Key telephone system with distributed control.

In a multiple station intercom and telephone system the stations are interconnected through paired conductors which simultaneously handle voice communications and control functions on the intercom bus. Main exchange or PBX paired conductors may be connected to all telephone stations in the same order, or each telephone station may be connected so as to have a unique combination of main exchange or PBX lines available to the system.

Previous multiple station telephone and intercom systems (key systems) have utilised separate conductor pairs for signalling and control function isolated from any audio communications. These systems have also included a centralised control unit using either hook-flashing or other signalling schemes as the data transmission medium.

In prior art systems using centralised control, expansion of the system by adding new stations required some degree of modification of the central control unit.

The present invention seeks to provide a station which can be used with a plurality of main exchange or PBX lines and an intercom line connected to similar stations.

According to one aspect of the invention there is provided a communication system, comprising an intercom line, a plurality of stations, each connected to the intercom line and a plurality of telephone lines, which stations each include means for transmitting and receiving information, switch means for selectively accessing the transmitting and receiving means to one of the telephone lines and intercom line, and control means for generating and receiving control signals and for controlling the switch means, and means for exchanging the control signals between stations, whereby the system is distributively controlled by the control means in each station and there is no need for centralised control.

The system may permit conversations at different stations to be conducted in privacy.

The system may be easily expandable because each station has its own distributed control means which operates completely independently.

The invention may provide a system wherein the existing equipment automatically recognises newly added stations and adapts its operation to include them.

According to another aspect of the invention there is provided a station for selectively exchanging signals on several telephone lines and an intercom line, comprising a telephone subset, a microprocessor, and switch means for selectively accessing the telephone subset to the telephone and intercom lines, the switch means being activated by the microprocessor.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is a diagram of the different elements of a station constructed in accordance with the invention,</li><li>Figure 2 shows details of a hold circuit and relay activating circuits,</li><li>Figure 3 shows a relay switching matrix which enables the connection to the main exchange and intercom lines,</li><li>Figure 4 shows details of some high frequency amplitude modulating control circuits,</li><li>Figure 5 shows details of some station configuration switches,</li><li>Figure 6 shows an off-hook detection and intercom dial pulse detection circuit,</li><li>Figures 7a and 7b together form a flow chart for the program in a microprocessor, and</li><li>Figures 8a, 8b and 8c show several of the possible interconnections of individual stations to form a working system.</li></ul>

The different elements of the invention are shown in Figure 1. The station is connected to three main exchange or PBX lines, and an intercom line, designated as C01, C02, C03 and IC or INTERCOM respectively. The various functions of the station are controlled by a microprocessor 10, which is a standard commercially available component such as an MC 146805 manufactured by Motorola. The purpose of the microprocessor 10 is to control the connection of voice transmission equipment such as subset 12 selectively to one of the lines through switch matrix 120 as required and to perform various other control functions described more fully below. Subset 12 has all the elements of a standard subset such as a ringer 14, a network and dialling element 16, a numerical pad 18, a hookswitch 20 and a handset 22. A dial pulse detection circuit 60 is interposed between subset 12 and microprocessor 10.

Incoming CO call: <ul id="ul0002" list-style="none"><li>The presence of a ringing signal on any one of the CO lines due to an incoming call will activate an appropriate ring sense element 24, 26 or 28 indicated on the diagram and signal the microprocessor 10 that a call is coming in. The microprocessor 10 will then send a call announce tone through a speaker drive circuit 30 to indicate an incoming call and at the same time cause a corresponding CO line status indicator 32, 34, 36 to flash. When the user goes off hook and presses a corresponding CO button 42, 44 or 46 on the front cover of a station, the microprocessor connects the subset 12 across the proper CO line via a switch matrix 120 which is shown in more detail in Figure 3 and described hereinafter. Ringing is terminated by the CO and a corresponding status indicator is flashed by the microprocessor at a different cadence. At this time a message would be sent by the microprocessor via the IC bus to inform the other stations that the particular CO line is seized. All respective CO line status indicators in all stations of the system are then turned on continuously by their respective microprocessors and access to that CO line is inhibited.</li></ul>

A relay control circuit 50 energises one of a group of relays 121, 122, 123, 124. Each of these relays is provided with two single pole double throw contacts which are connected between the main exchange and intercom lines, and the subset 12 as shown in Figure 3. In this figure contacts 121' and 121" are the contacts of relay 121 and so on. It is obvious from this figure that the TIP and RING wires from subset 12 are connected to one of the incoming lines by selectively energising relays 121-124 as follows: <tables id="tabl0001" num="0001"><img file="EP0176291A2_D0001.tif" /></tables>

Of course it should be understood that other switch means could be used in place of relays such as solid state switches.

