Switch Controller for Obtaining a Plurality of Functions From a Single Switch in a Two-Way Transceiver and Method Therefor

1. Field of the Invention

This invention relates generally to the field of control circuits for electronic equipment. More particularly, this invention relates to a controller which permits a single switch to perform a plurality of transceiver functions which may be dependent upon the status of the receiver or the transmitter, or the recent actuation history of the switch itself.

2. Background of the Invention

As the state of the art in the electronic industry progresses, the trend is towards ever smaller electronic devices which must reliably perform a variety of functions. An example of this trend may be found in the field of electronic calculators. While the predecessor of the modern pocket calculator literally occupied rooms, devices with similar computational power which will fit in a shirt pocket are now commonplace. Such devices often have control buttons which electronically address a number of different features per button in order to provide the user with a large number of features in a small package. This is accomplished by using a "second feature button" which is actuated prior to the actuation of a button which addresses a plurality of features. Therefore, in order for the user to address the features of his calculator, two button actuations are required to address a single feature.

In the field of portable radio transceivers, there is a similar size reduction trend taking place. As the size of these devices diminishes, the number of control features incorporated in a single control switch must also increase in order for the user to control the additional electronic features incorporated therein.

This is particularly true for selective calling portable radio transceivers (radio transmitter/receivers) such as those commonly used throughout Europe. In these systems the user is required by regulation to monitor his receiver prior to making any transmission to assure that there is no channel activity taking place on the communication channel he intends to use. Once the user establishes that the channel is free of activity, he must normally transmit a series of tones or a digitally encoded message in order to address the receiver or repeater he desires to contact. For example, in the well known ZVEI (Zentral Verband der Elektro-Industrie) system commonly used in Germany, an address code consists of a sequence of five audio frequency tones between 1060 Hz and 2600 Hz which are consecutively transmitted. Each tone has a 70 millisecond duration with no pause between tones. Each tone represents a single digit 0-9 (or a repeat tone indicating that the preceeding digit is repeated). Each user or repeater is assigned a five digit address which must be correctly decoded to access that receiver or repeater. Encoding and decoding circuits for such systems are well known. Having established receiver or repeater access, the user is then free to carry on normal two way voice or data communications with another transceiver.

One scheme often used to accomplish the above sequence of steps is to provide the portable transceiver with three switches. In operation, the user firsts actuates a "push-to-monitor" switch which enables the receiver section audio so that the user may determine if there is activity taking place on the desired radio frequency channel. When the channel is clear, the user actuates a second switch which transmits an appropriate code word in tones or binary digits, as for example, a set of five sequential tones of predetermined length and frequency, to access the desired receiver. The user then actuates a third switch to enable his transmitter and microphone in order to carry on normal conversation.

In the above transceiver scheme, a total of three switches are required to perform even the most basic selective calling transceiver functions. In addition to these controls, the transceiver must also include a volume control, an on/off switch, a squelch control, a channel selector control, and quite possibly many other controls such as a telephone style keypad for more sophisticated transceivers. It is evident that utilizing three separate switches simply to control the sequence of steps necessary for initialization of a conversation is highly inefficient. As these transceivers incorporate more and more features and options, the switch requirement becomes a severe size limitation in a portable or mobile transceiver.

Another scheme often used in portable transceivers, such as Motorola's European version of its MX300 series of transceivers, utilizes a momentary push-to-talk type switch in conjunction with a three position toggle switch. This exemplifies the utilization of a two switch requirement for the selection of any transceiver function. In this scheme a first position of the toggle switch places the radio receiver in a coded squelch mode. This makes the radio reciever responsive only to tone or digitally encoded messages bearing an appropriate user address code. The center position of the toggle switch places the receiver in the carrier squelch mode which allows the receiver to turn on its audio amplifier and speaker upon receipt of any message on the communication channel without regard to coding. The third position of the toggle switch enables the transmitter and transmits the encoded message required to access a receiver or repeater.

