Abstract:
An encoder for transmission of a digital deactivation code at the end of a variable length message transmission. The encoder adds a deactivation code to the end of an operator&#39;s transmitted message in order to inform the designated message receiving device that the message has concluded and thereby enable transmission of additional information immediately. Preferrably, the encoder transmits a digital deactivation code immediately following a variable length voice message transmission.

Description:
BACKGROUND OF THE INVENTION 
     The concept of squelch is to automatically quiet or mute a radio receiver in response to a specified input signal characteristic and circuits for providing this operation are well known in the art. In high gain receivers the speaker noise produced by the absence of carrier on a given frequency can be very annoying to a operator. Normally such squelch circuits continuously monitor for a presence of an RF signal or carrier. When no signal is detected the squelch circuit is activated and the audio channel becomes inoperative. 
     One of the principal difficulties with such detection systems is that they require constant monitoring for the detection for the presence of carrier and can cause a substantial power drain. When applied to a paging receiver system with a large number of pagers with a correspondingly high message traffic rate, the transmitter carrier signal may never be shut down because of the volume of traffic. Therefore the absence of carrier would never be detected so taht such squelch circuits could not operate correctly. A further difficulty is that the usual analog circuitry to achieve squelch requires some period of time to confirm detection and activation. Thus there frequencly occurs a tailing off of the audio channel. The deactivation control word of the present invention operates to selectively squelch the paging receiver which has been selectively addressed. Thus, only the pager which has been correctly addressed and received the message undergoes deactivation. 
     The use of a deactivation control word for achieving this function with respect to a paging system has the distinct advantage that not only can the voice channel and thus the operator reception be clear of the annoying noise sound but it permits messages to be more tightly packed and thus increase the message throughput. 
     In the system of the preferred embodiment, the coding format to achieve paging is digital in nature and therefore the coding signal which is utilized to provide the deactivation control is itself a digital signal. Thus the squelch operation of the paging recevier has the same sensitivity as for normal the paging. The use of a digitally encoded word to achieve squelching in such a circuit provides many advantages. The use of a positive deactivation contol word at the conclusion of a variable length audio message provides utilization of the now wanted normal time out allocated for fixed length message systems and allows a more tightly packed message arrangement, thus increasing throughput for the entire system. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved encoder for transmitted coded information. 
     It is a further object of the invention to provide an encoder which can control the termination of processing of transmitted coded information by a receiver. 
     An encoding device for generating sequential code signals containing information which is to be transmitted by means of electromagnetic radiation to selected receivers in a plural population of receivers comprising means for generating coded signals to address at least one receiver, in said plural population, means for producing coded information to be processed by addressed receivers, and means for generating coded deactivation signals subsequent to the coded information to cause the addressed receivers to terminate processing of coded information. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 A-C are timing diagrams for the encoded message system for the present invention. 
     FIG. 2 is a functional block diagram of a hardware embodiment of the present invention. 
     FIG. 3 is a state diagram representing the functions for the diagram of FIG. 2. 
     FIG. 4 is a functional block diagram of the firmware embodiment of the present invention. 
     FIG. 5 is an electrical schematic of a portion of FIG. 5. 
     FIG. 6 is a flowchart for the firmware embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a timing diagram depicting the use of a deactivation control word in conjunction with a message system. FIG. 1A shows in time sequence extending from left to right that an individual receiver is addressed from a transmitter terminal and at the conclusion of the address the receiver is actuated to receive a message. The message is generally a voice message and during the first portion of which there is an alert time followed by a real time or otherwised modified voice signal which could also include data information. It is assumed a priory that the message to be sent by such a terminal system can be of a variable length. 
     It is well known in the art that when such message systems provide a fixed length period that frequently the entire fixed time is not used thereby causing a waste of system time and a resultant diminishment of the the number of messages per unit time that can be handled by the system. At the conclusion of the message portion there is included as shown in FIG. 1A a deactivation control signal which for the preferred embodiment is a digital word which causes the individually addressed receiver and only that receiver to disable the audio channel so that there is no disturbance to the receiver in the form of carrier or other background noise and ensure that the broadcast transmitter can then be immediately used for addressing additional receivers. The advantages of such a deactivation code are that the variable length messages can be positively terminated with the same distinction as the initial activation of a receiver upon a receipt of its correct address. 