Outgoing CO Call: <ul id="ul0003" list-style="none"><li>To make an outgoing call on any CO line the user goes off hook, i.e. lifts handset 22 from hookswitch 20 and presses the desired CO line button 42, 44, 46 on the station, at which time the microprocessor 10 will connect the subset 12 across the selected CO line via relay contacts of switch matrix 120 as previously described, providing the line was not previously seized. The microprocessor 10 also activates intermittently the corresponding CO line status indicator 32, 34 or 36. The microprocessor 10 of the calling station sends a message to the microprocessors of the other stations via the IC bus indicating the seizure of the CO line. Line status indicators 32, 34 or 36 of the corresponding CO line in all other stations are similarly activated by their respective microprocessors. Dial pulses are then generated by dialler 16 in response to the buttons activated on pad 18 and are collected by the CO and the call is processed in the normal manner.</li></ul>

Hold: <ul id="ul0004" list-style="none"><li>Once an outside call has been completed through the main exchange, that CO line can be placed on hold by depressing a hold button 40 at the station. The microprocessor 10 will then remove the subset 12 from that line via the relay contacts of switch matrix 120 and activate an appropriate hold device 52, 54 or 56 (one per CO line) such as an optically coupled thyristor which maintains a proper d.c. hold current on the CO line and provides galvanic isolation between CO lines and the microprocessor. The microprocessor 10 also will cause the proper CO status indicators (34, 36 or 38) to flash intermittently. Further, the microprocessor 10 will cause a message to be transmitted to all other stations via the IC bus which will inform their microprocessors that the CO line has been placed in a hold state and to intermittently flash the corresponding status indicator. The hold function is automatically released when any user pushes the button (42, 44 or 46) for that CO line which is held at which time the microprocessor re-connects the off hook station to the specific CO line, dropping the hold. Going on hook terminates the call.</li></ul>

Transfer: <ul id="ul0005" list-style="none"><li>A call on any of the CO lines can be transferred from any station to another station of the system via the hold and intercom functions. The CO is placed on hold by depressing the hold button, at which time the appropriate status indicator (32, 34 or 36) for that CO line is flashed intermittently in all the stations via a message from the microprocessor 10 in the holding station to all other stations over the intercom line. The user wishing to make the transfer then pushes the IC button. The microprocessor connects the station across the IC circuit via the switch matrix 120 as previously described. The station which is to receive the transfer is then dialled by the user making the transfer and the microprocessor sends another message to alert the other user by flashing the IC indicator 38 in that particular station and also generating an audible tone in speaker 58. When the IC call is picked up at the receiving end, the IC indicator 38 is lit continuously in all the other stations via microprocessor messaging to indicate IC voice communication over the IC line. The operator at the first station announces the transfer and then hangs up. The operator at the second station pushes the button of the transferred CO line and the microprocessor 10 of the second station connects the subset 12 across the proper CO line while the subset is off hook thus releasing the hold. The call has then been successfully transferred to the receiving station. The IC at this time is no longer busy, and the microprocessor 10 of the second receiving station sends a message to all stations that the IC status indicator 38 should be turned off. The CO indicator will remain lit until the call is terminated by an on hook operation at the transferred set.</li></ul>

