JP2000269993A - Data transmission method and transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent transmission delays or aborted data due to occurrence of collision by enhancing a transmission protocol to a CSMA/CD system. SOLUTION: After application of power, transmission control is conducted by a transmission protocol of a CSMA/CD system, when transmission of a data frame is successful, the transmission depending on the CSMA/CD system in not used after that, a transmission timer 13 is started when the transmission is successful, and the transmission is conducted when the timer expires. These operations are repeated. By setting time identical to the transmission timer 13 for all the transmitters, no collision takes place among the transmitters after the transmission is successful, and then no transmission delay or aborted data is not generated. Even if a fault takes place in any transmitter and the data frame cannot be sent, the other transmitter can continue data transmission.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data transmission method and a transmission apparatus, and more particularly, to a data transmission method and a data transmission apparatus for performing data transmission in a CSMA / CD system.

[0002]

2. Description of the Related Art Conventionally, a plurality of transmission devices are commonly connected to a transmission line, and a data transmission system for mutually transmitting and receiving data frames between the transmission devices through the transmission line, particularly, each transmission device has a shared memory. The data transmission method in which a transmission area is assigned to each of the shared memories for each transmission device is already known.

FIG. 2 shows an outline of such a data transmission system, and FIG. 3 is a diagram showing a data storage state of a shared memory in the system of FIG. In this system, as shown in FIG. 2, the transmission devices 301 to 303 are commonly coupled via a transmission line 300, and the transmission devices 301 to 303
Are connected to the computers 311 to 313, respectively. Each of the transmission devices 301 to 303 has a shared memory, and a writable area is assigned to these shared memories in advance by the connected computers 311 to 313. That is, as shown in FIG.
The computer 311 includes two areas A1 and A2.
12, two areas B1 and B2 are allocated to the computer 313, and two areas C1 and C2 are allocated to the computer 313. Each computer periodically transmits data to the allocated area of the shared memory. Write. Further, each of the transmission devices 301 to 303 transmits the data frame written in the area allocated to itself to the transmission line 300 at a predetermined transmission cycle and writes the data frame into the shared memory of another computer. The contents of the respective shared memories of the respective transmission devices 301 to 303 that are commonly coupled to 300 are matched.

[0004] For example, the computer 311 writes data to the areas A1 and A2 to which the shared memory is allocated. The transmission device 301 adds a memory address indicating the head address of the area A1 or A2 of the shared memory as an identifier to the data written in the area A1 or A2, forms a frame, and forms a frame on the transmission path 300 at a predetermined period. To broadcast (to all transmission devices). In this case, the data frame has a format as shown in FIG. 5, for example, a header indicating a delimiter of each frame, a DA indicating a destination address of a transmission device to be transmitted, and a source address of a transmission device that has transmitted. , Data length information, a data part, and an FCS part for detecting a frame transmission error. The data portion includes the shared data written in the areas A1 and A2, respectively, and the memory addresses a1 and a2 of the shared memory corresponding to the shared data. Other transmission devices 302, 30 that have received this data frame
3 looks at the memory addresses a1 and a2 in the data frame and writes the data in the corresponding area of its own shared memory. Areas B1, B2 and area C of shared memory written by computers 302 and 303
Similarly, the data of C1 and C2 are transferred to other transmission devices and written in the corresponding areas. , As well,
When the data in the areas C1 and C2 is transmitted, the data is written in the corresponding area of the shared memory of the other transmission devices 301 and 302. The transmission cycle of the data frame from each of these transmission devices is asynchronous with the cycle at which each computer writes data to the shared memory.

In such a data transmission system, CSM standardized on the basis of Ethernet (registered trademark) is used.
A / CD method (carrier detection multiple access / collision detection,
IEEE 802 committee standard) is widely used as a transmission control method. Since the transmission timing of data in each transmission device is asynchronous, collision may occur between data frames due to simultaneous access to the transmission path. However, in the CSMA / CD system, collision occurs between data frames. When this occurs, the transmission of the data frame is interrupted, and after a specified time has elapsed, retransmission is performed.If a data frame collision occurs even during this retransmission, the same retransmission procedure is repeatedly executed, and the data frame is continuously transmitted more than the specified number of times. When a collision occurs, the data frame is discarded. For this reason, CS
The MA / CD method has a disadvantage that the transmission delay time of the data frame increases and the transmission data frame is discarded.