     FIG. 1B shows a variant form of the use of the deactivation code signal in which a plurality of addresses of individual receivers may be sequenced together in the form of a batch. At the conclusion of the last address in the batch the message is commenced and at the conclusion of the message the deactivation control word operates to terminate the audio operation of all of the previously addressed receivers. In FIG. 1B it is tacitly assumed that the number of receivers that constitute such a batch would effect the message duration in that if there were an alert time which were to be part of the message operation that sufficient time have to be allocated so that the receiver user of the last addressed receiver would have sufficient alerting time for the message. At the conclusion of the variable length message as in the case of voice, all of the addressed receivers would have the audio channel turned off by a reception of the deactivation control word thus the broadcast channel would immediately become available for additional message transmission. 
     While it is clear that many types and formats of signal coding may be utilized for the present invention the preferred embodiment utilizes a digital signal system designated as the Golay Sequential code. The Golay Sequential Code (GSC) is a selective call paging protocol based largely on the current Golay binary paging format. A full description of the Golay code may be found in a paper entitled &#34;Selective Signalling for Portable Applications&#34; by Leonard E. Nelson, 28th IEEE Vehicular Technology Conference, Denver, Colo., Mar. 22-24, 1978. The Golay Sequential Code is an NRZ binary signaling format that has been greatly modified from an earlier format to accommodate intermixed tone only, tone and data, as well as tone and voice paging and now improved battery saving. 
     The GSC is an asynchronous paging format which allows pages to be transmitted individually or in batches. Maximum message throughput for tone only and tone and data pages is achieved in the batch transmission mode; while the individual call mode is useful in tone and voice paging. 
     The single call address format consists of a preamble, a control word, an address code, and for voice paging, an Activation Code (AC). The preamble serves to divide pagers within the system into groups for improved battery life, as well as to uniquely identify GSC transmissions from other coding schemes to faciliate channel sharing without sacrificing battery life or false call integrity. The control word delimits the end of the preamble and it supplies timing information for the batch mode decoding. The address uniquely identifies each pager and the AC is used to control the pager audio circuits in voice paging. The batch mode of operation allows a string of addresses to be transmitted following the control word. 
     A data message consists of an address followed by one or more data blocks. Data messages may be transmitted individually in the single call mode or intermixed with address only pages in the batch mode of transmission. Address and data blocks are exactly the same length. The address information is constructed from words selected from the Golay (23, 12) cyclic code while the data information is encoded using the (15, 7) BCH code. Address information is transmitted at 300 bits/second while data information is transmitted at 600 bits/second. 
     In addition to enabling pagers to operate in a battery saver mode, the polarity of the preamble identifies the transmission mode single call or batch. For instance, when the preamble words are transmitted with one predetermined bit polarity, the single call mode is identified; if the preamble bits are inverted, the batch mode is indicated. 
     The control word activation code and address code all use a two word format consisting of 28 bits of comma followed by two (23, 12) code words. The comma is a 1, 0 bit reversal pattern transmitted at 600 bits/second. The two Golay code words (Word 1 and Word 2) are separated by a 1/2 bit space. The polarity of the 1/2 bit space shall be opposite the first bit of the second word and the starting comma bit must be of the same polarity as the first bit of the first word. The control word and activation code are predetermined for the preferred system. Word 2 of the control word and activation code are the inverses of the fixed words. 
     The address format is identical to the control word and activation code formats regarding the number of bits, the rules for comma and the 1/2 bit space. The address Word 2 may be chosen from any word of the (23, 12) code set except the all .0.&#39;s and all 1&#39;s combinations. Thus, there are 4094 potential second words made up of 12 information bits and 11 parity bits. The first words are chosen from a 100 word subset of the Golay code. To generate the binary bit patterns for the (23, 12) Golay code, the decimal representation of the code word is converted to binary. This binary representation is rewritten LSB to the left. 
     Tone only pages are those pager addresses which don&#39;t involve a voice message. Although the single call mode can be used, the batch mode of operation is the preferred method of address transmission for tone only and tone and data pages. The activation code is generally not used in tone only paging, but it may be and an extended batch mode is especially useful in the high traffic periods. 
     The batch transmission format begins with an inverted preamble followed by the control word and up to 16 pager addresses or data blocks. The arriving page requests should be grouped as a function of preamble and transmitted on a time or traffic basis at the discretion of the terminal manufacturer and his customer. 
