GB2053534A - Linked computer system - Google Patents

Abstract

In a linked computer system, information is stored in a plurality of units, any one of which is operable to recall information from its own store 14 or from that of another unit. The units are linked in a ring and each unit is conditioned to receive and to transmit a system free message which is circulated around the ring so that only the unit in possession of the system free message can send an enquiry message around the ring in order to obtain the response message from one of the other units. The units can distinguish errors and also transmit the system free message in the event of an error. Local information retrieval is also possible from each unit. <IMAGE>

Description

SPECIFICATION Linked computer system This invention relates to a linked computer system in which information is stored in a plurality of units, any one of which is operable to recall information from its own store or from that of another unit. Fields of application for the invention include point of sale terminals for recalling and/or updating purchase and stock information, and library systems with a distributed issue file.

Conventional systems have a centralised store, or master unit, to which a plurality of satellite units are connected. However, a disadvantage of such a system is that it fails together with failure of the central store or master unit. This problem may be avoided by connecting similar satellite units to a common bus, each satellite unit containing part of an effective central store. However, such a system requires an arbitration scheme to resolve contention problems. Moreover, a simple interface between the systems cannot be used, and the units cannot be conveniently linked at a distance.

A further problem is that a unit which enquires for information, must wait for a positive or negative acknowledgement from each other unit and it is necessary for the enquiring unit to know how many units there are in the system. This creates a problem if one of the units are removed.

The invention solves the problem of master unit failure, in that at least part of an effective common store remains operational. It also solves the contention problem which exists if the common bus system, since there is no pre-emptive enquiry other than the one which is being made by a unit which is capable of reaching the store.

According to the invention, a plurality of units, which each include part of an effective common storage of information as well as means to transmit an enqiury to storage, are connected in a ring. A "system free" message is continuously circulated around the ring and only the unit in possession of this message is allowed to send an enquiry. On receiving the system free message, a unit can send an enquiry message requesting a response from another unit. After receiving the desired response, the "system free" message continues to circulate the ring to the next unit. The units in the ring may be interconnected with conventional serial interfaces.

The advantages offered by the invention are mainly that of simplification in giving access to an effective central store, and that contention problems do not arise.

One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment, in which; Fig. 1 is a schematic block circuit diagram of the principle elements in a typical unit, which is one of a plurality of units connected in a ring in a linked computer system according to the invention, and Fig. 2 is a schematic diagram representing data flow in two of the units in the system.

Referring to the drawings, Fig. 1 illustrates a typical unit which is one of several connected in a ring (not shown). The construction of the units comprises conventional microcomputers with a serial data transmission facility and programmed to carry out the functions described below. Each unit is provided with a message input 1 connected to the unit as shown, and a message output (not shown) which provides output messages on any one of a series of terminals referenced 2-1 3 in the drawings.Terminals 1-13 are only shown schematically in this figure to assist explanation, since the messages may be collectively considered as belonging to one type of the following group and they are all sent on the same transmission line; STX ACK ETB - system free STX NAK ETB - negative response STXEDATAETB-enquiry STX R DATA ETB - response The messages each consists of blocks of ASC II characters as generated by a teleprinter or VDU.

The four types of messages (ACK, NAK, E and R) are identified by these characters.

The message output of each unit is connected to the message input of the next succeeding unit in the ring (not shown) in which the messages are circulated in either one direction, or the other.

Each unit contains its own local store 14, which forms a "part" of an effective common store shared by each unit in the system. Thus, each unit may be used by itself, for recalling information in the local store without affecting the operation of the other units (except when the information required by another unit is contained in the local store of a unit being used to recall local information). In order to provide access to the common store, the "system free" message ACK is circulated in the ring and the unit in possession of this message is able to contact and thus recall information from the local store of another unit.

Information may also be circulated in the ring to update a respective unit so as to keep the common store abreast of each transaction.

Generally speaking, the information contained in the local store of each unit is unique and so each unit does need to be updated to reflect the current status of transactions.

A printout system (not shown) may be provided at one or more units in the ring so as to provide information relating to a particular transaction, or the status of the transactions carried out by the system.

Each unit is provided with a series of "switches" 1 5-20, such as logic gates, which are normally in the state shown in fig. 1, wherein 1 represents a line which is open to transmit a message, and 0 represents a line which is closed to that message. The state of these switches 1 5-20 is controlled by software with regard to the state of devices connected thereto, which devices cause a change of state wherein 1 becomes 0 and 0 becomes 1. For example, n output from device 23 will cause a change of state at switch 15.

Each unit also includes a device 21, which is capable of distinguishing between any one of the 4 messages noted above and of transmitting the particular message on the appropriate line. For example, the output lines from device 21 are designated as NAK for a negative response message; RSB for a response message; ACK for a system free message; and ENQ for an enquiry message. These messages are routed to different parts of the unit as will be explained below.

Each unit also contains devices 23-26 for respectively setting one of the following flags; MRTN - return of enquiry message expected MRSP - response message expected NAKR - local response negative TMSG - enquiry waiting An enquiry device 27 enables an input 27a to be made to the unit in order to recall information from its local store or from the local store of another unit. Device 27 is connected to device 26 to set the "enquiry waiting" flag.