[0006] In order to eliminate such adverse effects, a data transmission method for preventing the occurrence of data frame collisions has recently been developed. As an example, a method disclosed in Japanese Patent Application Laid-Open No. 1-152836 is disclosed. There are means. This means
The present invention relates to a data transmission method for performing a broadcast transmission in which a plurality of transmission devices (stations) are commonly coupled to one transmission path, and the remaining transmission devices receive data transmitted from one transmission device. Wherein one of the transmission devices is defined as a master transmission device, the other transmission device is defined as a slave transmission device, and the master transmission device determines the transmission timing time of each transmission device including itself, and Notifying the transmission device, the master transmission device transmits a data frame at each transmission timing time of itself, and each slave transmission device transmits itself to the transmission device from the time of reception of the data frame transmitted from the master transmission device. Data frames are transmitted according to a set transmission timing time, thereby eliminating collision between data frames. , It is a kind of TDMA system.

[0007]

SUMMARY OF THE INVENTION The aforementioned Japanese Patent Laid-Open No. 1-1
The means disclosed in Japanese Patent No. 52836 is designed to prevent collision by transmitting data at different timings from other slave transmission devices with reference to a data frame transmitted by the master transmission device. However, in such a data transmission system, a method of determining a master transmission device,
There are problems such as relocation of the master transmission device, a method of determining an alternative master transmission device at the time of failure, and different transmission timings set for the slave transmission devices. Also, if the master transmission device cannot transmit a data frame at its own transmission timing time for any reason, or if the data frame does not reach the slave transmission device even if the data frame is transmitted. However, there is a problem that each slave transmission device cannot transmit a data frame during its own transmission timing time.

[0008] The present invention eliminates the above-mentioned problems, and its purpose is to determine the master transmission device without determining it.
Also, without setting a different transmission timing time for each transmission device, each transmission device transmits a data frame in synchronization with its own transmission cycle, and by preventing a collision from occurring continuously, transmission delay time is reduced. It is an object of the present invention to provide a data transmission method and a transmission apparatus which prevent the increase of the number of frames and discard of frames.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a data transmission method for mutually transmitting and receiving data frames by a CSMA / CD transmission control between a plurality of transmission devices commonly connected to a transmission line. Each of the transmission devices is provided with timer means, and when the power is turned on, each of the transmission devices sets a timer time set to the same value for each of the transmission devices in the timer means in its own device, and then sets an arbitrary value. When the data frame is transmitted at the timing and the data frame transmission succeeds for the first time under the transmission control of the CSMA / CD system, the timer means is started, and after this time, the data frame transmission is performed when the timer means times out. Do
Disclosed is a data transmission method characterized by repeating an operation of activating the timer means when transmission of a data frame is successful.

Further, according to the present invention, when there is no transmission data or when transmission data is not prepared when the timer means times out, a dummy data frame prepared in advance is transmitted. Is disclosed.

Further, according to the present invention, the length of a data frame to be transmitted is determined in advance as a fixed frame length,
Disclosed is a data transmission method, characterized in that when the data length requested to be transmitted is shorter than the fixed frame length when the timer means expires, the data frame having the fixed frame length is transmitted by adding dummy data. I do.

Further, the present invention is characterized in that when transmission data is not present or not prepared when the timer means times out, the time corresponding to the data frame length at the time up is set to the timer time set to the same value. The data transmission method is characterized in that the next transmission timing is determined using the value added to the above as the timer time of the timer means.

Further, according to the present invention, when the timer time set to the same value is Ts, and the reference transmission time of the predetermined data frame is T0, the data requested to be transmitted when the timer means times out is set. If the transmission time of T is T, the next transmission timing is determined by setting the timer time of the timer means activated after the transmission of the data frame to Ts + T0-T.

Further, the present invention relates to a transmission line coupled to a transmission line.
A transmission apparatus for transmitting and receiving data frames to and from each other by SMA / CD transmission control, comprising: first control means for performing transmission control according to a CSMA / CD procedure; and all transmission apparatuses coupled to a transmission path. Timer means for setting the same timer time with respect to, and second control means for activating the timer means at the first successful data frame transmission after power-on and at each subsequent successful data frame transmission. And a third control means for controlling the time-up time of the timer means to be a transmission timing after the transmission of the first data frame.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing a configuration example of a transmission device which is a feature of the present invention, and is used as each transmission device of a transmission system as shown in FIG. Note that FIG.
Is a transmission path such as a LAN (local area network) and has a bus shape.