In operation, if the user wishes to initiate a call he switches the toggle switch from the coded squelch position to carrier squelch position in order to monitor the channel for activity by other users. He then moves the toggle switch to the encode mode to transmit an appropriate address code to access an appropriate receiver. The user may then utilizes his "push-to-talk" (PTT) switch in a normal manner as necessary to carry on the desired conversation. This system has the advantage of forcing the user to monitor the channel prior to transmitting the code to access a receiver since he has to pass through the carrier squelch position of the three position switch in order to initiate a call. After completing the conversation, the user must switch his transceiver back to the coded squelch mode.

Another two switch system is used in radios such as the GRUNDIG MODEL FK103. In this system a first switch turns ON the receiver audio so that the user may monitor the channel. Deactuation of the switch places the transceiver in a coded squelch mode. Actuation of the second switch while in the coded squelch mode reverts the receiver to carrier squelch and transmits the address code. All subsequent transmissions are without encoding and the first switch is actuated again to place the radio back in the coded squelch mode.

Although these systems reduce the number of switches required to two, operation of the transceiver with one hand can be quite awkward. They inefficiently utilize valuable transceiver controls to actuate only basic transceiver functions not to mention increasing their cost. Also, the user is required in each case to manually place the transceiver back in a coded squelch mode after conversation has ended.

It is an object of the present invention to provide an improved controller for a multiple function switch.

It is another object of the present invention to integrate a plurality of switching functions in a single switch for a transceiver.

It is another object of the present invention to provide an efficient switch scheme for initiating communications with a portable transceiver in a selective calling communications system.

It is another object of the present invention to provide a controller which allows a single switch to perform a plurality of functions in a portable transceiver as a result of receiver conditions, transmitter conditions, and activity of the switch itself.

It is a further object of the present invention to provide a controller for efficient utilization of switches in a miniature portable transceiver.

In one embodiment of the present invention. A transceiver has a multiple function switch controller including a timer for a establishing a time interval. A first circuit produces a first control signal when the switch is actuated during the time interval, and a second circuit produces a second control signal when the switch is actuated outside the time interval.

In another embodiment of the present invention, a multiple function switch controller includes an apparatus for detecting the actuation of a switch and a circuit for monitoring the status of a communication channel. A timer responsive to either the monitoring circuit or the switch detecting apparatus establishes a time interval. A transmitter enabling circuit enables the transmitter upon actuation of the switch during the time interval and an annunciation circuit annunciates the status of the communication channel upon actuation of the switch outside the time interval.

In another embodiment of the present invention, a transceiver includes a multiple function switch controller having an apparatus for decoding a received signal having a predetermined address. A timer responsive to the decoder establishes a time interval following each received signal having a predetermined address. A first circuit responsive to the actuation of the switch produces a first control signal if the switch is actuated during the time interval. A second circuit produces a second control signal if the switch is actuated outside the time interval.

In another embodiment of the present invention, a transceiver has a multiple function switch controller including a circuit for detecting actuation of the switch and a decoder for decoding received signals of a predetermined type. A timer responsive to either the decoder or the switch detector circuit establishes a time interval following each decoding of a received message of a predetermined type. A controller circuit causes the switch to activate a first function if actuated during the time interval and a second function if actuated outside of the time interval.

In another embodiment of the present invention, a method of controlling a multiple function switch includes the steps of providing a timer to establish time intervals, detecting whether or not the timer's output is predetermined active, counting the number of switch actuations occuring while the output is a predetermined signal, enabling an encoder if the count equals a first predetermined count and inhibiting the encoder if the count is equal to a second predetermined count.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, as to organization, method of use, and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of a hardware implementation of the present invention embodied in a transceiver.

FIG. 2 is a detailed electrical schematic of a hardware embodiment of the controller portion of the present invention.

FIG. 3 is a timing diagram of the conroller of FIG. 2.

FIG. 4 is a system block diagram for a microcomputer implementation of the present invention embodied in a transceiver.

FIG. 5 is a flow chart detailing the operation of the controller portion of the present invention.

FIG. 6 is a combined functional block diagram and electrical schematic showing the hardware to utilize a preferred microprocessor embodiment of the present invention for FIG. 4.

