Abstract:
In an exemplary communications system a multiplicity of mobile terminals are to share a communications link with a primary processor. The primary processor may transmit a multiterminal polling signal defining a series of successive response time slots which the terminals may randomly select. A terminal wishing to send a message transmits a brief response burst in the selected time slot giving its identification. The primary processor may then sequentially poll specific individual terminals having messages to send. The primary processor may maintain a record of currently active terminals and/or of collisions (where more than one terminal seeks to transmit in the same response time slot), and as needed may automatically send e.g. with its multiterminal poll, a time slot number change, creating a greater or lesser number of time slots for future use by the active terminals. The system is thus highly adaptable on a dynamic basis during operation so as to readily maintain optimum throughput under changing conditions. As a consequence additional terminals may become active simply by transmitting their respective identifications in randomly selected response time slots. The multiterminal poll may include a listing of one or more terminals for which the primary processor has a message so that the primary processor may be assured that such terminal is available (as indicated by its response in a random time slot) before a lengthy message is sent.

Description:
AUTHORIZATION PURSUANT TO THE COMMISSIONER&#39;S NOTICE OF MAR. 20, 1987 (1077 OG 22) 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a communication system suitable for the transmission of messages from a large number of secondary units to a primary unit, and in a presently preferred implementation relates to improvements in radio data communication systems wherein a number of mobile transceiver units are to transmit data to a base station under a wide range of operating conditions. The invention is preferably to be applicable as an upgrade of an existing data capture system wherein a number of hand-held transceiver units of an earlier design are already in the field representing a substantial economic investment in comparison to the cost of a base station. 
     In communication systems, various approaches have been taken to allocating use of a communications link among a large number of terminals. In a sequential polling technique, each of the terminals may be polled in sequence. Each terminal may be required to wait its turn when it has a message to send, and many terminals may be polled which have no messages to send. In a time slot type system, a general poll is sent, and terminals with messages to send respond in random ones of a series of time slots following the general poll. It is conceived that a more efficient and adaptable communication system would result from a fusion of these seemingly incompatible polling techniques. 
     SUMMARY OF THE INVENTION 
     An important object of the invention is to provide a multiterminal system and method having increased throughput efficiency in comparison to a system based on either the sequential polling approach or the slotted polling technique as heretofore practiced. 
     Another important object is to provide such a multiterminal system and method which is automatically dynamically adaptable to varying numbers of active terminals and/or other changing parameters during a given operating time period. 
     A more specific object of the invention resides in improving the data throughput of low-cost FM or FSK data communications equipment used on existing narrowband channels. 
     Further objects reside in the provision of a radio data communication system which provides a reliable and efficient communication system which is dynamically adaptable to a wide range of operating conditions. 
     Other objects, features and advantages of the present invention will be apparent from the following detailed description taken in connection with the accompanying drawings, and from the respective features of the appended claims taken individually and in cooperative combinations. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of an existing prior art radio data system which may be upgraded to incorporate features of the present invention; 
     FIG. 2 is a diagrammatic illustration of the sequential polling procedure which is utilized with the prior art system of FIG. 1; 
     FIG. 3 is a diagram illustrating the system timing for the prior art system of FIGS. 1 and 2; 
     FIG. 4 is a diagram similar to FIG. 3 and illustrating system timing for a prior art system having nearly twice the polling speed of the system of FIGS. 1-3; 
     FIG. 5 is a diagram similar to FIG. 2 but illustrating a basic protocol in accordance with an aspect of the present invention, and which is to be utilized with a high performance type of mobile terminal unit representing the next generation in comparison to the system represented in FIG. 4; 
     FIG. 6 is a diagrammatic illustration of an embodiment of multiterminal communication system in accordance with the present invention, applied to the field of mobile radio data communication; and 
     FIG. 7 is a diagram useful in explaining the logic structure and information flow in a communications system such as represented by the embodiment of FIG. 6. 