     It may be desirable to transmit more than 16 addresses within a single preamble batch. The extended batch mode is intended for these situations. The extended batch scheme extends the batch mode in multiples of 16 addresses without requiring the retransmission of the preamble. To accomplish this extension, the terminal need only send the control word. In theory, the batch could be extended indefinitely; however, a very slight degradation in pager sensitivity will occur with each extension. 
     The GSC format allows data pages to be intermixed with tone only or tone and voice pages. A data page consists of a pager address followed by one or more data blocks. A data block is identical in length to an address block and may be freely substituted for addresses in the batch operating mode. The signal call mode can also be used by following the pager address with the data message. Data information is transmitted at 600 BPS to minimize the cross falsing probability between addresses and data. 
     Referring now to FIG. 1C the normal voice paging format involving a deactivation code of the preferred embodiment is shown on a time sequence basis beginning at the left hand side. A preamble signal is supplied for the purpose of designating a particular segment of the population of receivers. This is followed by a control word which in the preferred system is used for a function unrelated to the present invention. In sequence the address for the first designated pager is transmitted followed by an activation code whose function is to cause the addressed pager to immediately react to the reception of its address. Consistent with the normal alert time for voice systems there is an approximate two second gap during which time the addressed and activated pager provides a beep signal to alert the pager user that a voice message is about to be transmitted. At the conclusion of the alert time the variable length voice message is transmitted and at the conclusion of the variable length voice message the deactivation control word is transmitted to disable the audio channel for the addressed receiver. 
     The preamble for the next pager address is immediately transmitted followed by the appropriate control word and in sequence the address for the second pager the corresponding activation code. This is followed by the two second alert for the second addressed and activated pager after which time the corresponding voice variable length voice message for that second pager is transmitted. At the conclusion of the variable length message for the second addressed and activated pager there is the positive deactivation control word to shut down the audio operation of only the addressed and activated pager and allow further transmission of information for other pagers. 
     As may be seen for by comparison of FIGS. 1A, B, and C although the preferred embodiment includes additional signaling words to achieve other purposes the basic construction is identical to FIG. A in which case a variable length voice message is followed by a deactivation control word which in the digital system allows for an immediate shut down of the audio channel and enables the entire message system to be utilized for the transmission of additional addresses and subsequent additional information so as to increase the throughput of the overall message system. 
     FIG. 2 is an encoder according to the invention for transmitting a deactivation code to be received by a pager receiver or a similar device. Control logic 119 is preferably a PLA which controls the sequential timing of the encoder. Key pads on a keyboard 121 address file memory 123 through keyboard decoder 125. Four keys are shown on keyboard 121 for purposes of illustration only. The particular number depends on the operator&#39;s needs. Keyboard decoder 125 also signals the control logic 119 at its H input when a key pad has been pressed. A time counter 127 provides the timing for both control logic 119 and a shift register 129. Shift register 129 receives parallel data from file memory 123 in accordance with a file memory address A 0 , A 1  and A 2  which is correspondingly received from the keyboard decoder 125 (A 0 , A 1 ) and control logic 119 (A 2 ). A talk light 141 signals to the operator of the encoder that the microphone has been enabled for transmission. When the talk light 141 is lit, the T input of control logic 119 is sensitive to a depression of a talk button 131. In response to a depression of talk button 131, control logic 119 causes a selector 133 to output the signal on selector 133 input A or B. The signal on the A input of selector 133 is a voice audio signal from microphone 135 by way of amplifier 137. The signal on the B input of selector 133 is the serial digital output of shift register 129. The control logic 119 keys the transmitter 139 in the appropriate time sequences as will be more fully explained in connection with FIG. 3. A programmable counter 143 serves to count encoder internal time intervals which will be more particularly described in connection with FIG. 3. 
     FIG. 3 shows the state diagram for the encoder of FIG. 2 transmitting the address of a pager followed by a voice message and a deactivation code. Turning on the encoder of FIG. 2 from an off state (state 0) results in the encoder entering state 1 as shown in FIG. 3. In state 1 the encoder is sensitive to depression of a key pad of keyboard 121. Shift register 129 continuously circulates and loads binary data from file memory 123 as timed by time counter 127, but since there is no transmission in state 1 the data can be arbitrary. Therefore, the state of file memory 123 can also be arbitrary. Transmitter 139 is keyed off by control logic 119 while selector 133 is selected into the B position by control logic 119 so that talk light 141 is off. Control logic 119 is not monitoring the state of talk button 131 while in state 2. Upon the depression of a key pad on keyboard 121, the keyboard decoder latches onto the key number and keyboard 121 and sends a key detect signal to input H of control logic 119. In response the control logic 119 causes the encoder to move to state 2 shown in FIG. 3. The control logic 119 outputs at A 2  a signal to file memory 123 telling the file memory 123 that the outputs A 0  A 1  from keyboard decoder 125 are an address code for 1 of the 4 pagers (Rather than a deactivation code). The file memory 123, in response to the input A 0 , A 1  and A 2 , outputs to shift register 129, the appropriate binary address code which corresponds to the pager whose key pad was pressed. 