Turning now to FIG. 1, a receiver 10 is coupled to an address decoder 20 and receives modulated coded information from a radio channel. An address decoder 20 examines that information to determine if the transceiver is being selectively addressed by an appropriately encoded transmission. An indication of a correctly encoded address is then provided to an input 25 of a switch controller 30 for processing. This information is also provided to a receiver audio circuit 40 so that voice information received after the detected coded address may be transferred to a speaker 50 in the preferred embodiment. Receiver 10 also provides a squelch circuit 60 with information, usually audio noise, for the purposes of determining whether or not there is an on channel signal, correctly encoded or not, being received by receiver 10. This squelch information is transferred by switch controller 30 at a controller input 65 where it may be processed and delivered to the receiver audio via an output 75.

A transmitter 80 can receive inputs either from a microphone 90 or an address encoder 100, for modulation and transmission on the radio frequency communication channel. It is understood by those skilled in the art that transmitter 80 as well as receiver 10 may be AM, FM, PM or any combination or variation thereof without loss of generality. In the preferred embodiment, a narrowband FM system is used.

Transmitter 80 is enabled by a signal from switch controller 30 at a controller output 105. Switch controller 30 also determines when encoder 100 should generate an address code by providing encoder 100 with an enabling signal from a controller output 115. In the preferred embodiment a momentary switch 120 is normally grounded on one side. Actuation of the switch connects that side of the switch to a logic high in the form of a DC supply 130. The other side of switch 120 is coupled to an input 135 to switch controller 30.

In operation, the system functions in the following manner. If the user desires to initiate a transmission, he first actuates switch 120. If switch controller 30 has received an indication of channel activity at its input 65, the receiver's audio is turned on by output 75 so that the user may hear that channel activity on speaker 50 thereby annunciating the channel status to the user. In the alternative, other ways of annunciating the channel status such as a visual display or light may be preferred in some cases. When the user has determined that the channel is free of activity he actuates the same switch 120 for a second time. The second actuation must occur within a predetermined time interval established at deactuation of the first actuation of switch 120. Upon receiving the second switch actuation, switch controller 30 sends a control signal at output 115 enabling address encoder 100. It also sends a signal at output 105 actuating transmitter 80. Encoder 100 provides transmitter 80 with the address code, such as a ZVEI tone sequence, for the desired receiver and preferrably instructs transmitter 80 to mute any inputs from microphone 90 while the address is being encoded. Microphone 90 is muted at this time to prevent voices or noises entering the microphone from corrupting the encoder generated address.

After the brief period of time required for the encoder 100 and transmitter 80 to send out the address, microphone 90 is unmuted and the user can begin his conversation. If the party being called responds within a predetermined period of time established by the last deactuation of the user's switch 120, he may do so in an uncoded transmission mode in the preferred embodiment. The present user's receiver will acknowledge receipt of that message by transferring voice information from receiver audio 40 to speaker 50. After receiving that transmission the user will typically respond with more voice information in a manual two-way conversation. This is accomplished by once again actuating switch 120 within a predetermined time interval established at the end of the received transmission as indicated by loss of radio frequency carrier.

Upon a third actuation of switch 120 within a predetermined time interval of receiving the last message, a signal at output 105 once again enables transmitter 80 and voice transmission may once again occur. In one embodiment of the present invention, the user may select at his descretion whether or not encoder 100 is actuated at such a time. This is the type of system option that may be useful in a number of communication systems. The system operates in a manner identical to its operation during the last reception and transmission on subsequent receptions and transmissions as long as they occur within the time interval established by the switch controller 30.

In all cases of the preferred embodiment this time interval begins upon deactuation of switch 120 or the end of a received message. In the preferred embodiment this time interval is controlled by a programmable timer which can be set anywhere from several milliseconds to several minutes. A time interval of approximately 7 seconds has been found to be convenient. If either the receiving or transmitting party fails to respond within the predetermined time interval, the transceiver automatically reverts back to the coded squelch mode. The next actuation of switch 120 causes the transceiver to operate as if it is the first actuation of switch 120 thereby restarting the operation sequence without the necessity of manual intervention by the user.