    
    
     DETAILED DESCRIPTION 
     Description of FlGS. 1-4 
     FIG. 1 shows an existing radio frequency data transmission system 10 wherein a base station transceiver means 11 has a number of mobile transceiver units such as 12A, 12B, . . . , 12N in radio communication therewith. 
     By way of example, the base station may be comprised of a radio base unit 14 such as the model RB1212 of Norand Corporation, Cedar Rapids, Iowa which forms part of a product family known as the RT1200 system. In this case, the radio base 14 may receive data from the respective mobile RF terminals, e.g. of type RT1210, and transmit the received data via a multiplexor 15, e.g. type RM1211, and a communications link 16 (e.g. utilizing an RS--232 format) to a host computer 17. 
     The data capture terminals 12A, 12B, . . . , 12N may each be provided with a keyboard such as 18, a display as at 19, and a bar code scanning capability, e.g. via an instant bar code reader such as shown in U.S. Pat. No. 4,766,300 issued Aug. 23, 1988 and known commercially as the 20/20 High Performance Bar Code Reader of Norand Corporation. 
     The RT1200 system utilizes time division multiplexing on a single frequency channel (e.g. in the 450 megahertz band) to provide access to the respective terminals. The RT1200 communications protocol is based on a sequential polling method that transmits a query addressed to each portable terminal in succession, and allows a specified amount of time for the addressed terminal to respond in case a data message is ready for transmission. 
     A transmitted message consists of an initialization sequence, unit address, message identifier and system information, message data and/or control commands, error control, and end of message indication. 
     The basic sequential polling procedure of a base station such as the RB1212 is illustrated in FIG. 2 wherein level TX represents transmit mode of the base station, and level RX represents receive mode. In FIG. 2, solid line 21 represents a transmit time interval wherein the base station transmits a polling signal for a mobile unit of a first address, e.g. #0, at a predetermined data rate e.g. 4800 baud. The base station then allows a specified amount of time represented as time interval 22 for the addressed terminal to respond if communication activity is required. The intervals at 23 and 24 apply to a second terminal address, e.g. #1, the intervals 25 and 26 apply to a third terminal address, e.g. #2, the time intervals 27 and 28 represent the operation of a fourth terminal, e.g. #3, and so on. 
     The sequential polling process is timed by the multiplexor 15, FIG. 1, of the RT1200 system such that if a response is not received from the addressed mobile terminal within the allotted time such as indicated at 22, a poll is issued to the next terminal in the sequence, e.g. as indicated at 23. In the RT1200  system, if the designated mobile unit has some information to send to the host computer 17, that information is immediately transmitted to the host as the response, in which case the base station remains in reception mode. In effect, for the RT1200 system, any of the reception intervals 22, 24, 26, etc. will be extended so as to receive complete messages from the respective addressed terminals where such messages are ready to be sent in response to the respective polling transmissions 21, 23, 25, 27, etc. In FIG. 2, it is assumed that mobile unit #3 is the first unit with a message to send. In this case, the base station while in receive mode as indicated at 28, will actually receive a transmission from mobile unit #3 indicating that a message will follow. The base station in response to the transmission from mobile unit #3 (which is signified by a second line 28A in FIG. 2) remains in receive mode for the duration of the message from unit #3. The message itself from unit #3 may occur over a time interval as represented by dot-dash line 28B, and may be followed by an end of message transmission from unit #3 as indicated by a second line at 28C. In response to the end of message signal at 28C, the base station switches to transmit mode and transmits an acknowledgement message as indicated at 29 for advising unit #3 that the message was properly received. 
     The base station then resumes polling, e.g. transmitting a polling signal at 30 addressed to a mobile unit #4 and switching to receive mode for an interval 31, and so on. In order to minimize channel capacity that is wasted polling inactive terminals, activity time-outs may be employed so that units that have not transmitted are dropped from the polling sequence and placed in a contention queue. The assumption would be that inactive units are either not being used at all or that the operator is busy at some other activity for significant amounts of time. 
