US3633166A

US3633166A - Data transmission method and serial loop data transmission system

Info

Publication number: US3633166A
Application number: US34781A
Authority: US
Inventors: Jean L Picard
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1969-05-16
Filing date: 1970-05-05
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04
Also published as: JPS5026243B1; FI53519C; DE2022096A1; FI53519B; FR2044650A1; SE365629B; DE2022096B2; BE749291A; NO133735C; GB1244297A; NO133735B; FR2044650B1; DE2022096C3

Abstract

A method and system for transmitting data between a central control unit and a plurality of terminals arranged in a loop configuration in which each terminal includes a shift register and a two-position switch, the former short circuiting the terminal and the latter interposing the shift register in the loop. When the central unit wants to know which terminals have data to transmit to it, it sends a B character on the loop, which is intercepted by the registers of all the terminals. The terminals which have data to transmit place their switch in its second position and load their register with their identification number. The central unit then sends a sequence of A characters which are intercepted by the terminals which have their switch in the second position and which shift the identification numbers from register to register and so forth to the central unit, thus enabling the central unit to determine which terminals have data to transmit.

Description

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority DATA TRANSMISSION METHOD AND SERIAL LOOP DATA TRANSMISSION SYSTEM Primary ExaminerI-Iarold I. Pitts AttorneysI-Ianifin and J ancin and John B. Frisone ABSTRACT: A method and system for transmitting data between a central control unit and a plurality of terminals arranged in a loop configuration in which each terminal includes a shift register and a two-position switch, the former short circuiting the terminal and the latter interposing the shift register in the loop. When the central unit wants to know which terminals have data to transmit to it, it sends a B character on the loop, which is intercepted by the registers of all the terminals. The terminals which have data to transmit place their switch in its second position and load their register with their identifi- 5 Claims 7 Drawing Figs cation number. The central unit then sends a sequence of A US. Cl 340/163, characters which are intercepted by the terminals which have 340/147 their switch in the second position and which shift the identifi- Iut. Cl H04q 5/00, cation numbers from register to register and so forth to the H04q 9/00, H04q 1 1/00 central unit, thus enabling the central unit to determine which Field of Search 340/163 terminals have data to transmitv ,24 4 A 1 F is 'l 4 REGISTER L 26 20w T O 1 2 n I Z I I I -22 21 l I DECODER MEMORY l C O N T R OL PATENTEDJm 4m sleaailse swim 1 UF 5 rt FIG/1 TERMINAL I TERMINAL JERMINAL A 12 M/ I 4 CONTROL D 1 3\ 13 J T CENTRAL TRANSMISSION CONTROL UN IT I DECODER MEMORY I T 23 CONTROL J msmmm 41972 3633.166

SHEET 3 UF 5 REGISTER so; 1355 4 FIG 4 o 1 2 n a. I a 37 %V I l 31 REGISTER as; v l DECODER o 1 2 n M ILQ l l 52 MEMORY V I I as I CONTROL FIG. 6

I REGISTER 4o; 4 o 1 2 n t t III:

41 42 'DECODER MEMORY 45 [M CONTROL M PATENTEU JIII M972 SHEET 5 OF 5 FIG. 7

RECEIVED BY T C U CHARACTER TRANSMITTED BY T C U DATA TRANSMISSION METHOD AND SERIAL LOOP DATA TRANSMISSION SYSTEM BACKGROUND OF THE INVENTION This invention relates to a method for transmitting data between atransmission control unit'and a plurality of serially connected terminals arranged in closed loop.

As the business expands, many loan companies or banks have beenforced to open geographically separated branches so that their customers may,"without inconvenience, complete transactions. But with such widely separated network of branches, it becomes more difficult'to complete transactions in any one branch. Thus, if a customer has an account in one branch and if he wants to make a transaction at another one, the clerk of the bank will have to call the fonner in order to obtain information relative to the customer account in order to complete thetransaction.