If, rather than a user initiated transmission, a conversation is initiated by receipt of a properly encoded message, address decoder 20 provides switch controller 30 with a signal at input 25. This signal directs switch decoder 30 to enable transmitter 80 upon the first actuation of switch 120 and places the receiver in a non-coded squelch (carrier squelch) mode. The initial switch actuation which is necessary to prevent interference when initiating a call is bypassed automatically when a conversation is initiated by receipt of a properly encoded message. Since that initial actuation is carried out by the party initiating the call. Thus the controller operation is made dependent on both user initiated switch actuations and messages received from calling parties.

If receiver 10 receives a transmission which is on a proper channel but does not possess an appropriately encoded address, address decoder 20 will not respond and switch controller 30 will not turn on receiver audio 40. Actuation of switch 120 during the time that this improperly encoded signal is being received however, will cause the receiver audio 40 to turn on so that the user is alerted to the presence of an active or busy channel. The receiver audio therefore serves as an annunciation circuit in the preferred embodiment. It will be clear to those skilled in the art that other ways of alerting the user to the presence of channel activity are readily implemented.

Turning now to FIG. 2 for one embodiment of switch controller 30, input 135 is coupled to one input of an OR gate 200 and one input of an AND gate 205. Input 25 is coupled to one input of an AND gate 210. The output of an AND gate 210 is coupled to a second input of an OR gate 200. The output of an OR gate 200 drives one input of an AND gate 215 and one input of an OR gate 220. Input 25 is also coupled to a second input of OR gate 220. The output of an OR gate 220 is coupled to a reset input 225 of a timer 230. An output 232 of timer 230 is coupled to the input of an inverter 235 and one input of an AND gate 240. The output of AND gate 240 is coupled to a third input of an OR gate 220.

The output of inverter 235 is coupled to the reset input 245 of a counter 250 and the output of AND gate 215 is coupled to a clock input 251 of counter 250. Counter 250 in this embodiment is preferrably a commercially available Johnson counter with code converter having four digital outputs shown in FIG. 2 and designated 0, 1, 2, and 3 respectively and numbered 252, 253, 254 and 255, respectively in FIG. 2. However, it will be evident to those skilled in the art that many other types of counter circuits may be substituted for the Johnson counter of this embodiment.

It is understood that counter 250 is reset upon power-up. That is, 1 0 0 0 appears at outputs 0, 1, 2, and 3 respectively upon initial power-up of controller 30. Also, it is understood that timer 230 is not timing upon power-up of the system. That is, when power is applied a logic zero appears at the output of timer 230. Although further circuitry which is not shown is required to establish such an initialization, addition of that circuitry is well known and will not add materially to the understanding of the present invention. It will be evident to one skilled in the art that the design addition of that circuitry is readily accomplished.

The output of inverter 235 is coupled to one input of an OR gate 256 and one input of an AND gate 210. A second input of an OR gate 256 is coupled to squelch input 65 and the input of an inverter 260. The output of inverter 260 drives a second input of AND gate 240. The output of OR gate 256 is the audio enable output 75 of decoder 30.

Output 255 of counter 250 is coupled to one input of an OR gate 265 and to the input of an inverter 270. The output of inverter 270 is coupled to a second input of AND gate 215. Output 254 of counter 250 is coupled to a second input of an OR gate 265. The output of OR gate 265 is coupled to a second input of AND gate 205. The output of AND gate 205 is the transmit enable output 105. Outputs 252 and 253 of counter 250 are not used in controller 30 but are shown here to complete the discription.

Encoder enable output 115 is selectively coupled by a two position jumper or switch 280 to either counter output 254 (if switch 280 is in position A) or transmit enable output 105 (if switch 280 is in position B). If switch 280 is in position A, the address encoder 100 of FIG. 1 will be operative only on the first actuation of switch 120 after the timer starts timing and subsequent transmissions will be unencoded. If switch 280 is in position B, the encoder will be enabled each time a transmission occurs during the timer interval (each time the transmitter is enabled).

FIG. 3 is a timing diagram of the circuit embodiment of FIG. 2. Although outputs 252 and 253 of counter 250 are not utilized in the circuit of FIG. 2, they are included in timing diagram FIG. 3 for the purpose of illustrating the operation of the particular type of Johnson counter used in this embodiment. Commercially available Johnson counters such as the MC14022 counter produced by Motorola Inc. as well as other counters are entirely suitable for this application. Similarly, numerous timing circuits, such as analog one-shot type timers and clocked digital counter based circuits, to be used for timer 230 will occur to those skilled in the art.