     FIG. 3 illustrates standard system timing (in milliseconds) for the RT1200 system as represented in FIGS. 1 and 2. As indicated, a polling sequence such as indicated at 21 and 22 in FIG. 2 may occupy an overall time interval of 127 milliseconds, with a poll interval corresponding to interval 21 in FIG. 2 requiring twenty-two milliseconds, a transmit/receive turnaround interval such as represented at 33 in FIG. 2 requiring forty-five milliseconds, and a receive interval such as indicated at 22 in FIG. 2 being allotted fifteen milliseconds. 
     FIG. 4 illustrates the corresponding standard system timing (in milliseconds) for a second product family known as the RT2200 system of Norand Corporation. In this case, a standard cycle corresponding to the intervals 21, 33, 22 and 34 in FIG. 3 requires a total of sixty-seven milliseconds, polling intervals such as 35 and 36 requiring twenty-two milliseconds, and intervals 37, 38 and 39 each requiring fifteen milliseconds. The shorter transmit to receive and receive to transmit times offered by the RT2200 system result in nearly twice the polling speed in comparison to the RT1200 system. The modulation method and communication bit rates are identical in the two system families so that it is possible for the components of the RT1200 and RT2200 systems to be mixed subject to some limitations. For example, any mix of terminals or bases that includes an RT1211 multiplexor or an RT1212 base requires that the forty-five millisecond intervals such as 33 and 34 be included during communication involving these components. More particularly, if either the RT1212 base station or RT1211 multiplexor is used with a mixture of RT1210 and RT2210 terminals, all terminals must respond using the slower (45ms) delay. If these units are replaced with both the RB2212 base, and RM2216 multiplexor, the system has the capability of adjusting the delay to match the particular terminals in use. For example, if units #1, #5, and #7 are RT2210&#39;s, and units #2, #3, #4, and #6 are RT1210&#39;s, the former three will be polled using the 15ms delay, the latter four will be polled using the 45ms delay. 
     Description of FIG. 5 
     In a more recent commercial development, a type of mobile terminal unit is to be utilized that includes a high performance 16-bit microprocessor and a memory capacity that allows large and complex application programs to reside and be executed within the terminal. With the introduction of this terminal, which is identified as the RT2210 Radio Data Terminal of Norand Corporation, it is particularly convenient to load the basic terminal with programming so that the resultant programmed terminal will operate as part of an embodiment of a communication system in accordance with the present invention. Such a programmed terminal may interact with a primary communications controller according to a sequence of events such as illustrated in FIG. 5. Implementation of this embodiment of the invention may be effected by means of a new read only memory (firmware) to be loaded into the RT2210 basic terminal, for example. 
     As shown in FIG. 5 a multiterminal polling message such as indicated at 40 is transmitted to all terminals in the system, and the terminals are allowed to respond in random &#34;time slots&#34; such as indicated at 41-44 to indicate to the controller that communication activity is desired by the terminal. The example in FIG. 5 provides four time slots 41-44 in which the terminals may respond to the poll. In practice, the number of time slots are varied depending on the traffic load and other conditions. Each terminal may respond to the poll 40 in any of the desired time slots with its address and some positive acknowledgement. The particular time slot in which the terminal chooses to communicate is selected at random by the terminal. In the event that several terminals--potentially more than the number of time slots available, desire communications, a good probability may remain that at least one of the terminals will transmit its response in a time slot which is free of other responses. Under light loading conditions, it is likely that more than one slot will contain an intelligible response message, which further improves the efficiency of the polling procedure. An additional efficiency improvement may be realized because of a &#34;capture&#34; phenomenon of radio frequency modulation that allows recovery of a relatively strong signal in the presence of a relatively weak interfering signal. 