The unit also includes devices 21,28, 29 and 30 for distinguishing between certain types of the above mentioned messages, as will be explained below.

The function of the unit shown in fig. 1 will now be described, having regard to different search conditions wherein any one or more of the flag setting devices 23-26 may be set. There are 4 main search conditions namely, (a) no local search being made and system functioning without error, (b) a local search being made, (c) a distant search being made i.e. not in the unit in question, and (d) a search system error. Each of these conditions is described below and this is followed by a summary of the message routing with respect to flag setting.

It will be understood, that each unit, connected in the ring functions in a similar manner.

No Search In this passive condition, a message input at 1 is directed through switches 1 5, 16 to device 22 which determines the type of message. These messages are passed on from device 22 as shown abbreviated by the following designations: NAK - negative response RSP - response ACK - system free ENQ - enquiry The previous designations of these messages (for example, STX ACK ETB) are program designations wherein STX represents a system transmission and ETB represents termination.

With switches 1 7, 1 8 and 19 in the states show in fig. 1 and no flags set, these messages are suplied to terminals 6, 8 10 and 13 respectively.

Device 30 is capable of passing on the ENQ message to terminal 13 as well as to the local store 14. In the event of an error, where device 30 receives a message which is not an ENQ message, it derives its own "system free" ACK message at terminal 12. The ENQ message on terminal 13 enabes the next or second unit in the ring to process the ENQ message from the first unit through the store of the second unit. Device 14 is a conventional construction, for example, it is capable of searching a file of stored information and of transmitting data pertaining to a particular enquiry. The data is made available either at an output 14 a to the local enquiry device (as explained below under "Local Search"), or it is passed via a device 27 through switch 18 to terminal 7 for transmission to the next or second unit in the ring (as explained below under "Distant Search").Enquiry device 27 is also of conventional construction, for example, it is provided with a keyboard input for entering the ENQ message. If the local store 14 does not contain the required information, it sets the NAKR flag of device 25. The state of switch 19 then changes, whereby a response message RSP, received from a preceding unit in a ring, at input 1 is routed through switches 1 5, 16 and device 22 and then through switch 19 to terminal 8. This enables the RSP message to be transmitted around the ring until it reaches the unit at which a "distant" enquiry was made.

Local Search An enquiry at input 27a of device 27 sets the TMSG flag of device 26 and changes the state of switch 20 so as to route the ENQ message through device 29 to the local store 14. In the event of an error, where device 29 receives a message which is not an ENQ message, device 29 generates a "system free" ACK message which is transmitted from terminal 10 the the next unit in the ring. In a local search, the required information is retrieved from store 14 and is supplied to the enquiry device 27 via output 14a.

Distant Search There are two operating conditions of the unit shown in fig. 1 namely that where the unit illustrated seeks information in the store of another unit in the ring, and that where the unit illustrated passes information from its local store to another unit in the ring.

In the first case, an enquiry at device 27 sets flag TMSG and the ENQ message is routed, as explained in the "Local Search" above through device 29 to both the local store 14 and to terminal 11. However, in this case, the ENQ message terminal 11 needs to be transmitted and processed by each unit in the ring in order to reach the store of the unit which contains the information required. A signal is also transmitted through switch 20 to device 23 to set the MRTN flag indicating that a return of an enquiry message is expected. This changes the state of switch 1 5 whereby messages received at 1 are routed to device 28. If the ENQ message is correctly received back a NAK message is transmitted at terminal 3 to elicit a response and MRSP flag 24 is set to (change the state of) switch 1 6. If an ENQ message is not correctly received back then an ACK message is transmitted at terminal 2 to clear the error.

If none of the other units in the ring provide a positive response then a NAK message will be received at input 1 and routed to device 21 which will inform the enquiry device 27 and transmit an ACK message at terminal 4. If device 21 receives a RSP message, it is routed to device 1 7. If the local search device 14 has set the NAKR flag the RSP message is routed to device 27 and an ACK message is transmitted at terminal 4. However, if the local search was positive, the NAKR flag will not have been set, thereby indicating an error as there should only be one positive response. Device 17 then indicates an error and transmits an ACK message at terminal 5.

In the condition where an ENQ message has been received from another unit in the ring, a NAK message will next be received at input 1 and routed through device 22 to switch 18. If the local search is positive, the NAKR flag will not be set and device 18 will transmit the RSP message at terminal 7.

In the condition under which the illustrated unit does not contain the information required by another unit in the ring, the store 14 sets the NAKR flag which causes a change of state of switches 1 7, 1 8 and 19. Thus, when flags MRTN and MRSB are not set, input messages at terminal 1 pass through switches 15 and 16 and device 22 whereby an NAK message is retransmitted from terminal 6 and an RSP message is retransmitted from terminal 8. The state of switch 20 does not change, since no enquiry has been made to set flags TMSG, and hence the "system free" message ACK is routed to terminal 10. An ENQ message passes through device 30 to terminal 1 3.