The timing diagrams of FIG. 3 includes FIGS. 3A through 3M wherein FIG. 3A represents the signal at switch input 135, FIG. 3B represents the signal present at the reset input 225 of timer 230 and FIG. 3C represents the signal present at the output 232 of timer 230. FIGS. 3D, 3E, 3F and 3G respectively represent the signals present at outputs 252, 253, 254, and 255 of counter 250, respectively. FIG. 3H represents the signal present at transmit enable output 105. FIG. 3I represents the signal present at encode enable output 115 in the case of switch 280 set in the A position. FIG. 3J represents the signal present at squelch input 65. FIG. 3K represents the signal present at audio enable output 75. FIG. 3L represents the signal present at encoder enable output 115 in the case of switch 280 set to the B position. FIG. 3M represents the signal present at decoder input 25.

At a time T1 OF FIG. 3 the user actuates switch 120 for a first time to initiate a call causing a logic high to appear at input 135. This causes a logic high to appear at the reset input 225 of timer 230 which in turn causes the output 232 of timer 230 to become active and go to a logic high. Counter 250 is clocked at this time causing output 253 to go high, output 252 to go low and outputs 254 and 255 to remain at a logic low. Assuming the squelch input is at a logic high, indicating no channel activity, the user will hear that he has a clear channel and will release switch 120 at time T2. This causes a low going logic transition at the reset input 225 of timer 230 which causes the timer to begin timing its predetermined time interval.

At time T3, the timer interval started at time T2 has not yet expired, and the user once again actuates switch 120 causing a low to high logic transition at input 135. This once again resets timer 230 holding its output high and causes counter 250 to be clocked to its next state wherein output 254 is at logic high and outputs 252, 253 and 255 are at logic lows. This state of the counter causes a logical low to high signal transition at transmitter enable output 105 and encoder enable output 115. This causes the transmitter to be active and the encoder to provide the transmitter with the address of the party being called. This occurs whether switch 280 is in either position A or B. Since the output of the timer is held high by the constant reset input caused by actuation of switch 120, the audio is always disabled when the transmitter is enabled. Normally the encoder will require only a very brief period of time (typically less than 0.5 seconds) to encode an address to be transmitted by the transmitter. During this time, the transmitter will normally mute the microphone 90 of FIG. 1 and transmit the code address.

After the code address has been transmitted, the microphone 90 will be unmuted and voice transmission may proceed. Alternately, if data transmission is desired it may take place after the addressing process is completed. At the end of the voice or data transmission, switch 120 is deactuated at a time corresponding to T4 of FIG. 3. The deactuation disables the transmitter and, if switch 280 is in position A, disables the encoder. The low transition at the reset input 225 of timer 230 once again causes the timer to begin its predetermined time interval.

At time T5 the party being called responds with a transmission of his own. This causes squelch input 65 of the formerly transmitting and now receiving unit to go to a logic low causing audio enable output 75 to go low turning on the receiver's audio allowing the user to hear the message being received. The received audio also turns on the reset input 225 of timer 230 thereby holding the output at a logic high. At time T6 the received message ends causing the squelch input 65 to go back to a logic high which in turn causes the audio to be disabled by the high going transition at output 75. The change at input 65 causes timer 230 which had been reset by the logic high on input 65 to start its time interval once again.

At time T7, the user once again actuates switch 120 to respond to the message received between T5 and T6 causing input 135 to go high and reseting the timer. The counter is once again clocked to the next count causing its output 255 to go to a logic high while outputs 252, 253 and 254 are at a logic low. The transmitter is enabled by output 105 and, if switch 280 is in position B, the encoder 100 of FIG. 1 is once again enabled. If switch 280 is in position A an unencoded transmission will occur. If the encoder 100 is enabled, voice or data may be transmitted after the address is encoded and transmitted. If the encoder is not enabled voice or data information may be transmitted immediately.