Therefore, it is necessary to unite such a network of branches by creating a central office wherein all the transactions completed by the different branches may be centralized. In the case of a network of banks, for instance, all the customers will have theirbank accounts at the central office, and'any credit or debit operation made in a branch will automatically be sent to the central office in order to update the account of the customer as a transaction is completed.

The central office is generally provided with one or more computers, which will be called hereinafter transmission control unit," having a very large capacity disk or tape units for memorizing all the incoming information. The transmission control unit communicates with the branches through telephone cables, by means of modulator-demodulator systems, or modems, whichare well-known in the art. In a like manner, in each branch, a terminal which operates the various incoming transaction machines, transmits the information over the same telephone lines via the modems.

DESCRIPTION OF THE PRIOR ART In the prior art, a plurality of systems have been usedto connect the transmission control unit to the different terminals.

In a first system the terminals are radially connected to the transmission control unit. This is the so-called point-topoint configuration. Such a system requires one modem per line at the transmission control unit and one modem per terminal. The cost of all these modems is a high percentage of the total cost of the installation. 7

Another well-known system, the so-called multiple terminals" configuration, consists in connecting in parallel all the terminals on a single line. This reduces the total number of the modems used since only one modem is required at the transmission control unit. Such a system is wasteful of transmission capacity since it must start its modulator before it can start transmission.

A third well-known system, the so-called loop configuration can be used. In such a system, the terminals are connected in series by a single line coming from and leading back to the transmission control unit. The line reaches each terminal through the demodulator of a modern and leaves it through the modulator of the same modem. Thus, the information sent over the loop goes through each of the modems. Said modems, therefore, are always in operation. This avoids the loss of time mentioned previously with regard to the multiple terminals" configuration.

In the loop configuration, it is obvious that several terminals cannot communicate simultaneously with the transmis sion control unit. When a terminal wants to transmit a message, the other terminals must-therefore be informed that the loop will be occupied. A prior art method sends a plurality of predetermined instruction characters over the loop from the transmission control unit, which enables the first rank terminal on the line which wants to transmit a message to the transmission control unit, to get ready for a transmission operation and which controls the switching off of the other terminals. Once this terminalhas transmitted its message; it

LII

sends the same predetermined instruction characters aspreviously and the next following terminal which wants to send a message in its turn, can be operated. With, such a method, once the transmission control unit has infonned the first terminal that it may proceed to complete'a transmission, and this due to the predeterminedinstruction characters, data transmission is under the control of the various terminals in sequence of physical arrangement aridlcontrol is not restored to the transmission control unit. Furthermore, the transmission order of the terminals is determined solely by the rank or physical position they occupy onthe loop which makes the use of some terminals in priority over others impossible.

SUMMARY OF INVENTION Therefore, one object of the invention is to provide a data transmission method between a transmission controlunit and a plurality of terminals, wherein the transmission controlunit reassumes control of transmission after each terminal has completed a single transmission.

Another object of the invention is to provide a-data transmission method between a transmission control unit and aplurality of terminals, wherein the control unit can request apri-. ority transmission of any terminal over the others.

The objects of the invention are fulfilled in a preferred embodiment wherein each terminal includes a register and a twoposition switch, the first position short circuiting the tenninal, the second one inserting the register into the loop'. When there is no information from or to the transmission control unit, the switch is in its first position and the transmission control unit is continuously sending characters of a first configuration over the loop. When the transmissioncontrol.unit wants to know which terminals are in a condition to transmit information to it, said unit sends a character of a second configuration which is received by each terminal. The terminals which have information to be transmitted to the transmission control unit then, switch their switches into their second positions and transfer their identification code or character to their registers, respectively. Then, the transmission control unit continuously sends first configuration characters over the loop. The terminal identification characters are thus transferred from one register to the next one onto the tra'nsmissioncontrol unit, the terminal switches being respectively switched into their first positions when the first character of the first configuration. is received in the corresponding register. It should be :noted that after the second configuration character, the transmission control unit can continuously send characters of a third con.- figuration, said characters being used only for the transfer of the identification numbers from one register to the next one. Therefore, the transmission control unit receives the series of the identification character of all the terminals which want to transmit information to the control unit, which enables said unit to start data transmission of the different terminals according to a transmission order chosen by it while reassuining control of the transmission of data after each terminal completes transmission.