     Referring to FIG. 5, it may be that a terminal unit with an address of #3 responded during the first time slot 41, and that a terminal unit #0 responded in time slot 43. Several terminal units may have simultaneously responded in time slot 42 such that none was identified by the base station. In such a case, the base station after elapse of time intervals 41-44, may first transmit a polling signal 51 addressed to terminal unit #3, and then receive a message from unit #3 as indicated at 52. As in FIG. 2, the message received at 52 may include a preamble indicated at 52A, a message proper 52B, and an end of message signal 52C. The base station may then transmit an acknowledgement message as represented at 53, and thereafter proceed during time interval 54 to poll the next terminal unit, e.g #0, which successfully responded to the general poll at 40. The message from terminal unit #0 may include message components such as described for message 52. After each successfully responding mobile terminal has been polled and its respective message received, the procedure may be repeated with a further multiterminal polling cycle as at 40-44. The technique illustrated in FIG. 5 is herein termed slotted reservation-access polling since secondary terminal units with a message to communicate issue responses in randomly selected slots which merely seek entry to a queue of secondary units desiring to transmit, (that is, the responding secondary units seek to reserve a future time allocation from the primary unit, at which future time each successful secondary unit will be granted access to the primary unit in accordance with its reservation). 
     Description of FIG. 6 
     FIG. 6 shows an exemplary multiterminal communications system in accordance with the present invention. In this embodiment a type RC2250 network controller of Norand Corporation is programmed according to a protocol embodiment known as the RTC protocol so as to provide a programmed network controller means 6-10. In this particular embodiment which is in successful operation, a base transceiver 6-11 is in a separate housing from the controller means 6-10 and utilizes an existing product of Norand Corporation known as the Model RB2212 base station transceiver. 
     In the specific example of FIG. 6, the network controller means 6-10 manages all communication and processing of data between a group of radio terminals such as indicated at 6-12A, 6-12B, . . . ,6-12X, . . . 6-12N, and a host computer 6-14. Each radio terminal may be a separate hand-held or vehicle-mounted unit comprised of a model RT2210 commercial product of Norand Corporation with firmware programming so as to coact with the controller system 6-10 according to a protocol embodiment known as the RTC protocol. 
     Description of FIG. 7 
     In APPENDIX A to the present specification, a detailed document covering the RTC protocol embodiment is reproduced. This document presents the information required to readily implement the embodiment of FIG. 6. 
     For the sake of an introduction to descriptive terminology, the following discussion in relation to FIG. 7 is presented. While the present discussion includes aspects dependent on the particular example of FIG. 6, the application of the teachings herein e.g. to a local area network without radio links, will be apparent to those skilled in the art from the present disclosure. 
     Referring to Section 2.0 COMMAND LINE of APPENDIX A, an exemplary command for the system of FIG. 6 might appear as follows: run rtc -d90(H)-iF0(H)-q3 where the symbol (H) simply indicates that the preceding two characters are in hexadecimal notation for purposes of the present description. Referring to APPENDIX A, the command line indicates that the address of the data port is 90(H), the interrupt number to use is F0(H), and the queue number to use for the queue indicated at 7-15 is three. It will be noted that for the present implementation the data packet size may have a value of 128 bytes while the number of secondary terminal units such as 7-12A, 7-12B, . . . , 7-12X . . . , 7-12N may be any number up to 128. Where the symbol (H) is omitted, ordinary decimal notation is intended in the present text. 
     Referring to Section 3.0 RTC PROGRAM INTERFACE of APPENDIX A, the system of FIG. 7 uses two queues 7-14, 7-15 for communication with a higher layer, termed host means 7-10. The queue 7-14 is used to pass messages into the RTC task means 7-11 from host means 7-10, and queue 7-15 is used to pass messages to the host means 7-10. 
     The messages are passed in control structures called buffers as defined in Section 3.0. (All Section numbers herein refer to APPENDIX A.) For queues 7-14 and 7-15 a chain of buffers according to the present example are to be on successive entries in the queue. 