At time T8 switch 120 is deactuated causing input 135 to once again return to a logic low. This causes a high to low transition at the timer reset input 255 which restarts the timing of the predetermined time interval. The signal at output 255 of counter 250 is fed back through inverter 270 and AND gate 215 to prevent further actuations occurring while the output 232 of timer 230 is active (logic high) from clocking counter 250. Therefore, there is no change in counter outputs 252, 253, 254 or 255 on any subsequent transmission unless timer 230 times out to the end of its time interval. This will cause the counter 250 to reset to its original state prior to time T1.

The time interval from T9 to T10 represents a received message similar to that occuring between time T5 and T6. Although counter 250 is at a different count, the controller 30 responds to this incoming message in a manner identical to its response between T5 and T6. Similarly, the decoder responds to subsequent transmissions such as that occuring between time T11 and T12 the same as transmissions occuring between time T7 and T8 as long as the timer output 232 is at a logic high.

At time T13 the output 232 of timer 230 becomes inactive and goes to a logic low indicating that the predetermined time interval of timer 230 has expired. This resets counter 250 to its initial state just prior to time T1. Actuations of switch 120 occuring subsequent to time T13 will cause controller 30 to respond as it did at time T1 restarting the entire cycle.

At time T14, the response of controller 30 to correctly encoded incoming messages is shown. That is, the conversation is inititated by another transceiver addressing the transceiver of the present user. At this time input 25 makes a low to high transition as a result of a correctly decoded address by decoder 20. This causes a logic low to logic high transition at the timer reset input 225 which in turn causes the timer output 232 to go high. The incoming signal at input 25 also causes counter 250 to be clocked causing its output 253 to go high and its output 252 to go low. Outputs 254 and 255 remain at a logic low. Since a decoder output implies that a signal is being received, squelch input 65 makes a logic high to a logic low transition and audio enable output 75 goes low turning on the receivers audio circuits.

At time T15 the incoming message ends causing the timer 230 to begin timing its predetermined interval and the audio to be disabled. It should be noted that after time T15 controller 30 is in exactly the same set of logic states that it was in after time T2. Therefore it is evident that a response by the user of actuating switch 120 will cause the same response as that which occurred at time T3. That is, a transmission with an encoded address will occur. It will be evident to those skilled in the art, that the minor modification of causing no address encoding when communication is initiated by receipt of a correctly encoded incoming message may be readily implemented by clocking counter 250 more than once as a result of a logic high at the output of AND gate 210.

Turning now to FIG. 4, it will be evident to one skilled in the art that a microprocessor or microcomputer is ideally suited to perform the functions of controller 30 in an equivalent embodiment shown in FIG. 4 as system 300. In this system a microcomputer 310 along with its associated "code plug" ROM 320 will preferrably perform the functions not only of controller circuit 30 but also of other radio functions such as that of the address decoder 20 and address encoder 100 of the system of FIG. 1 but this is not intended to be limiting. In this system, micro-computer 310 accepts incoming information from receiver 10, squelch circuit 60, and switch 120 and delivers appropriate signals to receiver audio 40 and transmitter 325. It will be appreciated that transmitter 325 may alternately include an encoder such as encoder 100 and receiver 10 may alternately include a decoder such as decoder 20. Preferrably, however, the micro-computer would handle these functions. Micro-computers such as the widely available Motorola MC146805G2 as well as others is suitable for performing these functions.

In this embodiment, ROM 320 serves as a "code plug" which is used to program the transceiver with various options and information necessary to the transceivers standard operation. Information such as tone duration, tone frequency, etc may be stored therein and programmed to meet various user or system requirements. The switch 280 of controller 30 is preferrably replaced by one bit of digital information in ROM 320 for system 300.

FIG. 5 shows a flow chart of one method of programming micro-computer 310 to perform the functions of the present invention. This flow chart is designed to parallel the operation of hardware switch controller 30 and the reader should be aware that decision blocks do not uniformly show the result of a "yes" answer at the bottom of the diamond shaped blocks. It will occur to those skilled in the art that many other flow charts will result in firmware which will equally well perform the desired functions, therefore, the flow chart of FIG. 5 is not intended to be limiting as the only program sequence which would perform the function of the present invention.

