device 52 at station 1 transmits frame I1C to the transmission device 52 at station 3, the transmission device 52 at station 1 transmits transmission data frame I1C to the center transmission device 51 when receiving a polling signal from the center transmission device 51 as shown by *1 in FIG. 3. The center transmission device 51 converts the received frame I1C into frame IC3 addressed to the transmission device 52 at station 3 and transmits the resulting frame to the transmission device 52 at station 3. When data is exchanged between secondary stations like this, the data never fails to pass through the primary station before it reaches the destination secondary station as shown in FIG. 3.
Accordingly, when the SDLC procedure has been used, the following shortcomings are pointed out:
(1) More than one frame is not allowed to exist on the loop transmission path 53. As a result, empty time always takes place in the transmission path 53.
Generally, when the distance of a single loop is short, each transmission device has no delay, and the frame transmission speed is so fast that the head of a frame comes back, making one round of the loop, before the transmission of the trailing end of the transmitted frame has not been completed yet, empty time Ts will not take place in the transmission path. Even if a little empty time occurs, it may be left almost out of consideration.
However, in a network system with transmission devices having delay elements, such as a railroad control data transmission system, it takes considerable i i ;, , time for the head of a frame to come back, making one round of the loop, after the transmission of the frame has been completed. Since each secondary station can transmit a frame only when the primary station gives permission to transmit data, only one frame can exist on the loop transmission path at a time. Consequently, frames cannot be transmitted consecutively, making empty time in the transmission path longer, which leads to a lower transmission efficiency.
(2) N-to-N data transmission cannot be performed because the data transmission scheme for railroad stations is based on one-to-N correspondence.
Specifically, to realize N-to-N data transmission for railroad stations, it is necessary to carry out direct data transmission from station 1 to station 3.
With a conventional data transfer scheme, since a data frame is transmitted from station 1 to the center once and the center decodes the contents of the frame .and transfers it to station 3, the scheme is nothing short of a one-to-N data transmission scheme. Therefore, the procedure is complicated and it takes a long time to transmit data.
BRIEF SUMMARY OF THE INVENTION
The invention enables the implementation of a loop data transmission system which enables N-to-N data transmission, and which minimizes empty time in the transmission path to increase the transfer efficiency.
In accordance with one aspect of the invention, there is provided a system of a data transmission system comprising transmitting devices configured for bidirectional communication on a ring network. Each transmitting device is configured to receive and copy a i i I !

frame of data transmitted on the ring network. Each transmitting device is also configured to retransmit the frame in opposite directions on the ring network when a source address of the frame does not coincide with an address of the each transmitting device. Each transmitting device is also configured to not retransmit the frame of data when the source address does coincide with the address of the each transmitting device.
In accordance with another aspect of the invention, there is provided a data transmission device comprising a bidirectional modem operable to perform bidirectional communications on a ring network. The apparatus also comprises a processor circuit configured to cooperate with the bidirectional modem to receive and copy a frame of data transmitted on the ring network and to retransmit the frame in opposite directions on the ring network when a source address of the frame does not coincide with an address of the data transmission device. The processor circuit is configured not to retransmit the frame of data when the source address does coincide with the address of the data transmission device.
The data transmission device may be configured to withhold transmission of a frame originating at the transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.
The data transmission device may be configured to withhold transmission of a frame while another frame is being received from the network.
The data transmission device may be configured to read a flag in the frame and to not retransmit the frame when the flag is set.

The data transmission device may be configured to read a flag in the frame and to copy the frame into a data buffer for use by the data transmission device, when the flag is set.
The data transmission device may be configured to read a flag in a received frame and to save a frame being transmitted while the received frame is being received, when the flag is set, to interrupt occupation of the ring network by the data transmission device.
In accordance with another aspect of the invention there is provided a method of operating a data transmission device. The method comprises receiving and copying a frame of data transmitted on a ring network and retransmitting the frame in opposite directions on the ring network when a source address of the frame does not coincide with an address of the data transmission device.
The method also comprises not retransmitting the frame of data when the source address does coincide with the address of the data transmission device.
The method may further comprise withholding transmission of a frame originating at the transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.
The method may further comprise withholding transmission of a frame while another frame is being received from the network.
The method may further comprise reading a flag in the frame and not retransmitting the frame when the flag is set.
The method may further comprise reading a flag in the frame and copying the frame into a data buffer for use by the transmission device when the flag is set.

i j The method may further comprise reading a flag in a received frame and saving a frame being transmitted while the received frame is being received when the flag is set, to interrupt occupation of the ring network by the transmission device.
According to another aspect of the invention, there is provided a computer readable medium comprising codes for directing a processor circuit to receive and copy a frame of data transmitted on a ring network, retransmit the frame in opposite directions on the ring network when a source address of the frame does not coincide with an address of a data transmission device associated with the processor circuit, and not retransmit the frame of data when the source address does coincide with the address of the data transmission device.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 shows a conventional loop data transmission system;
FIG. 2 shows a frame used in the conventional loop data transmission system;
FIG. 3 is a diagram to explain a state where a frame is transmitted in the conventional system;
FIG. 4 is a block diagram of a railroad control data transmission system according to a first embodiment of the present invention;
FIG. 5 shows a frame used in the transmission system of the first embodiment;
FIG. 6 shows the modules in each transmission device of the first embodiment;
FIG. 7 is a time chart to explain a state where frames are transmitted in the data transmission system of the first embodiment;
FIG. 8 is a block diagram of a loop data transmission system according to a second embodiment of the present invention;
FIG. 9 is a block diagram of a transmission device in the second embodiment;
FIG. 10 shows the structure of a frame used in the loop data transmission system according to the second embodiment;
FIG. 11 is a flowchart to explain the operation of each transmission device;
FIG. 12 is a diagram to explain a state where more than one frame exists on the transmission path at the same time;
FIG. 13 is a flowchart to explain the operation of a frame transmission system in a loop transmission system according to a third embodiment of the present invention;
FIG. 14 is a flowchart to explain the operation of the frame reception system in the loop transmission system of the third embodiment;
FIG. 15 shows a state where the transmission devices at the individual sub-stations are copying a specific frame in sequence;