     Section 3.1 enumerates the types of messages which may be supplied to task means 7-11 via queue 7-14. Such messages may of course originate at a host computer such as indicated at 6-14, FIGS. 6 and 7. Referring to Section 3.1.1, data for a given terminal unit such as 7-12B may be supplied by task means 7-11 to a corresponding terminal queue such as the queue indicated at 7-20B, and will thereafter be transmitted in its turn e.g. on a first-in, first-out basis. 
     In Section 3.1.3, the version inquiry may be used to identify the particular version number of programming in a selected terminal where different terminals may have different versions of the basic RTC programming. 
     In Section 3.1.7, the RTC MUX may be implemented by programming of the processor of network controller means 6-10. Such processor may be a type 80186 microprocessor in this particular embodiment. In this way a separate multiplexer unit (such as the RM 1211 multiplexer of FIG. 1) is not required. The function of a multiplexer is represented at 7-21 in FIG. 7. 
     As indicated in Section 3.1.8, the slot configuration as represented at 41-44 in FIG. 5 may be changed during system operation, e.g. by changing the specification as to the numbers of slots which may be randomly selected by the secondary terminal units. The number of slots of a given duration may be specified as part of each multiterminal message such as 40, FIG. 5, but an initial slot or an initial set of slots may be made unavailable by selecting as the initial slot value a slot position such as 42 (initial slot value equals e.g. two) or such as 43 (initial slot value equals three). The effect is to introduce added delay between the multiterminal poll e.g. at 40 and the first time slot which may be selected by a responding terminal or responding terminals. The system may automatically and dynamically select the number of available slots e.g. as registered at slot size means 7-25, beginning with the initial slot value e.g. as registered at initial slot value means 7-28. Such number of available slots may be automatically adjusted, e.g. for each multiterminal poll, within the limits specified by the minimum and maximum slot values registered at 7-26 and 7-27. 
     Referring to Section 3.2 of APPENDIX A, the RTC task means 7-11, places messages as enumerated in the Section in queue 7-15 for supply via host means 7-10 e.g. to host computer 6-14. The host means 7-10 may be viewed as including a router means indicated at 7-30 for determining the disposition to be made of messages received from the queue 7-15. For example, referring to Section 3.2 1, the host means 7-10 may decide if terminal data received from queue 7-15 should be sent to the host or not. 
     Section 4.0 of APPENDIX A uses the terminology RTC Primary State Machine and Section 5.0 refers to a RTC Secondary State Machine. These sections discuss the programming and data structures employed in primary task means or primary processor means 7-11, and in secondary task means or secondary processor means 7-12A, 7-12B, . . . ,7-12X, . . . , 7-12N (and collectively designated 7-12), respectively. 
     The primary finite state machine of primary processor means 7-11 includes an UP send routine which is represented at 7-31 (SEND UP), and individual unit finite state machines collectively designated by reference numeral 7-41, and individually indicated at 41A, 41B, . . . , 41X, . . . , 41N. The routine represented as SEND UP means 7-31 is designated in the programming of the primary finite state machine with the notation rtc 13  send --  up and functions to send an UP command via link means 7-22 to the respective secondary finite state machines or secondary units 7-12A, 7-12B, . . . , 7-12X, . . . , 7-12N. In response thereto, the secondary units may randomly select time slots according to respective slot parameters from slot parameter tables as indicated e.g. at 7-32A and 7-33A for secondary unit 7-12A. 