     In state 2 shift register 129 loads this address when it next receives a load signal and begins to serially output the address into the B input of selector 133. The control logic 119 selects input B of selector 133. The transmitter 139 is keyed on by the K output by control logic 119. The talk light 144 is out and talk button 141 is inhibited. Automatic cycling time within the control logic 119 moves the state of the encoder from state 2 (send address state) to state 3 (beep time state) as shown in FIG. 3. At the end of transmission of the pager address, the control logic 119 loads the programmable counter 143 with the time duration of the beep that the pager outputs when it receives its proper address. 
     In state 3 the control logic 119 dekeys or deactivates the transmitter 139 and the keyboard decoder 125 input to H of control logic 119 is inhibited. At the end of the beep time as determined by the timing out of the programmable counter 143, the encoder changes from the beep time state (state 3 of FIG. 3) to the state of searching for the depression of the talk button 131 (state 4). 
     In state 4 control logic 119 selects input A of selecter 133. Correspondingly talk light 141 will be lit. When talk button 131 is depressed control logic 119 will move the encoder from state 4 to state 5 of FIG. 3. 
     In state 5 the control logic 119 keys on the transmitter 139 and sets the internal programmable counter 143 to time out a maximum talk length. The control logic 119 monitors the talk button input for its release. The keyboard decoder 125 input to control logic 119, H in FIG. 2, is inhibited. When the talk button 131 is released or when the programmable counter 143 times out the encoder will change from state 5 to state 6. 
     In state 6 the control logic 119 switches the selector 133 to input B. The control logic 119 addresses the file memory 123 to output the deactivation code. The deactivation code is outputted in a binary address from file memory 123 and parallel loaded into shift register 129 and serially outputted to input B of selector 123. The control logic 119 keys the transmitter 139 and the deactivation code is transmitted. Since the control logic 119 shows input B of selecter 133, the talk light 141 is out. The talk button 131 is inhibited. The internal timing of control logic 119 automatically transfers the state of the encoder from state 6 back to state 1 as shown in FIG. 3 and the cycle is repeated if a key pad of keyboard 121 is pressed again. 
     Table 1 on the following page is a time sequential logic table of the PLA comprising the control logic 119 in accordance with the invention. 
     
                                           TABLE 1__________________________________________________________________________LOGIC TABLE for PLAInputsControl  PresentTimer    State Next State                OutputsT H (.0.)    Q.sub.2      Q.sub.1        Q.sub.0          D.sub.2            D.sub.1              D.sub.0                A.sub.2                  K S Programmable Time Count__________________________________________________________________________X 0 X    0 0 0 0 0 0 X 0 0 XX 1 X    0 0 0 0 0 1 0 0 0 XX X X    0 0 1 0 1 0 X 1 0 Beep Time (2S)X X 0    0 1 0 0 1 0 X 0 0 XX X 1    0 1 0 0 1 1 X 0 0 X0 X X    0 1 1 0 1 1 X 0 X X1 X X    0 1 1 1 0 0 X 1 1 Voice Duration (2S)1 X 0    1 0 0 1 0 0 X 1 1 X1 X 1    1 0 0 1 0 1 1 1 0 X0 X 0    1 0 0 1 0 1 1 1 0 X0 X 1    1 0 0 1 0 1 1 1 0 XX X X    1 0 1 0 0 0 X 1 0 X__________________________________________________________________________ 
    
     FIG. 4 shows the functional block diagram of the encoder for the preferred embodiment of the invention in which a microphone 200 is coupled through an audio amplifier 202 to a preemphasis circuit 204 which finally produces an audio signal. This audio signal is supplied to a pair of transmission gates 206 which, as will be described in detail later, allow for the selection of either audio or data information to be transferred to the modulating input of a transmitter 208. An antenna 209 is coupled to transmitter 208. The transmission gates 206 are preferably of the type manufactured by Motorola and designated MC14551. A keyboard 210 is coupled to a CRT 212. This combination is preferably a Lear Siegler Model ADM-3A. The RS232 output of the CRT is coupled to a 6800 base computer 214 which further includes the several additional modules which are: a MEX 6820 I/O module, a 68MM19 6809 monoboard, microcomputer, a MEX6812-1 2K static RAM, and a M68MMCC05 Card Cage for a micro module. 