The transmission device 1 shown in FIG.
Transmission / reception control circuit 11, reception buffer 12, transmission timer 1
3, a DMAC 14, a transmission buffer 15, a dummy data transmission buffer 16, a selection circuit 17, an interface circuit 18, internal buses 19 and 20, and a shared memory 21. The transmission control circuit 10 is a CSMA / CD (IEEE).
802, 3 standard) for controlling based on an access method, and is connected to the transmission line 23 (the network 300 in FIG. 2). The interface circuit 18 is connected to a corresponding computer 22. The transmission control circuit 10, the reception buffer 12, and the selection circuit 17 are configured to be connectable to the internal bus 19 under the control of the transmission / reception control circuit 11, respectively.
The reception buffer 12, the DMAC 14, the transmission buffer 15, the interface circuit 18, and the shared memory 21
Under the control of the transmission / reception control circuit 11, the internal bus 20
It is configured to be able to be combined. Also, the transmission / reception control circuit 1
1 itself can also exchange data with the internal bus 20.

The shared memory 21 has an internal configuration as shown in FIG. 4 and is divided into unit length data (N words). Each of these blocks is assigned a transmission area for each transmission device, is assigned memory addresses a1 and a2 of the shared memory 21 when the transmission data frame shown in FIG. 5 is formed, and is assigned in block units (N words). Be incorporated. The allocation of the shared memory 21 and the information indicating the transmission cycle are set in the transmission / reception control circuit 11 by the corresponding computer 22. The transmission timer 13 is a circuit characteristic of the present invention, and its set value is set in the transmission control circuit 11 by the computer 22. This timer 13 may be realized by any means of hardware or software. Hereinafter, the operation of the transmission apparatus of FIG. 1 will be described in several cases.

(1) First data frame transmission after power-on (when there is no transmission collision) When power is applied to the transmission device 1, the transmission / reception control circuit 11 starts operating. Set the timer value that determines frame transmission. Next, the destination address DA, the source address SA, and the transmission data length are written into the transmission buffer 15 and the DMAC 14 is controlled, so that the data of the block allocated to the shared memory 21 as the transmission area of the transmission device is transmitted. Transfer to When this transfer is completed, the selection circuit 17 is set to select the transmission buffer 15 side,
After the above processing is completed, the transmission / reception control circuit 11 activates the transmission control circuit 10 for transmission. The transmission control circuit 10 that has been activated for transmission monitors the electric signal on the transmission line 23 (carrier sense), determines that there is no signal, and determines that the transmission data is free, and reads the transferred data from the transmission buffer 15. At the same time, generate and add header and FCS,
It is converted into a data frame and transmitted on a transmission path. When the transmission of the data frame is normally completed, the transmission control circuit 10 notifies the transmission completion to the transmission / reception control circuit 11, the transmission timer 13 is started, and the selection circuit 17 selects the dummy data transmission buffer 16 side. Set to.

(2) Data frame transmission for the second time or later after power-on (when there is no transmission collision) The transmission / reception control circuit 11 performs processing for giving special priority after the first data frame transmission after power-on is completed. If not, the data to be transmitted the second time after the power is turned on is transferred to the transmission buffer 15 in the same procedure as described above, and the selection circuit 17
Is set so that the transmission buffer side is selected. Normally, this setting ends before the transmission timer 13 times out. When the transmission timer 13 times out in this state, the transmission / reception control circuit 11 that has been notified of the timeout activates transmission to the transmission control circuit 10, and the transmission control circuit 10 that has received the activation command performs the same operation as in the first transmission. If the transmission path is idle, the data in the transmission buffer 15 is converted into a data frame and transmitted. When the timer 13 times out in a state where transmission data is not ready in the transmission buffer due to a high priority process such as an abnormal process, the selection circuit 17 sets the dummy data transmission buffer 16 side. Then, the dummy data is converted into a data frame and transmitted. If the destination address of the dummy data frame is set to an address that is not absolutely received by each of the transmission devices connected to the transmission line, the reception load of each of the transmission devices does not occur. The third and subsequent data frame transmissions after power-on repeat the same processing procedure as the second data frame transmission.