     For example, if the initial slot value is three, and slot size is twenty, a maximum of twenty units may succeed in sending response bursts in time slots such as 43,44, FIG. 5, which are correctly received by the primary processor means 7-11. A list of the addresses or identification means (such as 7-1201, 7-1202, . . . , 7-12FF) of the secondary units which transmit valid responses are then entered in an UP QUEUE means 7-50 (designated UP --  queue in Section 4.0). The primary processor means under the control of the send --  UP routine given by way of example in Section 4.0 may first send its slot size parameter at 7-25 to all secondary units, e.g. as part of a poll message. The secondary units then may select response time slots according to this transmitted slot size as stored at 7-33A, etc., and initial slot values as already stored at 7-32A, etc. The primary processor means 7-11 may count the number of secondary units represented in the UP QUEUE means 7-50 (termed up --  q --  count in Section 4.0), and if this number is greater than zero, a new slot size value may be entered into slot size means 7-25, equal to the value stored at minimum slot value means 7-27, e.g. ten. 
     For the illustrated embodiment, the link 7-22 can be simultaneously accessed by more than one secondary unit, the result being a garbled message burst at the receiver of 7-21. Such garbled messages will be recognized as invalid (using any suitable validity check such as the CRC-16 check value of Section 6.1), and will be counted as a collision by collision sense means of 7-21. As represented in Section 4.0, the send --  UP routine may respond to the presence of collisions in a selected number of available time slots to increase the slot value at slot size means 7-25 by an increment (slot --  inc, Section 4.0) which may be selected at start up of the system. As indicated in Section 4.0, if the new value of slot size is greater than the maximum value at 7-27 (max --  slot --  size, Section 4.0) then the contents of known queue means 7-51 is transferred to UP queue means 7-50. 
     Referring to Section 6.2.1, the primary processor means 7-11 may send a multiterminal polling message such as the following to all secondary terminal units: &lt;FF(H)&gt;&lt;FF(H)&gt;&lt;8A(H)&gt;. This is termed in Section 6.2.1 an UNNUMBERED POLL, and may also be referred to as a multiterminal poll to distinguish it from a poll directed to a specific secondary terminal (as at 51, FIG. 5). The first segment of the multiterminal polling message is the broadcast address (Section 6.2.1) or multiterminal address. The second segment is the RTC command byte (Section 6.2.1) which may represent the UP command (see Section 6.2). The third segment of the multiterminal polling message may represent the number of response time slots available for the response, e.g. ten such slots of forty milliseconds each. 
     The multiterminal polling message may include a fourth segment giving the addresses of terminals for which messages are pending e.g. at TXQ means such as 7-20B of primary processor means 7-11. 
     If a secondary terminal which receives the multiterminal message notes its address in the fourth segment, or itself has a message to send, such secondary terminal will randomly select which of the specified time slots to answer in. The secondary terminal will respond with the basic RTC frame (Section 6.1) inserting its address, e.g. 02(H) for terminal 7-12B, in the address field, and the UP command FF(H) in the command field. Such a response burst is of duration less than that of a time slot. 
     The timeout means 7-52 may control various timeout functions such as the waiting time of the primary processing means 7-11 after transmission of a multiterminal polling message in the absence of response bursts from the secondary terminals or before sending poll messages directed to specific terminals. The TIMEOUT interval may be based on the values in initial slot value means 7-28 and the slot size value in means 7-25 (which will reflect any new slot size specified in the multiterminal polling message). For example, where the initial value is three and the slot size is ten, timeout means 7-52 may effect a corresponding timing cycle (e.g. 520 milliseconds) before sending further messages. 
     Where a terminal such as 7-12B has successfully responded to a multiterminal poll, the primary processing means (before the timeout means 7-52 has provided a timeout interrupt), may be advised that the temporary buffer means 7-54 has received a valid response, termed &#34;receive&#34; in the polling procedure of Section 4.0. Thereupon the primary processor means may establish the event &#34;process buffer&#34; (event=process buffer, Section 4.0) whereupon the UP queue means 7-50 will be advised that terminal 7-12B is active, and the individual unit means 7-41B will be updated to show that terminal 7-12B is ready to be polled for its message. 
     If another secondary terminal 7-12X (e.g. with address 112, or 70 Hexadecimal) has a message at its TRFCB means 7-57X, and enters its address at another time slot in response to the multiterminal polling message, then the primary processor means will process this response also and revise the corresponding individual unit means 7-41X for this address. 