System Error If a message is not of an expected type, it will have been seen that an ACK message is transmitted to clear the error. A "System Error" message is also generated and sent to device 27.

The ACK message may also show a "system error" in other units in the ring, in particular warning the operator initiating the transaction. In normal operation of the system, errors should not occur.

Summarising the transmission of messages with regard to the setting of flags in each unit: If the MRTN and MRSP flags are not set, the message type is checked.

If the message ACK is received, it is retransmitted unless the TMSG flag is set in which case the ENQ message is set and the MRTN flag set. The message is also actioned locally and the NRKR flag is set if appropriate.

If flag MRTN is set, the NAK message is set to elicit the return of an RSP message and the flag MRSP is set unless the message received is not the return of the ENQ message in which case system error is displated as the ACK message transmitted.

If the MRSP is set, the message is checked for a negative response NAK. If not and the flag NAKR is not set, a system error is displayed as both a local and a remote resonse. The ACK message is transmitted and the response returned to the transaction program.

If flags MRTN and MRSP are not set and the message is ENO, it is passed on and locally actioned.

If a response message NAK, it is passed on unless the the NAKR flag is not set, in which case there is a system error.

If flags MRTN and MRSP are not set and the message is NAK, the RSP message to the previously received ENQ message is transmitted unless flag NAKR is set, in which case the NAK message is passed on. If the message is not NAK, ACK or E, then a system error is displayed and the ACK message is transmitted.

The link is maintained even after an error providing an ETB character is received.

The flags are automatically reset after they have been read.

Referring to Fig. 2, two units are shown which may form part of a ring containing other units. This figure is schematic only in showing that four message registers are required in each unit, and in illustrating the flow of data when unit 1 initiates an enquiry and unit 2 responds. The enquiry and search devices 27 and 14 respectively are those illustrated in the typical unit of Fig. 1. Registers E REG and R REF store the ENQ and RSP messages and the registers and the registers A REG and G REG respectively store the ENQ message to be processed through the local search device 14 and the response resulting from that search. Thus, an ENQ message from the E REG of unit 1 is stored in the G REG of unit 2. This is then processsed through the search device of unit 2 so as to deliver a response to its A REG which is subsequently transmitted via device D, to the R REG of unit 1. In the case of a local search in say, unit 1, the ENQ message in E REG is delivered to the G REG and then passed to the search device 14 which supplied the local response to the A REG. This response is then delivered to the R REG via device D. Device D is schematically illustrated so as to indicate the routing of these local and distant search responses to the R REG of unit 1.

Claims (9)

1. A linked computer system in which information is stored in a plurality of units, any one of which is operable to recall information from its own store or from that of another unit, said units being linked in a ring and each of said units being conditioned to receive and to transmit a system free message which is circulated around the ring so that only the unit which is in possession of said system free message, can send an enquiry message around said ring in order to obtain a response message from another one of the units in said ring.

2. A linked computer system in which information is stored in a plurality of units whereby each unit includes part of an effective common storage of information, each unit comprising input means to receive an enquiry message, means to pass the enquiry message to local storage for retrieving information, and means to pass the latter information, to output means, as a response message, the input and output means of said units being linked to form a ring, and each unit being conditioned:: (a) to receive a system free message from one of the preceding units in said ring; (b) to transmit said system free message to the following one of the units in said ring; (c) to transmit said enquiry message to a following unit in said ring, only when in posession of said system free message, in order to circulate said enquiry message in said ring to derive said response message from one of the other units in said ring having required information; (d) to elicit said response message after said enquiry message has circulated said ring; and (e) to transmit the system free message to the following unit in said ring subsequent to receiving the response message from said one of said other units in said ring.

3. A system according to claim 2 wherein each of said units includes means to condition the unit, in the event of a negative response, when said unit does not contain information in local storage to respond to said enquiry message, so as to pass the response message from the preceding unit to the following unit.

4. A system according to claim 2 or 3 wherein each of said units includes means to distinguish between the enquiry message and an error message and to transmit the system free message in the event of said error message.

5. A system according to any one of claims 2 4 wherein each of said units includes means to enable a local enquiry to be made for information to be retrieved in its local storage.

6. A system according to any of claims 2-5 wherein said means to pass the enquiry message to storage is operative to condition the unit to transmit the system free message and to indicate an error in the event that a response message is received from one of the other unit as well as from local storage.

7. A system according to any one of claims 2-6 wherein each of said units is conditioned to transmit the enquiry message and subsequently to set a flag which further conditions said unit to expect reception of the enquiry message after circulating said ring, said unit being further conditioned to transmit a negative acknowledgement message, on receiving back the enquiry message, to elicit the response message from one of the other units in said ring.

8. A system according to claim 7 wherein each of said units is conditioned to transmit a system free message if the enquiry message is not correctly received back from said ring, or if the negative acknowledgement message is received back from said ring.

9. A system according to any one of the preceding claims substantially as herein described with reference to the accompanying drawings.