Program step 400 of the flow chart of FIG. 5 encompasses the first steps of the program wherein a timer, counter and other circuitry will be initialized upon powering up the system. The program looks for a switch actuation at step 410 and if none is found proceeds to step 420 where the receiver squelch circuit is inspected to see if a radio frequency carrier is present. If carrier is present the timer is reset and a brief delay occurs at steps 430 and the program is returned to step 410. If the switch has been actuated step 440 checks to see if the timer is running. If not, step 450 enables the audio until step 460 detects a switch release. Until the switch release occurs periodic delays are encountered through steps 470 until the switch is released.

When the switch is released the timer is reset at step 480 and the program returns to step 410. If at step 440 it is determined that the timer is running, step 490 clocks the counter. If the counter's count equals 2 at step 500, the encoder is activated along with the transmitter at step 510. The transmitter remains activated until steps 520 and 530 determine that the switch has been released. At that point. Step 540 stops transmission and step 550 resets the timer. The program is then returned to step 410.

If at step 500 the count is not equal to two, step 560 disables the clock to the counter. Step 570 determines whether or not the user desires to encode upon each transmission or not. If so the program returns to step 510 and if not the program simply turns on the transmitter at step 580 without enabling the encoder. The program then proceeds to step 520.

If at step 420 it is determined that carrier is not present, step 590 determines if a message has been is properly encoded and correctly decoded. If not step 600 checks to see if the timer's interval has expired. If not a delay is encountered at step 610 prior to returning the program to step 410. If the timer's interval has expired at step 600, the audio is muted at step 620 and the timer is reset at step 630. The program then returns to step 410.

If at step 590 the signal was appropriately decoded, step 635 checks to see if the timer is running. If so, step 640 resets the timer and step 660 enables the receiver's audio. The program then returns to step 410. If at step 635 the timer is not running, step 665 clocks the counter, step 670 resets the timer, step 675 enables the timer and step 680 enables the audio. The program then returns to step 410. If it is desirable not to transmit an address code on the first actuation of the switch 120 occuring after receipt of a correctly encoded message, step 665 should clock the counter twice, otherwise an address will be encoded on the first transmit.

Turning now to FIG. 6 a diagram of the actual hook up for the preferred firmware embodiment of the present invention is shown. This embodiment utilizes the Motorola MC146805G2P microcomputer in conjunction with the MCM2802P programmable ROM. The details of using this particular widely available microprocessor/microcomputer family are well known and documented in the "M6805/M146805 Family Microcomputer/microprocessor User's Manual" published by Motorola, Inc., 3501 Ed Bluestein Blvd., Austin, Tex. 78721. The contents of the above referenced manual is hereby incorporated by reference. Details of the microprocessor itself may be found in the commonly published "Motorola Microprocessor Data Manual" in the section entitled "MC146805G2", the contents of which is also hereby incorporated by reference. In FIG. 6, the actual transceiver functions are represented in block diagram form and one skilled in the art will readily know how to accomplish the appropriate interfacing to those functions. Also, pin numbers for the plastic dual in-line package versions of the microcomputer and ROM are circled and shown adjacent the appropriate I.C. terminals. ##SPC1##

When the hexadecimal program code shown in Table I is loaded into the microcomputer's internal memory and the code shown in Table II is loaded into the programmable ROM, the circuit will perform in a manner substantially the same as that of controller 30 with switch 280 in the A position and in addition will perform the decoding function for a 9-9-9-9-9 ZVEI code on data entering the PB6 input. If an encode is desired on each transmission (switch 280 in the B position), the tenth byte in line 3 of Table II should be changed from 2E to 3E. Also, the encoding function is performed and the output appears in the form of a 350 millisecond, 1200 Hz tone in digital form at outputs PD2 and PD3 when the microcomputer is clocked at a bus speed of 1 MHz. This output is processed by a two bit D/A converter 690 and a low pass filter 695 prior to being transmitted by transmitter 325. The output of receiver 10 is processed by a low pass filter 700 and then limited by a limiter 710 prior to input into the PB6 terminal of the microcomputer.

Thus, it is apparent that in accordance with the present invention a method and apparatus that fully satisfies the objects, aims, and advantages is set forth above. While the invention has been described in conjucntion with specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.