     When the timeout means 7-52 shows the expiration of the unnumbered or multiterminal polling cycle time, the primary processing means may utilize the POLLx frame (Section 6.2.2) to poll specific secondary units such as 7-12X for data. Where there is data as at 7-20B to send, the primary processor means may proceed to send the data using the DATA FRAME of Section 6.2.3. 
     Where data is to be received from a terminal such as 7-12X, the terminal responds to the POLLx command (with the NO DATA bit not set, Section 6.2.3) by sending a data frame based on data stored at 7-57X. The primary processor means acknowledges receipt with a suitable POLLx frame (Section 6.2.2, item 2), and will store the data at RXQ means 7-58X of individual unit means 7-41X. 
     SUMMARY OF EXEMPLARY FEATURES 
     In accordance with an aspect of the present invention, the primary processor means may have a list of known secondary units (e.g. at 7-60) and a list of currently active secondary units (e.g. at 7-50). A count of currently active secondary units (e.g. at 7-61) may be maintained and each multiterminal poll may specify a number of slots (e.g. from 7-25) which is dynamically adjusted in accordance with such count (up-q-count, section 4.0) and in accordance with number of collisions (e.g. at 7-62) experienced in a sample number of recent polling cycles. 
     Since the system has provision for an initial slot value (e.g. at 7-28), and since the secondary terminals can register a corresponding initial slot value (e.g. at 7-32A, etc.), the primary processor means in its multiterminal polling message can assign a specific slot in which a given terminal can respond. Such specific slot could be the initial slot value minus one if there were sufficient priority messages to warrant a reserved slot during an operating time of relatively long duration. Where there are less than FF(H) actual terminals, a high order bit in the address segment (Section 6.1.1) could signal a secondary unit with address given in lower order bits to respond in the reserved time slot (e.g. initial slot value minus one or slot 41, FIG. 5, where the initial slot value was two). 
     SUMMARY OF OPERATION 
     In general, the configuration of a system such as indicated in FIG. 7 with suitable start up parameters, and the synchronization of the secondary units 7-12 with the primary processor means 7-11 may take place analogously as in a prior art system such as shown in FIGS. 1,2,3 and 4. 
     The detailed structure of the new types of messages is given in APPENDIX A hereto; see for example, Section 6.2.1 of APPENDIX A concerning the multiterminal polling message or UNNUMBERED POLL, which corresponds with the poll at 40 in FIG. 5. 
     In accordance with the teachings of the present invention, a multiterminal poll such as represented at 40 in FIG. 5 may include a slot size parameter (slot size, Section 4.0 of APPENDIX A) which specifies the number of time slots such as 41 to 44, FIG. 5, which may be utilized by responding terminals. This slot size may be dynamically varied during normal operation of the system according to a current number of active terminals, e.g. which are answering individually addressed polls within a specified number of retries (Max Retry means 7-70). See the procedure for processing each secondary unit 7-12 which is listed in UP QUEUE means 7-50, as found in Section 4.0 of APPENDIX A hereto. The dynamic variation of slot size is apparent from the send --  UP routine given in Section 4.0 of APPENDIX A. 
     Another feature apparent from Section 6.2.1 of APPENDIX A is the ability to include optional addresses in the multiterminal polling message. This is particularly valuable for a system having appreciable transmit/receive turnaround time e.g. as represented in FIGS. 3 and 4. The contemporaneous transmission of the multiterminal polling message and message segments concerning slot parameters and/or specific terminal addresses is a particularly important aspect of the preferred system and method when utilizing a half-duplex link means 7-22 based e.g. on single channel RF transmission or the like. 
     It will be apparent that many modifications and variations may be effected without departing from the scope of the teachings and concepts of the present invention. 
     APPENDIX A referred to in the present specification occupies the following twenty-one pages and contains Sections 1.0 through 6.2.6. ##SPC1##