The foregoing and other objects,.features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings:

BRIEF DESCRIPTION or DitAwmos and transmitted by a terminal according to a preferred embodiment of the invention. 7

FIG. 4 is a simplifieddia'gram ofa terminal for communication on the loop according to another embodiment of the invention.

FIG. 5 is a table showing the characters received and transmitted by the terminals on the loop according to the preferred embodiment shown in FIG. 4.

FIG. 6 is a simplified diagram of a terminal for communication on the loop according to another embodiment of the invention.

FIG. 7 is a table showing the characters received and transmitted by the terminals on the loop according to the preferred embodiment shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is the general arrangement of a data transmission system in a closed circuit, the so-called loop configuration.

The transmission control unit 2, or TCU, sends data over the closed circuit 4 via a modulator. Said modulator is part of the modulator-demodulator system, or modern. Likewise, the TCU receives the data from the circuit via the demodulator of modern 3. As shown in the figure, a plurality of terminals A, B, C, can be arranged in the closed circuit. For instance, if reference is made to terminal A, it can be observed that the data sent from the TCU 2 passes through the demodulator of modem 11 ad are then received in the control unit 12. Said control unit can operate on its own a plurality of input-output stations l3, l4, l5, generally placed at the same geographical location. Said stations which include the devices for the introduction of the operated transactions are not part of the invention, and therefore, they will not be described here.

The above used term transmission control unit may designate any device provided with a memory and a control unit capable of handling data transmission and data reception, such as a computer provided with adequate peripheral units, a communication controlling device or a line concentrator.

The preferred embodiment of the invention will now be described with reference to FIG. 2. This figure shows a terminal in a schematic form. The main units for the embodiment of the invention are a shift register the capacity of which corresponds to the format of the characters used during the transmission wherein the data are received or transmitted bit after bit over the loop and a switch 24 with two positions and 26. Position 25 is used to short circuit the terminal and position 26 causes the data to pass through register 20. While switch 24 has been shown in a simplified manner, it is obvious because of the rate of the data flow that it will include electronic circuits which are well known in the art.

Except for the data transmission to or from the terminals, the transmission control unit sends continuously characters of configuration A over the loop. If, for instance, the data transmission is made according to the synchronous mode, these characters are synchronization characters which cause the terminals to get synchronized with the TCU. As long as the A configuration characters are being received by the terminal, switch 24 is in position 25.

When the TCU wants to know which terminals desire to transmit information to it, it then sends a character of a second configuration B. This B configuration character is sent all along the loop since the terminal switches are in positions 25, respectively. But this character is also loaded bit after bit in the shift register of each terminal. When the entire character is loaded in register 20, it is parallel decoded by a decoding device 21 and the result is transmitted to the control device 23. Should the terminal have information to be communicated to the TCU, then the device 23 controls two operations, namely the parallel transfer of the identification number of the terminal from a memory 22 to register 20 on the one hand and the switching of switch 20 which switches from position 25 to position 26 on the other hand. If transmission is not required, switch 24 is maintained in position 25. These decoding and control operations are achieved within a very short time period with respect to the time duration of a bit. Since the sampling of the bits is made at about the midtime of the time period of a bit, there is enough time left after the sampling of the last bit in the character to complete all these decoding and control operations.