(3) Data frame transmission (when transmission collision occurs) When a plurality of transmission apparatuses try to start the first data frame transmission after power-on at the same time, the transmission of data frames starts simultaneously. Therefore, data frame collision occurs on the transmission path. When detecting the collision, the transmission control circuit 10 repeats several retransmissions according to the retransmission algorithm of the CSMA / CD system until the transmission is successful. When the transmission is successful, the transmission control circuit 10 notifies the transmission / reception control circuit 11 of the completion of the transmission, Start

(4) Data Frame Reception Next, the processing at the time of data frame reception will be described. The transmission control circuit 10 monitors the data frame transmitted on the transmission path, checks its destination address DA,
If it is addressed to its own transmission device, it transfers the transmission frame to the reception buffer 12 and outputs a reception notification to the transmission / reception control circuit 11. Then, the transmission / reception control circuit 11
Activates the DMAC 14 and transfers the data in the reception buffer 12 to the corresponding area of the shared memory 21.

Next, the operation of a system in which the transmission apparatus performing the above-described operation is connected to a network as shown in FIG. 2 will be described. FIG. 6 is a transmission timing chart showing an operation in the case where all the transmission devices are turned on at the same time and the first data frame transmission is started simultaneously in the data transmission system shown in FIG. In FIG. 6, Fa, Fb, and Fc are data frames transmitted by the transmission devices 301 to 303, respectively, and Ca1 and Ca2 are the first and second transmission frames of the transmission device 301, respectively.
The retransmission waiting time at the time of the second transmission collision, Cb1 and Cb2 are the retransmission waiting time at the time of the first and second transmission collisions of the transmission device 302, and Cc1 is the retransmission at the time of the first transmission collision of the transmission device 303. In the waiting time, each of these waiting times is randomly determined according to the CSMA / CD method.
Ts is a transmission timer 1 which is a component of the transmission apparatus.
3 is a timeout time measured, and the same value is set in each transmission device.

Here, in the transmission collision, it takes some time for one transmission apparatus to transmit a frame and receive it at another transmission apparatus, and thereafter it is detected that the transmission path is busy. . For this reason, if two or more transmission devices perform transmission operations at substantially the same time, transmission will occur before transmission path busy detection, and a collision will occur. In this case, the voltage level of the received signal generally decreases. The device recognizes the occurrence of the collision from the voltage level drop and performs the retransmission operation.

When the power of the transmission devices 301 to 303 is simultaneously turned on and the transmission of the first data frame after the power is turned on is started simultaneously, the data frames Fa,
A collision between Fb and Fc occurs (time t1). When a collision occurs on the transmission path, the transmission control circuit 10 of each of the transmission devices 301 to 303 detects the collision as described above, and randomly determines the retransmission waiting time Ca1, Cb1, and Cc1 according to the CSMA / CD method. , After that time elapses, retransmits the collided data frame. At this time, assuming that the retransmission waiting time Ca1 of the transmission device 301 and the retransmission waiting time Cb1 of the transmission device 302 are the same and the retransmission waiting time Cc1 of the transmission device 303 is shorter than Ca1 and Cb1, the transmission of the data frame Fc by the transmission device 303 takes place. Successful (time t2), the data frames F of the transmission devices 301 and 302
The transmissions of a and Fb collide again (time t3). After a successful transmission, the transmission device 303 thereafter repeats the operation of starting the transmission timer 13 → preparing the next transmission data → detecting the timeout of the transmission timer 13 → transmitting the data frame → completing transmission → starting the transmission timer.

The transmission devices 301 and 302 that have collided with the transmission again
Each transmission control circuit 10 detects the collision and randomly again determines the retransmission waiting times Ca2 and Cb2 according to the CSMA / CD method, and retransmits the data frame to be retransmitted again after the lapse of the time. At this time, if the waiting time Cb2 is shorter than Ca2,
The transmission of the data frame Fb of the transmission device 302 succeeds (time t4), and then the data frame Fa of the transmission device 301.
The order of transmission is successful (time t5). Transmission devices 301, 3
The operation of the transmission device 302 after successful transmission is the same as the operation of the transmission device 303 after successful transmission.

Looking at the operation of FIG. 6 as a whole, each transmission apparatus uses the conventional C until the transmission of its own data frame succeeds.
Attempts are made to avoid collisions in the SMA / CD system, and once transmission has succeeded, transmission is not random but is performed using a transmission timer, which is a feature of the present invention, with a fixed timer time Ts as a period. Therefore, if all of the transmission devices succeed in transmission once, no transmission collision occurs as long as normal operation continues thereafter. Also, even if any transmission device stops transmitting the data frame due to a failure or power loss,
Other transmission devices can continue transmission of data frames without collision.