     PBO output of computer 214 is coupled to the data input of transmission gate 206, the input of an audio data select circuit 216 and the input of an audio/data detect circuit 218. The output of the audio/data select circuit 218 is coupled to the selection input of the transmission gate 206. A zero logic level at this selection input terminal indicates that the data transmission gate is activated and a 1 level indicates that the audio information gate is activated to supply the corresponding information to the modulating input of transmitter 208. 
     The output of audio data detect circuit 218 keys the transmitter so that the operation of aduio data detect circuit 218 is similar to an OR function in that if either signal is detected as being present at its input, the transmitter is keyed so that the input signal may be modulated by the RF carrier. The combined signal is supplied to antenna 209 for broadcast. 
     FIG. 5 shows the detailed circuitry for audio/data detect circuit 218 and audio data select circuit 216. The PBO output of computer 214 is coupled to the input terminal of audio data detector 218 which comprises a retriggerable monostable. The circuit operates to maintain a constant signal output so long as a signal edge is detected within a predetermined time period of every 100 milleseconds. The absence of a detected signal edge during that time period would indicate that neither data nor audio signals were present. 
     In operation of the audio/data detector circuit 218, the PBO output terminal of computer 214 is coupled directly to a first input of an OR gate 230 and through an inverter 230 to the second input. The output of OR gate 230 is coupled to the trigger input of a monostable 234 which may be implemented using one-half of an IC designated MC14538. A timing network is provided for the monostable and VDD, the source of voltage, is applied to the clear input terminal of monostable 234. The VDD is also applied through a resistor 236 to a timing input of the monostable 234 and through a capacitor 238 connected to ground. The combination of the resistor 236 and capacitor 238 provide a timing circuit for monostable device 234. The Q1 output of monostable 234 is coupled directly to transmitter 208 as shown in FIG. 4 to provide the transmitter keying control signal. 
     The function of the audio data detector 218 is to receive within every 100 millesecond interval an edge indicating that either a data or an audio signal is present on the line. As each edge is detected in the OR gate, it retriggers monostable 234 for its predetermined time. As long as pulses are received at an interval that is less than the normal time out for the monstable 234, a Q1 output of monostable 234 in audio data detector 218 will remain at a logic level thereby keying the transmitter. 
     The PBO output of computer 214 is also connected directly to the first input of an OR gate 240 and through an inverter 242 to the second inut of OR gate 240. The first input of OR 240 is coupled to ground. The output of OR gate 240 is coupled to the trigger input of a second retriggerable monostable 244. Monostable 244 is preferably half of the same IC package which was utilized to implement audio/data detector circuit 218. For monostable 244, VDD is applied in the same manner to produce a somewhat different timing characteristic for its operation. This new time is chosen by the value of a resistor 246 and capacitor 248 which are coupled to the timing inputs of monstable 244. VDD is also connected to the clear input terminal of monostable 244. The Q2 terminal of monostable is coupled to the D input terminal of a data flip-flop 250. 
     The PBO output of computer 214 is also coupled to the clock input of D flip-flop 250. The set input of the data flip-flop 250 is connected to ground. Flip-flop 250 is preferably one-half of an IC designated MC14013. The Q output of flip-flop 250 provides the control signal output of audio/data select circuit 216 which is supplied to transmission gates 206 and causes the selective activation of one of the gates depending upon whether either the audio or data information is to be supplied to the modulating input of the transmitter. 
     When it is desired to transmit an audio signal, the encoder for the present embodiment is designed to respond to the presence of a 600 Hz signal so that the activation of an external microphone or other voice storage and forwarding device may be achieved to insert the voice message into the transmission at the appropriate time. There are several other reasons for using various square wave signals at predetermined frequencies but with respect to the present invention only the presence of a 600 Hz signal is used for this specific purpose. 