FIG. 16 shows a frame to explain another example of the third embodiment;
FIG. 17 shows a frame to explain still another example of the third embodiment;
FIG. 18 shows an example of performing a priority process on the basis of the priority flag in a received frame;
FIG. 19 shows a frame used in a loop data transmission system according to a fourth embodiment of the present invention;
FIG. 20 is a flowchart to explain the operation of the fourth embodiment;
FIG. 21 shows the structure of a frame to help explain another example of the fourth embodiment;
FIG. 22 shows the structure of a frame to help explain still another example of the fourth embodiment;
FIG. 23 is a flowchart to explain the repeat process of the transmission device in the fourth embodiment;
FIG. 24 is a flowchart to explain the repeat process of the transmission device in the fourth embodiment; and FIG. 25 is a flowchart to explain the repeat process of the transmission device in the fourth embodiment.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be explained.
<First Embodiment>
FIG. 4 is a block diagram of a railroad control data transmission system according to a first embodiment of the present invention.
In the railroad control data transmission system, a transmission device 3 and a management transmission device 4 in each station are connected in sequence to a center transmission device 1 in such a manner that they form a loop network.
Computers #1, #2 connected to the center transmission device 1 are computers that manage train service and output control signals and others to the individual railroad stations.
A personal computer 5 and a station controller 6 are connected to the transmission device 2 at each station. The personal computer 5 is used to manage information at each station and inform the center or other stations of the management information. The station controller 6 is composed of a device for controlling a signal mechanism, called an electronic interlocking device.
A network management device 7 composed of a workstation (WS) and a personal computer (PC) is connected to the management transmission device 4.
FIG. 5 shows a frame used in the network system of the first embodiment.

As shown in FIG. 5, the frame of the first embodiment is such that a source address SA is added to an SDLC frame shown in FIG. 2. Specifically, the frame of the first embodiment is composed of a flag, a source address SA, a destination address DA, a control section, an information section, FCS, and a flag.
The flag is frame synchronization sensing data, the control section is information used to control transmission, the information section is information for application, and the FCS (Frame Check Sequence) is frame error sensing data.
The transmission devices l, 3 in the data transmission system using such a frame perform the following processes in transmitting and receiving data.
When receiving data on the frame, each of the transmission devices l, 3 reads the first two bytes (the flag and SA) and checks the contents of the received data. If the device itself is not the sender, it will repeat the data. If the device is the sender, the frame will be considered to have made one round of the ring (the network loop) and the device will remove the frame from the ring without repeating it.
The transmission devices l, 3 are designed to be able to transmit the data unless they are in the course of repeating. When they are repeating the data, the first two bytes in the frame have been stored in a buffer. It is therefore judged on the basis of the contents of the buffer whether they are in the course of repeating. When they receive the data from another station (or another transmission device) while transmitting data, they store all of the frame data in a buffer. After having completed the transmission, they start repeating. Namely, each transmission device can transmit data unless it is in the course of repeating.
The configuration of each of the transmission devices 1, 3, 4 is shown in FIG. 6.
FIG. 6 shows the modules in each transmission device of the first embodiment.
In each of the transmission devices l, 3, 4, an MPU 12, a memory 19, serial interfaces 13, 14, 15, 16 are connected to a bus 11. Modems 17, 18 are connected to the serial interfaces 13, 14, respectively.
Each of the modems 17, 18 is connected to an adjacent transmission device 1, 3, or 4. Modems with a transmission speed of the order of about 600 bps to 288000 bps in the V.22, V.29, or V32 standard complying with the V series recommendation of the ITU-T
(International Telecommunication Union-Telecommunication Standardization Sector) are used as the modems 17, 18. In the first embodiment, a case where modems complying with the V.32 standard are used at 9600 bps mode will be explained.
Moreover, the modems 17, 18 are full duplex modems.

As described above, a loop network is formed by connecting the full duplex modems 17, 18 to adjacent transmission devices. This enables a frame of the same information to be transmitted clockwise and counter-s clockwise in the loop.
The serial interfaces 13, 14, 15, 16 are modules for serial data communication complying with RS-485 or RS-232C. The serial interfaces 15, 16 in the center transmission device 1 are connected to the computers #1, #2, respectively. The serial interfaces 15, 16 in each station transmission device 3 are connected to the personal computer 5 and station controller 6, respectively. The serial interfaces 15, 16 in the management transmission device 4 are connected to the network management device 7.
The MPU 12 operates according to a program stored in the memory 19 and is a microprocessor for realizing each of the functions of the transmission devices 1, 3 in the first embodiment.
The transmission devices 1, 3 realize the various functions of the data transmission system in the first embodiment. The transmission device 4 and network management device 7 are provided as a means for removing invalid frames appearing on the loop of the network.
Specifically, the transmission devices 1, 3 check for only the source address SA in a frame to keep the network transmission at the high speed and, if the address is not its own address, will repeat the data.
Therefore, if the wrong address was sent by some reason or the correct address changed to the wrong address, the wrong address could not be removed from the loop forever, which would have an adverse effect on the frame transmission.
Unlike the other transmission devices 1, 3, the transmission device 4 reads all the information once.
The network management device 7 has the function of checking frames on the network. The device 7 checks the frame read at the transmission device 4 for SA, DA, and FCS and informs the transmission device 4 of the result.
Only when being informed by the network management device 7 that the frame is valid, the transmission device 4 repeats the valid frame. The network management device 7 may be combined with the transmission device 4 integrally by incorporating the configuration shown in FIG. 6 into the same workstation.
The operation of the railroad control data transmission system according to the first embodiment will be explained.
At the request of the computer #1, personal computer 5, or station controller 6, the corresponding one of the transmission devices l, 3 transmits a data frame. When the transmitted frame has made one round of the network loop and returned to the corresponding one of the transmission devices 1, 3 (the transmitting party), the source address SA is checked and the transmitting party removes the frame from the ring.
While the frame is making one round of the loop, when any one of the transmission devices 1 and 3 has verified from the destination address DA that the frame is addressed to itself, the corresponding transmission device 1 or 3 copies the frame.
Specifically, the transmission devices 1, 3 can transmit a frame unless it is repeating the data. If receiving the data from another station (another transmission device) in the course of transmission, the transmission device will store all the frame in a buffer and start repeating at the time when its transmission has been completed. As a result, more than one frame can exist at the same time in the network loop.
FIG. 23 is a flowchart to explain the repeat process at each of the transmission devices 1, 3.
As shown in FIG. 23, each transmission device reads the flag data, SA data, and DA data in the received frame (5101). Then, the device judges whether the read DA coincides with its own address (5102).
If at step S102, the device has judged that the DA
and its own address coincide with each other, it will copy the frame (5103) and pass control to step 5104.