FIG. 7 shows all transmission devices 301 to 303.
Is a diagram showing an example of the transmission timing after successful transmission of the data frame. The transmission timer of the transmission device 302 has poor accuracy, and the time-out time Ts' is different from that of the other transmission device 30.
1, 303 shows a case where it is shorter than the timeout Ts of the transmission timer. The transmission of the data frames Fa, Fb, and Fc succeeds in the order of the transmission devices 301, 302, and 303 (time t1, t2, and t3), and then the transmission device 301 transmits the data frame Fa after the elapse of the transmission timer timeout time Ts.
(Time t4), and the transmission apparatus 302 further transmits the data frame F after the elapse of the transmission timer timeout time Ts'.
When transmitting b (time t5), Ts' is shorter than Ts, so the transmission timer of the transmission device 302 may time out during transmission of the data frame Fa by the transmission device 301. However, at this time, the transmission control circuit of the transmission device 302 detects that the data frame Fa is on the transmission line, delays the start of transmission of the data frame Fb according to the CSMA / CD method, and stops the transmission of the data frame Fa from the transmission line. At (time t6), the transmission of the data frame Fb is started, and after the transmission is completed, the transmission timer is started and the transmission / reception control circuit 11 is notified of the completion of the transmission. Even if the transmission time of the data frame Fb is delayed every time, the transmission device 302 starts its transmission timer after the transmission is completed each time, so that a timer error between other transmission devices does not accumulate. Also, if the timer error per transmission cycle is smaller than the data frame length, no transmission collision occurs. Therefore, transmission collision does not occur unless a hardware failure or the like occurs.

Next, if the next transmission data frame is not prepared when the transmission timer of the transmission device times out, a dummy frame is transmitted as described with reference to FIG. If the transmission timer is started, the transmission timing of the next data frame will not be out of order. However, when there is no transmission data frame, the transmission timing can be adjusted without transmitting the dummy frame. FIG. 8 is a time chart for explaining such a method, and shows a case where there is no transmission data frame of the transmission device 301 after the data frame transmission of all the transmission devices succeeds. Transmission device 30
The transmission timings of 2, 303 are omitted.
In this figure, when the transmission device 301 has completed the transmission of the data frame Fa (time t1), and when the timeout period Ts of the transmission timer elapses (time t2), there is no data frame to be transmitted next, or there is no preparation. If not, the transmission / reception control circuit 11
Then, Tfs, which is a value obtained by adding the time Tf for the transmission data frame length to the normal transmission cycle Ts, is set in the transmission timer 13 and activated. Accordingly, even when the dummy frame is not transmitted when there is no transmission data frame, the timing at which the next transmission timer 13 times out can be maintained in the same manner as when the data frame is transmitted.

FIG. 9 is a transmission timing chart showing a problem in a system in which the length of a data frame transmitted by any of the transmission apparatuses fluctuates greatly with each transmission. After the transmission of the data frames Fa, Fb, and Fc of the transmission devices 301, 302, and 303 is completed, each transmission device starts a transmission timer. Next, the transmission device 30
The transmission device 301 transmits the data frame Fa at this time, and the transmission data frame length at this time is longer than the previously transmitted data frame length, and during the transmission of the long data frame Fa. It is assumed that the transmission timers of the transmission devices 302 and 303 have both timed out. Then, the transmission devices 302 and 303 try to start transmission of the data frames Fb and Fc, respectively, but detect that the data frame Fa is being transmitted on the transmission path (carrier sense), and
The transmission of the data frames Fb and Fc is postponed until the transmission of the data frame is completed. Then, when the transmission of the data frame Fa is completed (time t2), the transmission devices 302 and 303 start transmitting the data frames Fb and Fc whose transmission has been postponed, respectively, and transmit the data frames Fb and Fc. Collisions occur. After this collision occurs, the transmission devices B and C
The transmission control circuit 10 according to the present invention, according to the CSMA / CD retransmission algorithm,
After elapse of c1, the data frames Fb and Fc are retransmitted. The retransmission of this data frame may be successful only once, but the collision and retransmission may be repeated several times, which is not preferable.

This problem can be solved by keeping the length of the data frame to be transmitted constant. In this case, the data frame length may be made constant for each transmission device, and there is no problem even if the data frame length between the transmission devices is different.
As a method of keeping the data frame length constant, for example, the length of dummy data added to the trailing end of the cyclic data portion in the data frame format shown in FIG. 4 may be adjusted. Control circuit 1
When 1 transfers the transmission data to the transmission buffer 15,
If the transmission data length is shorter than the predetermined length, the transmission data having the predetermined length may be transferred to the transmission buffer 15 by adding dummy data.