After the transmission control unit has sent the B configuration character, it resumes sending the A configuration characters. The transmission control unit could send characters having another configuration over the loop without modifying in any way the rest of the procedure. Since the terminals which had no information to be transmitted to the TCU have maintained their switches in positions 25, respectively, they are of no interest in the following description since they are short circuited. On the contrary, the first terminal which has its switch 24 in position 26 receives the first A configuration character transmitted by the TCU. As this character is being loaded into shift register 20, the identification character is transmitted bit after bit over the loop through line 27 due to switch 24 being in position 26. In the meantime, the bits of the identification character of this first terminal are loaded in the shift register of the second terminal having its switch in position 26 (or in other words, having information to be transmitted). Likewise, as the bits are arriving, the identification character of this second terminal is transmitted over the loop and so on. Therefore, there is a step-by-step shift and the transmission control unit receives the identification character of the last terminal of the loop that desires to transmit information.

Once the A configuration character has been loaded in the shift register of the first terminal, it is decoded by decoder 21 and control device 23 causes switch 24 to transfer from position 26 to position 25. On the other hand, no change in the switch positions occurs in the following terminals which receive the identification number of the preceding terminal which desires to transmit information since it is not an A configuration character which has been decoded.

When the transmission control unit sends the second character of configuration A, it is received by the second terminal which desires to transmit information. The shift of the identification characters occurs in like manner as seen previously, and the transmission control unit therefore receives the identification character of the next to last terminal on the loop that desires to transmit information. Likewise, upon reception of the A configuration character, the terminal which received it switches its switch back to position 25, thus causing the next A configuration character to reach directly the next terminal which has its switch in position 26.

In this embodiment, it has been supposed that the terminal identification codes were represented by a single character, but they could be assumed as being represented by a plurality of characters. In this case, each terminal will be provided with a buffer memory containing the different characters of the identification code before transmitting them via register 20.

Therefore, the transmission control unit receives consecutively, all the identification characters of the terminals having information to be transmitted according to an order which is opposite to their respective geographical positions on the loop. Said unit sends A configuration characters until it receives an A configuration character which is immediately following the last identification character corresponding to the geographically first terminal having information to be transmitted. The reception of this character indicates to the transmission control unit that it has received all the identification characters of the terminals desiring to transmit information. At this time, the TCU may ask any one of said terminals to transmit its information. Contrary to the prior art devices, the first terminal on the loop will not necessarily be the first to transmit its information. The information control unit has only to send a C configuration character followed by the identification number of the selected terminal. The addressed terminal will cause switch 24 to move to position 26. Then the terminal will load the shift register 20 with the information characters coming from memory 22. The continuous sending of A configuration characters will cause register 20 to send said characters over the loop bit after bit.

For a better understanding of the preferred embodiment of the invention, reference is made to FIG. 3. In this figure, it is supposed that there are five terminals on the loop. From amongst these five terminals, only three have information to be transmitted to the transmission control unit, namely terminals l, 3, 5 having identification numbers No. 1, No. 2, No. 3, respectively. It is obvious that these numbers are given only for a descriptive purpose and that, in fact, a number is represented by a single character of a specific configuration (or by a group of several characters as mentioned previously). In FIG. 3, the first column indicates the succession of the characters transmitted from the TCU and the last column, the succession of the characters received by the TCU. For each terminal, the contents of the register at the end of the reception of a character are indicated on the one hand, and the position of switch 24v at the end of the reception of this very character is on the other hand. It can be observed, as seen previously, that the TCU receives first the B configuration character, then the succession of the identification numbers of the terminals having information to be transmitted according to an order opposite to their respective geographical positions on the loop, the A configuration characters which indicate that the TCU has duly received all the identification numbers of the terminals having information to be transmitted.