If the device has judged that the DA does not coincide with its own address, it will pass control to step 5104.
At step 5104, the device judges whether the read SA coincides with its own address. If the device has judged that they coincide with each other, the device will remove the frame from the network. If the device has judged that they do not coincide, it will repeat the frame.
FIG. 7 is a time chart to explain a state where frames are transmitted in the data transmission system of the first embodiment.
In FIG. 7, the center transmission device 1 (the main station), station 1's transmission device 3 (sub-station 1), and station 3's transmission device 3 (sub-station 3) transmit frames IC2, IlC, and I3C, respectively, at the same time. Frame IC2 is addressed to railroad station 2, frame IlC is addressed to the center, and frame I3C is addressed to the center.
Frames IC2, IlC, and I3C existing on the transmission path 2 at time T1 in FIG. 7 are shown in FIG. 4.
These three frames are copied at their own addresses or subjected to frame data reception.
Thereafter, they return to their transmitting parties, which remove them from the loop. The time required for this is Ts, which is the same as empty time Ts in the SDLC procedure of FIG. 3. In the embodiment, because three frames are transmitted during Ts, empty time Tp in this case is about one-third of Ts.
The time required in each stage of the frame transmission is considered and examined in detail as follows.
For example, it is assumed that a network system is constructed using, for example, modems with a transmission speed of 9600 bps and a conversion delay of 20 ms and that 32 sub-stations are transmitting 16-byte frames. In this case, it takes about 800 ms for the transmitted frame to make one round of the loop.
If the time required for one frame of information to be transmitted is 40 ms, 20 frames can theoretically exist on the ring at the same time. The multiplexing improves the transmission efficiency.
Because the frames are multiplexed this way, if an invalid frame has appeared for some reason, the network management device 7 will remove the invalid frame. For this reason, only the management transmission device 4 takes in all the frames and checks for invalid frames.
Furthermore, as shown in FIG. 7, the data transmission has realized N-to-N transmission. For example, since only the main station (the center transmission device) can unload the frame transmitted onto the loop in the conventional SDLC procedure, any sub-station cannot transmit a frame freely.
In the first embodiment, however, a source address SA is inserted in the frame structure and each of the transmitting parities' transmission devices 1, 3 unloads the frame that has made one round of the loop.
This enables any of the transmission devices 1, 3 to transmit a frame to any of the transmission devices 1, 3, 4 at the destination address, which realizes N-to-N
transmission.
In the data transmission system of the first embodiment, use of two full duplex modems 17, 18 in each of the transmission devices 1, 3, 4 makes it possible to continue data transmission even when any one of the transmission devices l, 3 has failed.
Specifically, one of the transmission devices l, 3 has failed, the remaining ones of the transmission devices l, 3, 4 receive a frame in either the clockwise direction or counterclockwise direction. This enables the remaining good transmission devices l, 3, 4 to continue frame transmission properly.
If all the transmission devices 1, 3, 4 function properly, the same frame will be received twice.
Because data on the transmission sequence number has been stored in the control section in the frame of FIG. 5, this makes it possible to judge whether one frame is the same as another frame. As a result, the same frame received later is discarded at the transmission devices l, 3, 4.
As described above, with the data transmission system of the first embodiment, a frame including a destination address and a source address is used in data transmission. The transmission devices l, 3 receive the frames addressed to them and remove the frames they have transmitted themselves. Each of the transmission devices l, 3 can transmit frames any time unless they are receiving frames. Therefore, even in a loop network including transmission devices with temporal elements, not only can frames be transmitted consecutively over the transmission path, but N-to-N
transmission can be achieved. Therefore, it is possible to realize a network system which has a good transmission efficiency and a fast response time and is suitable for controlling railroads.
Furthermore, the data transmission system of the first embodiment includes the management transmission device 4 and network management device 7. The relevant section reads all the information in a frame, judges whether the data is valid or invalid, and transmits only the proper frame to the transmission path 2.
Therefore, when the data has been sent using the wrong source address for some reason or the correct address has been changed to the wrong one, such an invalid frame appeared on the loop can be removed efficiently and reliably.
This prevents transmission from being disabled because the invalid frame has not been removed from the loop forever. Providing such a mechanism in only one place on the loop prevents the transmission time from getting long as compared with the case where all the transmission devices 1, 3, 4 make such checks.
Furthermore, the data transmission system of the first embodiment uses full duplex modems 17, 18 as the transmission devices l, 3, 4 and transmits the same frame clockwise and counterclockwise. Therefore, even when one transmission device has failed, the remaining transmission devices can continue transmission properly without an adverse effect, such as a delay in the transmission time, which realizes a highly reliable network.
Accordingly, even when highly reliable control is required, a highly reliable network system that meets the requirement can be provided. With the present invention, even when transmission devices are connected in a loop, the entire system will not stop as a result of the malfunction of one transmission device.
When the data transmission system of the first embodiment is compared with the conventional SDLC data transmission system, the effects they produce are as follows:

(1) Prerequisites ~ Modem: V.32 modem with a transmission speed of 9600 bps ~ Number of stations (the number of transmission devices at the center and railroad stations): 32 stations Volume of information: the capacity of the information section in a frame Center -~~ station: 5 bytes/station frequency to each station once/2 seconds Station ~---j center: 10 bytes/station frequency from each station once/second Transmission delay: delay in modem about ms/station 15 ~ Delay in the time needed for processing by MPU in a transmission device is neglected (2) Calculating the time required to make one round of the loop in the SDLC procedure and the method of the present invention 20 In a network system including 32 stations (31 railroad stations plus the center), the time from when a transmission device transmits 10 bytes of information until the information has come back after making one round of the loop will be determined.
1) Time T cycle in which one frame has made one round in the SDLC procedure is:
TSDLC cycle = 20 ms x 32 = 640 ms In SDLC, repeating requires almost no time and therefore only a delay in the modem is taken account of.
2) The time required to make one round of the loop in the method of the present invention will be determined.
If the header of the frame is 7 bytes, the volume of information in one frame will be 10 bytes +
7 bytes = 17 bytes.
Time t1 required for the first 2 bytes in the header to make one round is (delay time in modem +
delay time in the repeat by transmission device) x the number of transmission devices = (20 ms +
2 x 8/9.6 ms) x 32 = 693.3 ms, that is, about 700 ms.
Time t2 required to receive one frame of the remaining data is (17 - 2) bytes x 8/9.6 ms = 12.5 ms.
Therefore, time T cycle required to make one round of the loop in the method of the present invention is:
T cycle = t1 + t2 = 705.8 ms (3) Shortening the transfer time:
When a given transmission device sends 10 bytes of information in the information section to the transmission device 16 stations beyond the given device (assuming a station in the middle of 32 stations/loop), the transmission time needed for the head information to reach the destination device is used for comparison.
1) The time required to transmit the data to the device 16 stations ahead in the SDLC procedure Transmission by the SDLC procedure requires a permit signal from the transmission device at the main station. The average waiting time is determined to be half the time required to make one round of the loop.
Average waiting time + transmission delay x 16 stations = 640/2 ms + 20 ms x 16 = 640 ms 2) The time required to transmit the data to the device 16 stations ahead in the method of the present invention (20 ms + 2 x 8/9.6) x 16 stations = 346.7 ms 3) Effect of shortening the transmission time in the present invention In the above example, transmission can be achieved in a little more than half the time needed in the SDLC
procedure, specifically, 346.7 ms/640 ms = 54.20. The method of the invention makes a good response and is suitable for real-time transmission.
(4) Improvement of transmission efficiency More than one frame can be transmitted by the method of the invention, whereas only one frame can be transmitted by the SDLC procedure during the time when a frame makes one round of the loop.
When a frame with 10 bytes of information in the information section is sent, the time needed for the transmission device to process information including the header is 17 x 8/9.6 ms = 14.2 ms minimum. Even if rough consideration is given to the software processing, the time is about 20 ms.
Consequently, with the method of the invention, during the time that a frame makes one round of the loop (705.8 ms), 35 (705.8 ms/20 ms) frames can be transmitted. The improvement of the transmission efficiency by the present invention is 35 times.
Therefore, the transmission efficiency in the invention is improved remarkably as compared with that in the SDLC procedure.
The present invention is not limited to the first embodiment and may be practiced or embodied in still other ways without departing from the spirit or essential character thereof.
The approach described in the first embodiment may be stored on a storage medium, such as a magnetic disk (e. g., a floppy disk or a hard disk), an optical disk (e.g., CD-ROM or DVD), or a semiconductor memory, in the form of programs (software means) executable on a computer. Alternatively, the approach may be transmitted and distributed via a communication medium.
The programs stored on a storage medium includes a setting program that constructs software means (including not only executable programs but also tables and data structures) in a computer. A computer that realizes the device reads the programs stored on a storage medium, constructs software means using the setting program, and executes the above-described processes under the control of the software means.
As described above in detail, with the present invention, it is possible to provide a data transmis-sion system and method which enable N-to-N data transmission and have a high transmission efficiency with a minimum of empty time in the transmission path.
<Second Embodiment>
FIG. 8 is a block diagram of a loop data transmission system according to a second embodiment of the present invention.
In the railroad control loop data transmission system, transmission devices 103 at the individual stations acting as secondary stations are connected serially via a transmission path 102 to a center transmission device 101 acting as a primary station.
They forms a loop network.
Computers #l, #2 for managing train service are connected to the center transmission device 101. Under the control of the computers #1, #2, the center transmission device 101 transmits a control signal to the transmission device 103 at each station.
A personal computer 105 and a station controller 106 are connected to the transmission device 103 at each station. The personal computer 105 is used to manage information at each station and inform the center or other stations of the management information.
The station controller 106 has the function of controlling a signal mechanism, called an electronic interlocking device.
FIG. 9 shows the modules in each of the transmission devices 101, 103.
In each of the transmission devices 101, 103, there is provided a microprocessor (hereinafter, referred to as an MPU) 111. Connected to a bus line 112 drawn from the MPU 111 are a recording medium 113 on which programs and fixed data needed to execute the programs are recorded, a data buffer 114 in which received data and processed data are recorded, and serial interfaces 115 t 118. A memory 121 in which programs read from the recording medium 113 and executed by the MPU 111 are stored is connected to the bus line 121.
The recording medium 113 may be not only a magnetic disk, magnetic tape, or CD-ROM but also DVD-ROM, a floppy disk, M0, MD, CD-R, or memory card.
Modems 119, 120 with a transmission speed of about 600 bps to 28800 bps are connected to the serial interfaces 115, 116, respectively. The modems 119, 120 are connected to adjacent transmission devices 101, 103, respectively. They form a loop network.
The computers #l, #2 or the personal computer 105 and station controller 106 are connected to the serial interfaces 117, 118, respectively.
The transmission devices 101, 103 can transmit data any time unless they are transmitting frames. If they are transmitting frames, they will store the data being transmitted in the data buffer 114 and place it in a temporary transmission wait state and start the transmission when the repeat has finished. If they receive a frame to be repeated in the course of data transmission, they will store the frame data in the data buffer 114 and place it in a temporary repeat wait state and start the frame repeat when the data transmission has finished.
The second embodiment has a configuration including the basic configuration of FIGS. 8 and 9.
Furthermore, the contents of the SDLC frame are modified. By carrying out frame repeat or frame removal after copy according to the contents of the frame, N-to-N data transmission is effected.
Specifically, the following two modifications are provided.
(1) In one modification, a structure as shown in FIG. 10 is used as an SDLC frame used in the loop data transmission system. The SDLC frame includes not only a destination address but also a source address SA.
Specifically, the SDLC frame includes a flag, a destination address DA, a source address SA, a control section, an information section, FCS, and a flag. The flag is frame synchronization sensing data, the control section is information used to control transmission, the information section is information for application, and the FCS (Frame Check Sequence) is frame error sensing data.
The transmission system in each of the transmission devices 101, 103 has the following configuration.
The MPU 111 of each of the transmission devices 101, 103 has the function of, when receiving a frame transmit request from the computers #1, #2, personal computer 105, or station controller 106, creating a data frame at any time according to the request unless it is repeating a frame.
The MPU 111 in the reception system of each of the transmission devices 101, 103 has the function of reading the first two bytes (flag and DA) in the received frame, comparing the destination address DA
with its own address, and when the destination address does not coincide with its own address, repeating the frame at it is, and when the destination address coincides with its own address, copying the frame into the data buffer 114 and removing the frame from the transmission path without repeating it.
The operation of the system will be described by reference to FIG. 11.
After starting to operate, the MPU 111 of each of the transmission devices 101, 103 reads the program recorded on the recording medium 113 and clears the unnecessary data from the data buffer 114 (S1) according to the program. In this state, the MPU
judges whether the computers #1, #2, personal computer 105, or station controller 106 has made a frame transmit request (S2). If a frame transmit request has been made, the MPU will set a flag (S3) and then judge whether it is receiving (repeating) another data frame (S4) .
If another frame is not being received, the MPU
will judge whether a flag is present (S5). If a flag has been set, the MPU will judge that there has been a frame transmit request. Then, the MPU will create its own data frame and transmit it (S6). The MPU judges whether it has received a frame from another station while it is transmitting the data frame. If the MPU
has received a frame, it will taken in the frame and store it in the data buffer 114 (S7, S8). Then, the MPU judges whether the transmission of its own data frame has been completed (S9). If the transmission of its own data frame has been completed, the MPU will verify whether another data frame has been stored in the buffer (S10). If another data frame has been stored, the MPU will pass control to step 511.
At step 511, the MPU judges whether the destination address of another data frame coincides with its own address when a request for transmitting its own frame is made while another data frame is being received at step S4 or when another data frame is stored in the buffer while its own frame is being transmitted. If they do not coincide, the MPU will perform a repeat process by which the frame is repeated to a station on the downstream side (S12). If they coincide, the MPU will copy into the data buffer 114 and unload the frame from the transmission path without repeating it (513, S14).
When a request for transmitting its own frame is made while another data frame is being received at step S2, the MPU judges whether a flag has been set after the completion of steps 512, S13 (S14). If a flag has been set, the MPU will pass control to step S6, where the MPU will transmit its own data frame.
Therefore, as seen from the above explanation, more than one data frame exists on the loop network at the same time.
FIG. 12 is a time chart to explain a state where more than one data frame is being transmitted over the transmission path 102.
In the example, the center transmission device 101 acting as the main station, the transmission device 103 at railroad station 1 acting as sub-station 1, and the transmission device 103 at railroad station 3 acting as sub-station 3 transmit frames IC2, IlC, I3C respec-tively at the same time. At this time, frame IC2 is addressed to railroad station 2, frame IlC is addressed to the center, and frame I3C is addressed to the center.
The transmission device 103 at railroad station 2 and the center transmission device 103 copy the frame and remove the frame from the transmission path.
With the conventional SDLC, because only the transmission device at the center (main station) was able to unload the data frame transmitted onto the transmission path 102, the transmission devices at the sub-stations could not transmit data frames freely.
With the present embodiment, the data frame can be unloaded from the transmission path at the transmission device at more than one destination station. Therefore, a data frame can be transmitted from more than one transmission device at the same time and the data frame can be received by the transmission devices at plural destination stations, which realizes N-to-N data transmission.
When the data transmission system of the present embodiment is compared with the conventional SDLC data transmission system under the following prerequisites, the transmission performance is as follows:
(1) Prerequisites ~ Modem: V.32 modem with a transmission speed of 9600 bps ~ Number of stations (the number of transmission devices at the center and railroad stations): 32 stations ~ Volume of information: the capacity of the information section in a frame Center ~~ station: 5 bytes/station frequency to each station once/2 seconds Station ~-> center: 10 bytes/station frequency from each station once/second ~ Transmission delay: delay in modem about 20 ms/station ~ Delay in the time needed for the MPU 111 of a transmission device to process is neglected (B) Comparison of transmission performance between stations When a given transmission device sends 10 bytes of information in the information section to the transmission device 16 stations beyond the given device (assuming a station in the middle of 32 stations/loop), the transmission time needed for the head information to reach the destination device is used for comparison.
(a) SDLC data transmission system Data transmission by the SDLC procedure requires a permit signal from the transmission device at the main station. The average waiting time is determined to be half the time required to make one round of the loop.
Average waiting time + transmission delay x 16 stations = 20 ms x 32/2 + 20 ms x 16 =
640 ms (b) Data transmission system of the present embodiment (20 ms + 2 x g/g,6) x 16 stations = 346.7 ms (c) Effect of shortening the transmission time in the present embodiment Dividing the result in (b) by that in (a) gives 346.7 ms/640 ms = 54.20. From this, it can be seen that data transmission can be achieved in a little more than half the time needed in the SDLC procedure.
With the present embodiment, because data frames can be unloaded from the transmission path at the individual destination transmission devices 101, 103, N-to-N data transmission can be realized. In N-to-N
data transmission, any of the transmission devices 101, 103 can transmit a data frame at the same time. When a transmission device is in the course of transmitting its own data frame, it stores a data frame from another transmission device in the buffer and processes the stored data frame after completing the transmission of its own data frame.
The other modification will be explained.
This modification has the following configuration including the basic configurations of FIGS. 8 and 9. A
data frame used in the present modification has the same structure as in FIG. 10.
Each of the transmission devices 101, 103, when receiving a data frame, reads three bytes (a flag, a destination address DA, and a source address SA) in a data frame and compares the destination address DA and source address SA with its own address. If none of those addresses coincide with its own address, it will S repeat the frame to a transmission device on the downstream side. If one of the destination address DA
and source address SA coincides with its own address, it will unload the frame from the transmission path 102 without repeating it. In addition, only when the destination address DA coincides with its own address, it will copy the frame. In this case, the operation of each transmission device is carried out according to the flowchart of FIG. 11 except that step 11 is replaced with step 11A of FIG. 24.
With this configuration, if none of the transmission devices 101, 103 having the destination address set in the data frame should exist on the transmission path 102, the frame can be removed from the transmission path 102 when the source transmission device has received it, improving the reliability. In contrast, with the previous modification, the repeat of the frame is continued infinitely over the transmission path.
In the present modification, the time required to transmit a frame to the transmission device 16 stations ahead under the same conditions as in the previous modification is:

(20 ms + 3 x 8/9.6) x 16 stations = 360 ms Although the transmission time is a little longer than that in the previous modification, comparison of the transmission time with that in the conventional system gives 360 ms/640 ms = 56.30. Therefore, the transmission time in the present modification is shorter than that in the conventional system, improving the reliability.
<Third Embodiment>
(1) A first modification of a third embodiment of the present invention has the following configuration including the basic configurations shown in FIGS. 8 and 9. A data frame used in the first modification has the same structure as that in FIG. 10 of the second embodiment.
The transmission system in each of the transmission device is designed to judge whether it is a broadcast request when receiving a data transmission request from the computers #l, #2, personal computer 105, or station controller 106, and if it is a broadcast request, set a broadcast address in the destination address field in the frame, and transmit the frame.
The reception system in each of the transmission devices 101, 103 is designed as follows. In receiving a frame, the reception system copies the received frame when a broadcast address is present in the set value in the destination address field. Even if a broadcast address is absent, the system will copy the received frame when the destination address in the frame coincides with its own address. When a broadcast address is present and the destination address in the frame does not coincide with its own address, the reception system unloads the frame from the transmis-sion path 102 without repeating it, provided the transmitting party's address coincides with its own address.
The operation of the first modification will be explained.
In a frame transmission operation at each of the transmission devices 101, 103, when receiving a broadcast request or other data request from the computers #1, #2, personal computer 105, or station controller 106, the MPU 111 judges whether a transmission request is present (S22), after initialization (S21). If a transmission request is present, the MPU will judge whether it is a broadcast request (S23). If it is not a broadcast request, the MPU will create a data frame similar to that explained in the second embodiment (524, S25).
If it has been judged at step S23 that a broadcast request is present, the MPU will set a broadcast address (OFFh) in the destination address field and transmit the data frame (S26 to S28).