Intercom call: <ul id="ul0006" list-style="none"><li>Each station has a unique two-digit address assigned to it. The preferred embodiment may have up to 16 stations, however, this number may be varied depending upon design choice and microprocessor limitations. To make an outgoing call on the intercom circuit the user presses the IC button 48 before or after going off hook at which time the microprocessor 10 connects the subset across tne IC line via the relay contacts of switch matrix 120 and sends a dial tone to the subset 12 to indicate call progress. The microprocessor 10 also sends a message to all other stations in the system that the IC line has been seized. These other station microprocessors will indicate this seizure by lighting indicator 38. The two digit address of the station being called is then dialled. The microprocessor 10 then terminates the dial tone and collects the dial pulses via the dial pulse detector 60 and sends a message out on the IC line which informs all stations of the called station address. When the called station receives the message, its microprocessor 10 will flash the IC indicator 38 and send an audible tone to both the speaker 58 (ring) and to the IC line (ring back) to alert the called user that the call is coming in and the calling user that the called station is ringing. The call is completed when the called station goes off hook and presses the IC button 48, at which time all call progress tones are terminated by the called station microprocessor 10. The IC status indicator 38 is turned on continuously at all other stations to indicate that the IC line is seized. This is in response to a message transmitted by the called station when the called station went off hook. Note that during voice communications over the IC line at this time, messages are being sent by other microprocessors in the system over the same pair of conductors.</li></ul>

Signalling: <ul id="ul0007" list-style="none"><li>Messages are sent from microprocessor to microprocessor via amplitude modulated signalling over the IC line pair. Voice communications occur simultaneously over these same conductors and hence the modulated signal is chosen in a frequency range well beyond the voice band. Whenever the user goes off hook or pushes any of the station buttons 40, 42, 44, 46, the station will automatically send a message over the IC line to inform all other stations what change has occurred in the system. The microprocessor 10 at the sending station initiates a digital pulse code message which is used to gate the high frequency output signal of a signal generator 62, on and off, to obtain 100% amplitude modulation. This amplitude modulated signal is sent through a hybrid network 64 to the signalling coil of transformer 66, and then to the IC line. Thus an amplitude modulated version of the original digital signal is applied to the IC line. Message collision is detected by feeding the transmitted signal back to the transmitting microprocessor through a Signal Receive (SIG Rx) path 110. If the fed back signal matches the transmitted signal, it is assumed that no collision has occurred. Note that message collisions are a common problem in distributed control systems where message length approaches communication channel delay and there is high signalling activity.</li></ul>

The amplitude modulated signal is received on the signalling coil of transformer 66, and passed through a hybrid amplifier section 64, which is bi-directional for Tx or Rx transmission, and is input to a tone decoder 68. The tone decoder 68 reproduces the original digital code signal from the amplitude modulated high frequency signal and feeds it directly to the microprocessor's receive port. If that microprocessor happened to be transmitting a message at the same time it was receiving the message from the other microprocessor, its transmit monitor would indicate an error and a collision would be assumed. The message would then be sent again after a delay of random length. This retransmit procedure will be repeated until no collision is detected. All the stations receive information on the system's status each time a user initiates a new condition.

Paging: <ul id="ul0008" list-style="none"><li>Any station user can initiate an all-stations page by first accessing the intercom line as described above and, dialling a single digit such as "7" instead of the normal 2 digit station address such as "11" to "26". The microprocessor will recognise the "7" as the all station page command and will send a message to all stations via the IC line to activate their page enable controls 103. The paging station user simply speaks at this time into the handset 22 and his voice is transmitted to all other sets stations via the IC line, and is fed to speaker 58 through the hybrid amplifier circuit 64 and speaker drive circuit 30 which was previously enabled by the microprocessor 10.</li></ul>

Conference: <ul id="ul0009" list-style="none"><li>A conference call can be set up by the station user so that more than one station is connected to the same CO line. The station user wishing to set up the conference call would be in conversation with a party on a CO line and would activate the CO line hold as previously described. The station user then executes an IC call as described and dials the 2 digit station number of the station to be included in the conference. A message is sent via the IC line to the called station, the called party receives the audible and visual indications previously described, and answers the IC call. The station initiating the conference at this time announces to the called party the intention of setting up a conference and proceeds to press the hold button and then presses the appropriate CO line button. A message is sent to the called station over the IC line that a conference call is to take place on that CO line. This enables the called station user to join the conference by pressing the corresponding CO line button. If the conference is not desired after connection has taken place on the IC the station user simply presses the CO button to return to the held party.</li></ul>

A conference call can be set up by a station user so that more than one station is connected to the IC line. The station setting up the conference call would be in conversation with a party on the IC line and would press the IC button and dial the 2 digit station number of the station to be included in the conference. The message is then sent via the IC line to alert that station user of the IC call coming in. The microprocessor also alerts the system that a conference call will be set up thus cancelling IC privacy to that station that was dialled. The station dialled presses the intercom button, goes off hook, and joins the conference. Calling party control is in effect at this time, and if the station that initiated the conference goes on hook, all connections are released.