It is possible to envisage a variation of the invention by making use not only of a single shift register as seen in the previous embodiment, but of two shift registers. In that case, with reference to FIG. 4, it can be seen that register receives the characters coming from the transmission control unit, however, it is register 38 which is loaded from memory 32 with the characters that the terminal is transmitting to the TCU. As seen in the previous embodiment, the transmission control unit sends one B configuration character in order to ascertain which terminals have information to be transmitted. The B configuration character is received in register 30, is decoded by decoding device 31 and'should the terminal have information to be transmitted, the control device 33 which receives the decoding information from device 31, controls the parallel transfer of the identification number of the terminal from memory 32 to register 38 at the end of the reception of the B configuration character in register 30. In the same time, the device 33 controls the change in position of the switch 34 which passes from position 35 to position 37. Thus, all the terminals having information to be transmitted to the TCU have their switches in positions 37 and have loaded register 38 with their identification character, respectively. After sending the B configuration character, the transmission control unit resumes sending A configuration characters. If the first terminal on the loop ready to transmit information is considered, it will receive an A configuration character. As the register 30 is being loaded with this character bit after bit, the terminal proceeds to the sending of its identification number bit after bit from register 38 via position 37 of switch 34. Once the transmission of the identification number is over, the terminal switches its switch to position 35. On the other hand, the second terminal having information to be transmitted receives the identification number of the previous terminal in its register 30 as it is transmitting its identification number from register 38. But because this terminal does not receive any A configuration character from its register 30, its switches its switch 34 to position 36. The same holds true for each of the following terminals having information to be transmitted which switch their switches 34 to positions 36, respectively. For the rest of the procedure, the preferred embodiment of the invention is considered anew with a two-position switch.

The example shown in the table of FIG. 5 is used for a better understanding of the above explained embodiment. In this example, it has been supposed that the

terminals

1 and 3 have information to be transmitted from amongst the three terminals on the loop. In this embodiment as in the previous one, the transmission control unit receives the identification numbers of the terminals according to an order opposite to their geographical positions on the loop.

In this embodiment, it should be noted that if the transmission is carried out in the start-stop mode, it is the detection of the first bit in the character, i.e., the bit start received in register 30 by device 31, that will enable device 33 to control the beginning of the transmission, bit after bit, of the identification number contained in register 38; Whereas in the synchronousmode wherein the characters are occupying consecutive time intervals, the transmission of the identification number contained in register 38is made without taking the bits received in register30 into account.

Once the TCU has'all the identification numbersof'the te'rminals having information to be transmitted, it sends a C configuration character followed by the identification number of the terminal which is asked to transmit its information. But in this embodiment, the designated terminal switches its switch to position 37 in order to send the information characters from register 38.

Another embodiment of the invention will now be described with reference to FIG. 6. As shown in this figure, the shift register is directly on the loop and the characters coming from the transmission control unit are transferred from one register to the next one without being able to short circuit the terminals as seen in the previous example. When the TCU sends a B configuration character, this character is received by the register of the first terminal on the loop. Shouldthis terminal have no information to be transmitted to the TCU, the decoding operation of the character by decoder 41 entails no operation. On the contrary, should this terminal have information to be transmitted, the decoding of the B configuration character will cause the control device 43 to prepare for the transfer of the identification number of the terminal from memory 42 to register 40. But the identification characteris merely pending and it is only when the following A configuration character sent by the TCU is loaded in register 40 that the identification character is actually transferred in parallel into the register 40. In the meantime, the B configuration character is transferred into the register of the next following terminal. When a terminal has information to be transmitted, it prepares its identification character in order to transfer it into its register 40 when the first received A configuration character is decoded by decoder 41. But, in fact, said second terminal will receive first the identification character of the first terminal and it will proceed to no operation. The first terminal identification character will be transferred and only after decoding the following A configuration character which immediately follows the identification character of the first terminal will the second terminal identification character be inserted. Thus, the sequence of the identification numbers received by the transmission control unit will correspond to the order of the terminals on the loop.

For a better understanding of the embodiment of the invention, an example is shown in the table of FIG. 7. It is supposed that there are four terminals on the loop and that only No. l and No. 3 have information to be transmitted. The first column indicates the character sent by the TCU and the columns corresponding to the terminals indicate the character which is in the terminal register after the character has been sent by the TCU. The last column indicates the character received by the TCU after the same has transmitted a character.