There is another method for keeping the data frame length constant when the data frame length fluctuates. It assumes the longest data frame,
When the length is T0, when transmitting a data frame of a shorter length T, the timer time Ts set at the time of activation is changed to Ts + (T0−T), and the data frame of the length T is transmitted. How to start. According to this method, the transmission cycle of each transmission device is always Ts + T0, and the collision as described with reference to FIG. 9 can be avoided.

[0032]

As described above, a data transmission system free from transmission delays and data discards can be realized by performing transmission at a fixed period after successful transmission based on the CSMA / CD system. . In addition, since it is not a TDMA system using a master transmission device, even if a failure occurs in one of the transmission devices and the data frame cannot be transmitted, the other transmission devices do not interrupt the data transmission and perform data transmission. Has the effect of being able to continue.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of a data transmission device according to the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of a data transmission system.

FIG. 3 is an explanatory diagram of a shared memory used for a data transmission method.

FIG. 4 is a diagram illustrating a format example of a data frame.

FIG. 5 is a schematic diagram showing an internal configuration of a shared memory.

FIG. 6 is a diagram illustrating an example of a data frame transmission timing immediately after power-on of a transmission device.

FIG. 7 is a transmission timing chart showing an operation example when there is a transmission timer error.

FIG. 8 is a transmission timing chart showing an operation example when there is no transmission data frame.

FIG. 9 is an explanatory diagram of transmission collision when a transmission data frame length is different for each transmission cycle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 301-303 Transmission device 23, 300 Transmission line 10 Transmission control circuit 11 Transmission / reception control circuit 12 Reception buffer 13 Transmission timer 14 DMAC 15 Transmission buffer 16 Dummy data buffer 17 Selection circuit 18 Interface circuit 19, 20 Internal bus 21 Shared memory

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Tomizawa 5-2-1 Omikacho, Hitachi City, Ibaraki Prefecture Inside the Omika Plant, Hitachi, Ltd. (72) Inventor Yoshihiro Nakano 5-chome, Omikamachi, Hitachi City, Ibaraki Prefecture No. 1 F term in the Hitachi, Ltd. Omika factory (reference) 5K032 CA08 CA10 DB31 5K033 CA08 DB25

Claims (6)

[Claims] 1. A data transmission method for mutually transmitting and receiving a data frame by a CSMA / CD transmission control between a plurality of transmission devices commonly coupled to a transmission line, wherein each transmission device has a timer means. In addition, each transmission device, when its power is turned on, transmits a data frame at an arbitrary timing after setting a timer time set to the same value for each transmission device in its own timer means, When a data frame is transmitted successfully for the first time under the transmission control of the CSMA / CD system, the timer means is started,
After this point in time, the data transmission is performed when the timer is up, and the operation of activating the timer when the data frame transmission is successful is repeated. 2. The data according to claim 1, wherein a dummy data frame prepared in advance is transmitted when there is no transmission data or when transmission data is not prepared when the timer means times out. Transmission method. 3. The length of a data frame to be transmitted is predetermined as a fixed frame length, and dummy data is added when the data length requested for transmission is shorter than the fixed frame length when the timer means times out. 2. The data transmission method according to claim 1, wherein the data frame having the fixed frame length is transmitted. 4. When there is no transmission data or when no transmission data is prepared when the timer means times out, a value obtained by adding a time corresponding to a data frame length at the time up to the timer time set to the same value. The data transmission method according to claim 1, wherein the next transmission timing is determined as a timer time of the timer means. 5. The method according to claim 1, wherein the timer time set to the same value is T
s, and when the predetermined reference transmission time of the data frame is T0, and the transmission time of the data requested to be transmitted at the time of the timer means is T, the timer started after the data frame is transmitted. 2. The data transmission method according to claim 1, wherein the next transmission timing is determined by setting the timer time of the means to Ts + T0-T. 6. A transmission device coupled to a transmission path for mutually transmitting and receiving data frames under transmission control of the CSMA / CD system, wherein the transmission device performs transmission control according to a procedure of the CSMA / CD system.
Control means, a timer means for setting the same timer time for all the transmission devices coupled to the transmission line, and success of data frame transmission at that time and thereafter after the first successful data frame transmission after power-on. Second control means for activating the timer means every time, and third control means for controlling the time-up time of the timer means to be a transmission timing after the first data frame transmission. A transmission device characterized by the above-mentioned.