System Power Supply: <ul id="ul0010" list-style="none"><li>Each station could have its own power supply connected to an a.c. outlet for providing d.c. power to the station components. Alternatively one power supply such as 150 shown in Figure 1 could be provided for the entire system. In such case the power from supply 150 would be distributed over the IC lines to each station. Each station in turn would have its own power circuit 152 connected to the IC line through transformer 66 for providing power to the station components.</li></ul>

Default operation during power failure: <ul id="ul0011" list-style="none"><li>In the case of failure of the d.c. power source 150 indicated on the block diagram (Figure 1), each station will revert to the operation of a normal telephone on the CO priority line (CO line 1 on the block diagram - Figure 1). This is accomplished by having the relay switch contacts arranged such that in the de-energised state C01 is connected to subset 12 and C02, C03 and IC are disconnected. All other CO lines and the IC bus are inaccessible at that time.</li></ul>

Ring sense &amp; hold circuit: <ul id="ul0012" list-style="none"><li>Details of the ring sense circuit 24 and C01 hold circuit 52 are shown in Figure 2. It should be understood that the corresponding circuits for C02 and C03 are identical.</li></ul>

Ring Sense: <ul id="ul0013" list-style="none"><li>When a ringing signal from the CO is present on CO line 1, the ring current is passed through sense circuit 24 connected across CO1 line and comprising resistor 76, zener diode assembly 78, opto-isolator 80, and capacitor 82. Zener diode assembly 78 actually comprises two zener diodes 78', 78" connected back-to-back as shown. Ring current flowing through optical diode 80' in the opto-isolator 80 causes photo transistor 80" to conduct thus signalling an input 81 of the microprocessor 10 at the same frequency as the ring signal applied to C01. The microprocessor will then send the audible tone over line 94 (Figure 4) and visual indication drive over 92 to speaker driver 30 and to status indicator 32 respectively, as long as a signal is prsent on 81.</li></ul>

Hold: <ul id="ul0014" list-style="none"><li>If a conversation is in progress on CO line 1, that line can be placed in the hold condition by pressing the hold button 40 which signals the microprocessor to perform the hold function. Hold circuit 52 (Figure 2) comprises an opto-coupled thyristor 86", and an LED 86' which is activated by a signal from the microprocessor 10 on output port 87 through relay control 50. Simultaneously relay 124 is activated thereby removing the subset 12 from the CO line 1 as shown in Figure 3. Hold circuit 52 will then provide a path for the d.c. holding current from the CO line 1 through resistor 76, diode 78 and thyristor 86". Since device 52 is a photo-triac the thyristor 86 will conduct d.c. current in either direction to handle CO current of either polarity. The hold function will be released when any off hook telephone is connected against CO line 1. Since the voltage across CO 1 will collapse due to the low impedance of a subset, zener diode 78 will no longer conduct current hence thyristor 86" will turn off due to lack of holding current. The microprocessor is also programmed to release the hold function after a predetermined period of time.</li></ul>

The ring sense and hold circuits are implemented using opto-isolators and opto-couplers so that galvanic isolation is provided between the main exchange line and the station circuitry. Other types of isolators could also be used.