In a frame reception operation at each of the transmission devices 101, 103, after initialization (S31) as shown in FIG. 14, the MPU 111 judges whether a frame has been received (S32). If a frame has been received, the MPU 111 will judge whether the destination address in the data frame coincides with the its own address (S33).
If the destination address coincides with its own address, the MPU will set a coincidence flag (S34) and judge whether a broadcast address is present in the set value in the destination field (S35). If a broadcast address is present, the MPU will copy the received frame (S36). Even when a broadcast address is absent, if the coincidence flag has been set, the MPU will copy the received frame (S37).
If the coincidence flag is off or if the transmission address coincides with its own address when a broadcast address is present, the MPU will remove the frame from the transmission path 102 without repeating it (538, S39).
FIG. 15 is a time chart to help explain a state where a broadcast frame is transmitted in the data transmission system of the present modification.
In the first modification, when the center transmission device 101 acting as the main station transmits a data frame including a broadcast address, all the sub-station transmission devices 103 on the transmission path take in the data frame and return it to the center transmission device 101. The center transmission device 101 receives the frame after the simultaneous transmission and then removes it from the transmission path.
With the first modification, when the main station transmits an instruction to all the sub-stations simultaneously, a broadcast address is set in the destination address field and the frame is transmitted.
Because each sub-station copies the frame from the broadcast address in the destination address field, the specified information can be transmitted to all the sub-stations more efficiently than in the second embodiment, where the information is transmitted separately to all the sub-stations.
(2) A second modification of the third embodiment will be explained.
This modification has the following configuration including the basic configurations of FIGS. 8 and 9.
A data frame used in the second modification is assumed to have a structure shown in FIG. 16. The meaning of each field is basically the same as in FIG. 10 except that a source address is set when a broadcast flag has been in the destination address field.
The transmission system of each of the transmission devices 101, 103 has the configuration of the second embodiment and further is designed to set its own address and a broadcast flag in the destination address field when receiving a broadcast request from the computers #l, #2, personal computer 105, or station controller 106 and transmit the data frame.
The reception system of each of the transmission devices 101, 103, when receiving a frame, copies the frame into the data buffer 114 when a broadcast flag has been set in the destination address field. If the address set in the destination address field is its own address, the reception system will remove the frame from the transmission path 102 without repeating it.
This enables simultaneous communication.
As described above, with the second modification, a delay in repeat is only two bytes, enabling a more efficient broadcast transmission. In the previous modification, a delay in repeat is three bytes.
(3) A third modification of the third embodiment will be explained.
The third modification of the third embodiment has the following configuration including the basic configurations of FIGS. 8 and 9.
In a frame used in the third modification, a priority flag is set in the destination field as shown in FIG. 17. The priority flag may be set in the transmission party's address field.
The transmission system of each of the transmission devices 101, 103 is designed to set a priority flag in the destination address field in a data frame and transmit its own data frame when receiving a priority transmission request from the computers #1, #2, personal computer 105, or station controller 106. At this time, if the system is in the course of repeating a frame, it will transmit its own data frame as soon as it has processed the frame.
The reception system of each of the transmission devices 101, 103 is designed to save the frame being transmitted in the data buffer 114 when receiving a data frame, if a priority flag has been set in the destination address field, regardless of whether a frame is being transmitted or not, repeat the received frame in preference to others, and transmits it to a transmission device on the downstream side.
FIG. 25 is a flowchart to explain the operation of the transmission device when the priority flag has been set.
As shown in FIG. 25, it is judged whether a flame with a priority flag has been received (S111). When a frame with the priority flag has been received, it is judged whether another frame is being transmitted (5112). If it has been judged at step 5112 that another frame is being transmitted, the frame being transmitted will be saved in the data buffer 114 (5113) and control will proceed to step 5114. If it has been judged that another frame is not being transmitted, control will also proceed to step 5114. At step 5114, the frame with the priority flag is repeated.
Thereafter, another frame is transmitted (5115).
FIG. 18 is a time chart to help explain the operation of repeating the frame in preference to the others and transmitting it to the transmission device 103 at sub-station 2, when the transmission device 103 at sub-station 1 receives a frame with the priority flag on from the center transmission device 101 at the main station while transmitting frames to the transmission device 3 at sub-station 2 consecutively.
With the third modification, even if a transmission device continues occupying the loop transmission path, setting the priority flag enables the necessary data to be transmitted within a specific period of time by interrupting the current occupation temporarily.
<Fourth Embodiment>
(1) A first modification of a fourth embodiment of the present invention has the following configuration including the basic configurations of FIGS. 8 and 9. A
frame used in the first modification has a structure where a TTL (Time to Live) field is provided next to, for example, a flag as shown in FIG. 19.
The transmission system of each of the transmission devices 101, 103 is designed to set an initial value of 31 one smaller than 32 stations and transmit the data frame, when its transmission device transmits a frame with the number of stations on the transmission path being 32 in the TTL field in the data frame.
The reception system of each of the transmission devices 101, 103 includes a TTL content judging section that, when receiving the frame, takes in the set contents from the TTL field in the frame and judges whether the set contents are within a preset specified range from 1 to 31 and a section that, if the TTL
content judging means has judged that the set value is within the specified range, will subtract one from the set value and repeat the frame to a transmission device on the downstream side, and if the TTL content judging means has judged that the set value is outside the specified range, will remove the frame from the transmission path.
Hereinafter, the operation of receiving a frame will be described by reference to FIG. 20.
The MPU 111 of each of the transmission devices 101, 103 performs initialization according to a program (S41) and then judges whether a frame has been received in a specific period (S42). When receiving a frame, the MPU takes in the set value from the TTL
field in the frame (S43) and judges whether the set value is within the preset range from 1 to 31 (S44).