Though, in the preceding embodiments, the transmission control unit wanted to know which are the terminals that have information to be transmitted to it. It is obvious for the man skilled in the art that the invention applies also to the case when the transmission control unit has to transmit information to the terminals and must know which are the ones of said terminals that are susceptible to receive information.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of:

transmitting from said central station, characters having a first predetermined bit configuration followed by at least one character of a second predetermined bit configuration and immediately followed by characters having said first predetermined bit configurations;

at each said remote station monitoring the bit configuration on the loop for detecting the presence of said second predetermined bit configuration;

inserting a unique bit configuration identifying the remote station following the detection of said second predetermined bit configuration when the station has data to send to the central;

buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next remote station on the loop;

monitoring the buffered bits to detect the said first predetermined bit configuration; and

passing the received bit stream directly onto the next remote station in the loop when the said first predetermined bit configuration is detected amongst the buffered bits whereby said central station receives from the last station in the loop a plurality of unique bit configurations, each identifying one remote station which has data to send to the central and requires service which may be initiated by the central in any order which is appropriate since the central is advised of all remote stations requiring service.

2. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of:

transmitting from said central station characters having a first predetermined bit configuration followed by at least one character of a second predetermined bit configuration and immediately followed by characters having said first predetermined bit configuration; at each said remote station;

monitoring the bit configurations on the loop for detecting the second predetermined bit configuration,

inserting a unique bit configuration identifying the remote station following detection of the said second predetermined bit configuration when the remote station has data to send to the central station,

following detection of the said second predetermined bit configuration buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next remote station on the loop,

monitoring the buffered bits to detect the said first predetermined bit configuration, and

passing the received bit stream directly onto the next remote station in the loop when the said first predetermined bit configuration is detected whereby the said central station receives from the last remote station in the loop a plurality of unique bit configuration each identifying one remote station which has data to send to the central in the inverse order of their physical arrangement on the loop preceded by the said second predetermined bit configuration and followed by said first predetermined bit configuration, and

at said central station after receipt of said unique address followed by said first predetermined bit configuration transmitting a third predetermined bit configuration followed by a unique remote station address from amongst the unique station addresses received for enabling the addressed remote station to send data via the loop to the central station and repeating the sequence until all stations needing service have transmitted data to the central station. 3. A communications system for providing data transmisprising:

means at said central station for transmitting characters having a first bit configuration followed by at least one character having a second bit configuration and immediately followed by characters having said first bit configuration;

input means at each remote station for receiving data bits from the preceding station;

output means at each remote station for transmitting data bits to the next succeeding station;

shift register means at each remote station connected to the input means for registering a predetermined number of consecutive bits; 1

switch means for alternatively connecting the shift register to the output means in one state and the input means directly to the output means in its other state;

decoder means responsive to the contents of said shift register for detecting the said first and second bit configuration and providing outputs indicative thereof; and

control means responsive to the output indicative of the detection of the second bit configuration for causing said switch means to assume its said other state and for inserting a unique station identification code in the said shift register over the previously received said second bit configuration and for causing said switch means to assume its said first state when the output from the decoder means indicates the reception of the said first bit configuration whereby said central station receives from the output means of the last remote station on the loop a plurality of unique bit configurations each identifying one remote station which has data to send to the central and requires service which may be initiated by the central in any appropriate order since the central is advised of all the remote stations requiring service.

4. A communications system as set forth in claim 3 in which the first and second bit configurations and said unique station identification codes all have the same number of bits.

5. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of:

transmitting from said central station characters having a first predetermined bit configuration followed by at least one character having a second predetermined bit configuration immediately followed by characters having said first predetermined bit configuration;

at each said remote station buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next station on the loop;

monitoring the buffered bits for detecting the presence of said first bit configuration following the second bit configuration; and

inserting a unique bit configuration identifying the remote station when the station has data to send to the central station in place of the said first bit configuration which follows the said second bit configuration whereby the central station receives from the last remote station in the loop a plurality of unique bit configurations each identifying one remote station which has data to send to the central station and requires service which may be initiated by the central in any order which is appropriate since the central is advised of all remote stations requiring service.