     Audio data selection circuit 216 operates as a tone detector in which OR gate 240 is triggered on the detection of a squarewave signal edge to produce a relatively long duration output pulse at the Q2 output terminal of monostable 244. This signal is supplied to the D input of the flip-flop which is also coupled directly to the PBO output of computer 214 through its clock terminal. If the long duration pulse from the monostable continues during the time that a subsequent squarewave signal edge is detected from the PBO output of the computer, data flip-flop 250 is either maintained at or is toggled to the 1 logic level output at its Q terminal. This indicates detection of the 600 Hz signal. Termination of the long duration pulse from the monostable before an edge occurs at the clock terminal of the data flip-flop would cause it to toggle to zero or maintain it at zero thus indicating that no 600 Hz audio signal had been selected and therefore no audio signal was to be transmitted. 
     FIG. 6 represents the flowchart for the firmware operation of the encoder shown in FIGS. 4 and 5. When the encoder is activated, it goes through an initialization routine in which all registers are set so that the program can be loaded and executed. After initialization, control is transferred to a decision block at which time the operator selects the control mode. There are three possible control modes; first is the selection of multiple queue transmissions; the second is the selection of single queue transmissions; and most importantly the third is the selection for the block number and type of information to be transmitted. 
     The block number allows the operator to select in a given message sequence exactly where information is to be inserted. The type of information refers directly to the coding system utilized and allows the operator to select the preamble number and the duration of the preamble signal and to select the first and second address words for the echo coding system of the preferred embodiment. Completion of this operation establishes the addresses of the pagers which can be paged. In normal operation, subscriber addresses would be maintained in a memory file status so that individual pagers could be correctly addressed. 
     The next operator choice is the selection of whether or not to specify numeric data information which produces a block of numeric data suitable for transmission to a numeric or data pager. The last operator selection is a frequency signal which for the preferred GSC coding embodiment is indicative of a tone and voice operation for pagers. This has already been described in detail. 
     At the conclusion of the selection of the four types of information, control is transferred to a decision block to determine whether or not all of the selections are completed. If not, the control of the operation is again returned to the select block of number and type operation at which time additional locations for blocks of information may be selected and additional types designated. The selection of the block number and type of information and whether or not the message is to be data tone only or tone and voice would be made for each block of information. After all of the decisions are made and the operator is finished with the block selection, control of the operation is returned to the select mode decision block at which time the operator can choose among the three possible mode operations including adding additional information to the assemblage of blocks and information. 
     The selection of multiple queue transmissions or single queue transmissions enable information of the designated blocks in sequences of information to be sent singly, that is in one transmission, or to be repeated a number of times. Normally, only the single queue transmission would be used, however for the purpose of sending extremely long messages, the use of a redundancy can be built in to ensure even greater reliability of the reception of long complex messages. 
     At the conclusion of the select queue operation for the transmission, control is transferred to the transmit decision block. If at this time the operator elects no transmission, operation of the program is again returned to the select control mode. If the operator chooses to transmit, control is transferred to the start timer operation. 
     The start timer operation takes the blocks of information and formats them into various transmission patterns. At regular intervals of 1200 times per second, the information storage buffers in which the information has been formatted are accessed by a timer interrupt routine which takes the output of a given buffer and provides one bit of information to a sequence control operation so that the transmission pattern may be sent out one bit at a time. At the end of each interruption, control of the program returns to the formatting operation at which time additional information may be loaded into the buffers. 
     In operation the format control takes the blocks of information as designated and selected and loads them into temporary storage buffers so that they may be emptied on a time interrupt bit by bit basis and provided to the transmitter. The output from the transmitter is digital logic signals superimposed on a carrier at the rate of 1200 bits per second. 
     As the various information storage buffers are emptied on a bit by bit basis, the formatting routine continues to load the additional selected information. The buffers may be drained serially to produce the message sequence. At the end of the information in the last stored buffer, the decision to end transmission is made. If it is not the end of the transmission, control is returned to the formatting block so that additional transmission patterns may be loaded into the information storage buffers so that they may be accessed to provide the information for the transmission. If the decision from the end of the transmission decision block is to stop, then a stop timer routine is activated at which time control is transferred back to the select control mode operation. 
     Included with this primary flowchart is a separate secondary flowchart for the timer interrupt routine which on a regular 1200 times per second basis interrogates the information sequence stored in the storage buffers and provides an output to the transmitter. At the conclusion of the transfer of each bit of information control of the operation is transferred back to the formatting operation in the primary flowchart. 
     Table 2 shows the hexadecimal core dump of the entire firmware coding program consistent with the language appropriate for the computer shown in FIG. 4. Loading of this code into the ROM will provide the operation described by the flowchart in FIG. 6. ##SPC1## ##SPC2##