If the set value is within the preset range, the MPU
will subtract one from the set value in the TTL field (S45) and repeat the frame to a transmission device on the downstream side of the transmission path (S46). If the set value is outside the preset value, for example, if it has exceeded the initial value or is zero, the MPU will judge that the frame is invalid or unnecessary and remove the frame from the transmission path without repeating it (S47).
With the first modification of the fourth embodiment, a TTL field is provided in a data frame.
An initial value is set in the TTL field according to the number of stations. Each transmission device receives a frame and repeats the frame by subtracting one, thereby judging whether the frame has made one round or whether the frame is invalid or unnecessary.
This enables the frame to be removed from the transmission path without being repeated.
(2) A second modification of the fourth embodiment will be explained.
A second modification of the fourth embodiment has the following configuration including the basic configurations of FIGS. 8 and 9. A frame used in the second modification has a structure where a time information field is provided next to, for example, a flag as shown in FIG. 21.
The transmission system of each of the transmission devices 101, 103 is designed to set an internal time information variable value in the time information field in a frame and transmit the frame.
Each of the transmission devices 101, 103 includes an internal clock and increments by one in a period of, for example, one second in synchronization with the system internal clock. The set value of the internal time information variable can be changed.
The reception system of each of the transmission devices 101, 103 includes a time difference judging section that, when receiving a frame, compares the set contents in the time information field in the frame with the internal time information variable value in the transmission device at the time of receiving the frame and judges whether the difference between the two values is within a predetermined time, for example, two seconds and a section that, if the result of the judgment has shown that the set contents are within two seconds, will repeat the frame to a transmission device on the downstream side and, if the set contents are larger than two seconds, will judge that the frame is invalid or unnecessary and remove the frame from the transmission path without repeating the frame.
Therefore, with the second modification of the fourth embodiment, it is possible to judge whether a frame is invalid or unnecessary and remove the invalid or unnecessary frame from the transmission path reliably without repeating it.
(3) A third modification the fourth embodiment has the following configuration including the basic configurations of FIGS. 8 and 9. A frame used in the third modification has a structure where a serial-number field is provided next to, for example, a flag as shown in FIG. 22.
Each of the transmission devices 101, 103 has a transmission internal serial-number variable and a monitor internal serial-number variable for each source address and is designed to set a transmission internal serial-number variable in the serial-number field in a frame at the time of transmitting a frame and not only transmit the frame but also increment the transmission internal serial-number variable in the transmission device by one.
The reception system of each of the transmission devices 101, 103 is composed of a serial-number judging section that, when receiving a frame, judges whether the serial number in the serial-number field is larger than the monitor internal serial-number variable corresponding to the source address at the time of receiving the serial-number field and the source address SA, a serial-number changing section that, when the result of the judgement has shown that the serial number is larger (excluding the value of the monitor internal serial-number variable at that time), repeats the frame to a transmission device on the downstream side of the transmission path and resets the transmission internal serial-number variable or the contents of the serial-number field in the frame to the monitor internal serial-number variable, and a frame removal section that, when the transmission internal serial-number variable is a previous one (including the value of the monitor internal serial-number variable at that time), judges that the frame is invalid or unnecessary and removes the frame from the transmission path without repeating it.
With the third modification of the fourth embodiment, it is possible to judge whether the frame is invalid or unnecessary, on the basis of whether the transmission internal serial-number variable in the received frame is larger or smaller than the monitor internal serial-number variable corresponding to the source address SA in the received frame. If it has been judged that the frame is invalid or unnecessary, the frame is removed from the transmission path reliably without being repeated.
(4) A fourth modification of the fourth embodiment of the present invention has the following configura-tion including the basic configurations of FIGS. 8 and 9. A frame used in the fourth modification has the same structure as in FIG. 22.
Each of the transmission devices 101, 103 has a transmission internal serial-number variable for each destination address and a monitor internal serial-number variable for each combination of a destination address and a source address and is designed to set the value of a transmission internal serial-number variable corresponding to the destination address in the serial-number field in a frame and not only transmit the frame but also increment the transmission internal serial-number variable by one.
The reception system of each of the transmission devices 101, 103 is composed of a serial-number judging section that, when receiving a frame, judges whether the serial number in the serial-number field is larger than the value of the monitor internal serial-number variable corresponding to the combination of a destination address and a source address at the time of receiving the destination address field, source address field, and serial-number field and a serial-number changing section that, when the result of the judgment has shown that the transmission internal serial-number variable is larger (excluding the value of the monitor internal serial-number variable at that time), repeats the frame to a transmission device on the downstream side of the transmission path and resets the transmission internal serial-number variable or the contents of the serial-number field in the frame to the monitor internal serial-number variable, and a frame removal section that, when the transmission internal serial-number variable is a previous one (including the value of the monitor internal serial-number variable at that time), judges that the frame is invalid or unnecessary and removes the frame from the transmission path without repeating it.
With the fourth modification of the fourth embodiment, it is possible to judge whether the frame is invalid or unnecessary, on the basis of whether the transmission internal serial-number variable in the received frame is larger or smaller than the monitor internal serial-number variable corresponding to a combination of a destination address and a source address in the received frame. If it has been judged that the frame is invalid or unnecessary, the frame is removed from the transmission path reliably without being repeated.
<Fifth Embodiment>
A fifth embodiment of the present invention has the following configuration including the basic configurations of FIGS. 8 and 9. A frame used in the fifth embodiment has the same structure as that in one of FIGS. 18, 20, and 21.
The reception system of each of the transmission devices 101, 103, when receiving the destination address field in a frame in the normal mode, judges whether the destination address or the contents set in the field coincides with its own address. If they do not coincide, the reception system will repeat the frame to a transmission device on the downstream side.
If they coincide, the reception system will copy the frame into the data buffer and remove the frame from the transmission path without repeating it.
The destination alone is not always sufficient because an address may be set erroneously. To overcome this problem, a portion other than the address fields in a frame, or the variable explained in the third embodiment, may be used complementarily. Specifically, the validity of a frame is judged from TTL, time information, serial number, or others (FCS error or abort frame error), a portion other than the address fields. If it has been judged that the frame is invalid or unnecessary, the operation may be brought into the absorption mode where it is judged whether absorption or repeat should be effected at the time when reception has been carried out as far as the field at which the frame has been judged to be invalid or unnecessary. When the frame repeated to a transmission device on the downstream side has traveled over the transmission path and returned to the source transmission device, the frame may be removed from the transmission path and the operation be returned to the normal mode.
As explained above, with the present invention, because each transmission device judges the contents of a frame separately and, if necessary, removes the frame from the transmission path, N-to-N data transmission is realized.
Use of N-to-N data transmission enables empty time in the transmission path to be minimized, improving the transmission efficiency.
Furthermore, with the present invention, it is possible to provide a recording medium on which a program that realizes N-to-N data transmission and reduces empty time in a transmission path to improve transmission efficiency has been recorded.