Claims

1. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of: transmitting from said central station, characters having a first predetermined bit configuration followed by at least one character of a second predetermined bit configuration and immediately followed by characters having said first predetermined bit configuration; at each said remote station monitoring the bit configurations on the loop for detecting the presence of said second predetermined bit configuration; inserting a unique bit configuration identifying the remote station following the detection of said second predetermined bit configuration when the station has data to send to the central; buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next remote station on the loop; monitoring the buffered bits to detect the said first predetermined bit configuration; and passing the received bit stream directly onto the next remote station in the loop when the said first predetermined bit configuration is detected amongst the buffered bits whereby said central station receives from the lAst station in the loop a plurality of unique bit configurations, each identifying one remote station which has data to send to the central and requires service which may be initiated by the central in any order which is appropriate since the central is advised of all remote stations requiring service.

2. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of: transmitting from said central station characters having a first predetermined bit configuration followed by at least one character of a second predetermined bit configuration and immediately followed by characters having said first predetermined bit configuration; at each said remote station; monitoring the bit configurations on the loop for detecting the second predetermined bit configuration, inserting a unique bit configuration identifying the remote station following detection of the said second predetermined bit configuration when the remote station has data to send to the central station, following detection of the said second predetermined bit configuration buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next remote station on the loop, monitoring the buffered bits to detect the said first predetermined bit configuration, and passing the received bit stream directly onto the next remote station in the loop when the said first predetermined bit configuration is detected whereby the said central station receives from the last remote station in the loop a plurality of unique bit configurations each identifying one remote station which has data to send to the central in the inverse order of their physical arrangement on the loop preceded by the said second predetermined bit configuration and followed by said first predetermined bit configuration, and at said central station after receipt of said unique address followed by said first predetermined bit configuration transmitting a third predetermined bit configuration followed by a unique remote station address from amongst the unique station addresses received for enabling the addressed remote station to send data via the loop to the central station and repeating the sequence until all stations needing service have transmitted data to the central station.

3. A communications system for providing data transmission between a central station and a plurality of remote stations connected in a serial loop with said central station comprising: means at said central station for transmitting characters having a first bit configuration followed by at least one character having a second bit configuration and immediately followed by characters having said first bit configuration; input means at each remote station for receiving data bits from the preceding station; output means at each remote station for transmitting data bits to the next succeeding station; shift register means at each remote station connected to the input means for registering a predetermined number of consecutive bits; switch means for alternatively connecting the shift register to the output means in one state and the input means directly to the output means in its other state; decoder means responsive to the contents of said shift register for detecting the said first and second bit configuration and providing outputs indicative thereof; and control means responsive to the output indicative of the detection of the second bit configuration for causing said switch means to assume its said other state and for inserting a unique station identification code in the said shift register over the previously received said second bit configuration and for causing said switch means to assume its said first state when the output from the decoder means indicates the reception of the said first bit configuration whereby said central station receives from The output means of the last remote station on the loop a plurality of unique bit configurations each identifying one remote station which has data to send to the central and requires service which may be initiated by the central in any appropriate order since the central is advised of all the remote stations requiring service.

5. A method for establishing communications between a central station and a plurality of remote stations connected in a serial loop with said central station comprising the steps of: transmitting from said central station characters having a first predetermined bit configuration followed by at least one character having a second predetermined bit configuration immediately followed by characters having said first predetermined bit configuration; at each said remote station buffering a predetermined number of incoming bits from the loop before passing the buffered bits onto the next station on the loop; monitoring the buffered bits for detecting the presence of said first bit configuration following the second bit configuration; and inserting a unique bit configuration identifying the remote station when the station has data to send to the central station in place of the said first bit configuration which follows the said second bit configuration whereby the central station receives from the last remote station in the loop a plurality of unique bit configurations each identifying one remote station which has data to send to the central station and requires service which may be initiated by the central in any order which is appropriate since the central is advised of all remote stations requiring service.