Claims (24)

1. A data transmission system comprising:
transmitting devices configured for bidirectional communication on a ring network, each transmitting device being configured to receive and copy a frame of data transmitted on said ring network and to retransmit said frame in opposite directions on said ring network when a source address of said frame does not coincide with an address of said each transmitting device and to not retransmit said frame of data when said source address does coincide with said address of said each transmitting device.

2. The data transmission system of claim 1 wherein said each transmitting device is configured to withhold transmission of a frame originating at said each transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.

3. The data transmission system of claim 1 wherein said each transmitting device is configured to withhold transmission of a frame while another frame is being received from said network.

4. The data transmission system of claim 1 wherein said each transmitting device is configured to read a flag in said frame and to not retransmit said frame, when said flag is set.

5. The data transmission system of claim 1 wherein said each transmitting device is configured to read a flag in said frame and to copy said frame into a data buffer for use by said transmission device, when said flag is set.

6. The data transmission system of claim 1 wherein said each transmitting device is configured to read a flag in a received frame and to save a frame being transmitted while said received frame is being received, when said flag is set, to interrupt occupation of the ring network by said each transmission device.

7. A data transmission device comprising:
a bidirectional modem operable to perform bidirectional communications on a ring network;
and a processor circuit configured to cooperate with said modem to receive and copy a frame of data transmitted on said ring network and to retransmit said frame in opposite directions on said ring network when a source address of said frame does not coincide with an address of said data transmission device and to not retransmit said frame of data when said source address does coincide with said address of said data transmission device.

8. The data transmission device of claim 7 wherein said processor circuit is configured to withhold transmission of a frame originating at said transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.

9. The data transmission device of claim 7 wherein said processor circuit is configured to withhold transmission of a frame while another frame is being received from said network.

10. The data transmission device of claim 7 wherein said processor circuit is configured to read a flag in said frame and to not retransmit said frame when said flag is set.

11. The data transmission device of claim 7 wherein said processor circuit is configured to read a flag in said frame and to copy said frame into a data buffer for use by said transmission device, when said flag is set.

12. The data transmission device of claim 7 wherein said processor circuit is configured to read a flag in a received frame and to save a frame being transmitted while said received frame is being received, when said flag is set, to interrupt occupation of the ring network by said transmission device.

13. A method of operating a data transmission device, the method comprising:

receiving and copying a frame of data transmitted on a ring network;
retransmitting said frame in opposite directions on said ring network when a source address of said frame does not coincide with an address of said data transmission device; and not retransmitting said frame of data when said source address does coincide with said address of said data transmission device.

14. The method of claim 13 further comprising withholding transmission of a frame originating at said transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.

15. The method of claim 13 further comprising withholding transmission of a frame while another frame is being received from said network.

16. The method of claim 13 further comprising reading a flag in said frame and not retransmitting said frame when said flag is set.

17. The method of claim 13 further comprising reading a flag in said frame and copying said frame into a data buffer for use by said transmission device when said flag is set.

18. The method of claim 13 further comprising reading a flag in a received frame and saving a frame being transmitted while said received frame is being received when said flag is set, to interrupt occupation of the ring network by said transmission device.

19. A computer readable medium comprising codes for directing a processor circuit to:
receive and copy a frame of data transmitted on a ring network;
retransmit said frame in opposite directions on said ring network when a source address of said frame does not coincide with an address of a data transmission device associated with said processor circuit; and not retransmit said frame of data when said source address does coincide with said address of said data transmission device.

20. The computer readable medium of claim 19 further comprising codes for directing said processor circuit to withhold transmission of a frame originating at said transmitting device while another frame is being retransmitted to give priority to retransmission of received frames.

21. The computer readable medium of claim 19 further comprising codes for directing said processor circuit to withhold transmission of a frame while another frame is being received from said network.

22. The computer readable medium of claim 19 further comprising codes for directing said processor circuit to read a flag in said frame and to not retransmit said frame when said flag is set.

23. The computer readable medium of claim 19 further comprising codes for directing said processor circuit to read a flag in said frame and to copy said frame into a data buffer for use by said transmission device when said flag is set.

24. The computer readable medium of claim 19 further comprising codes for directing said processor circuit to read a flag in a received frame and to save a frame being transmitted while said received frame is being received, when said flag is set, to interrupt occupation of the ring network by said transmission device.