Relay control: <ul id="ul0015" list-style="none"><li>The output 87 from the microprocessor is used to control the relays 121, 122, 123, 124. Relay control 50, a BCD to decimal decoder driver such as the SN 74145 manufactured by Texas Instruments, is used to activate each of the devices. The microprocessor will output a BCD value on output ports 87 to relay control 50 which after being decoded, will pull one of the decoder outputs low. For a relay, 121 for example, output bit 1 would be pulled low, on relay control 50, causing current to flow from the 6V power supply through the Kl coil and the limiting resistor 88 into the output stage of the relay control 50, thus turning on the relay 121. Capacitor 90 provides an initial short across the resistor 88 to ensure the relay turns on fully before current limiting takes place for power conservation. Diode 91 provides a current path for the current generated by the Kl coil during turn off. Likewise for the hold functions, e.g. Hold circuit 52 an output on bit 5 of relay control 50 will cause current to flow from the power supply through resistor 89 and the optical diode 86' in the opto-isolator which will activate the photo-triac 86". A unique BCD value is assigned to each state of operation of the relay control circuit 50.</li></ul>

System status indicators: <ul id="ul0016" list-style="none"><li>System status is indicated visually in each station by appropriate indicators such as LED's 32, 34, 36, 38 which are controlled by the microprocessor 10 through corresponding output ports such as 92. The microprocessor can hold an LED on continuously by pulling the corresponding bit low, or can flash an LED by pulsing the appropriate bit low.</li></ul>

System status indications are given audibly via the speaker 58 and/or the earpiece of the handset 22. For an incoming call announce (e.g. ring), the microprocessor generates a cadence of tone in the voice band from output port 94 which is fed into the speaker drive circuitry 30 through an amplifier 96 (Figure 4). Different cadences of the same fundamental frequency are used to differentiate between CO and IC calls. A second output port 98 is used for call progress tones to the IC bus. These tones are dial tone and ring back to the calling party on the IC line. These tones are heard in the earpiece during call set up and are input to the IC bus through a buffer amplifier 100 and coupled to the IC line via transformer 66. The telephone subset 12 is connected during a call across the second winding of transformer 66 which enables the tone to be heard in the earpiece of handset 22.

All the stations of the system are connected to the intercom line by transformer interface 66. The a.c. impedance looking into the primary winding of each transformer 66 is set to 600 ohms by an impedance matching terminating network (not shown).

For paging, the telephone network is connected across the TRANSMIT coils of transformer 66 for transmitting paging audio signals from the microphone in the handset. In the receiving stations the paging signals are coupled to the speaker 58 through audio switch 101 from hybrid buffer amplifier 64. The switch 101 is controlled by port 103 of the microprocessor 10.

The microprocessor of a station generates a digital pulse code modulated message containing information for the control of the system via output port 102 which in turn will gate a high frequency signal from oscillator 104 onto the IC line as follows. Oscillator/amplifier 62 produces a carrier signal at a frequency such as 30 KHz. The oscillator is free running and its output is gated by the microprocessor digital signal stream from port 102 to amplitude modulator 106 which then passes an amplitude modulated carrier signal through amplifier 100 and hybrid buffer amp 64 to the IC line through the transformer 66. Some of the amplitude modulated signal is allowed to pass back through the hybrid buffer amp 64 attenuated to a lower monitoring level for the tone decoder 68. The transmitting station uses this attenuated signal to check for message collisions by comparing the message sent and the message received as previously described. If they are the same, the message is assumed good. The output of the tone decoder 68 is connected to the microprocessor input port 110. The balance of the hybrid amp 64 is offset slightly for the purpose of monitoring the transmitted signal.

Station identification: <ul id="ul0017" list-style="none"><li>The microprocessor in each station is assigned a unique two digit address for the purpose of IC identification. This address set by a 4-bit BCD input to the microprocessor via input port 74 and set through a manually selectable switch 70 as shown in Figure 2.</li></ul>

System configuration: <ul id="ul0018" list-style="none"><li>Each microprocessor has to know how many CO lines of the network are available to the station it controls, i.e. which three of these lines are connected to this particular station, and which of these three lines is its priority line. This information is input to the microprocessor via a write/read sequence from output ports 71 and input ports 73 as shown in Figure 5. Thus the above-mentioned system parameters (i.e. the selection of CO lines, and the priority line) are identified by the hardware selectable matrix switch 72.</li></ul>

The determination of the system parameters is as follows: <ul id="ul0019" list-style="none"><li>1. The microprocessor 10 outputs a code on port 71, shown in Figure 5, corresponding to the station push button 42, 44, 46 to be correlated with one of the plurality of CO lines available in the network. The codes required in Figure 5 are xyz = 011 for testing matrix 72 for the CO line corresponding to station push button 42; xyz = 101 for testing matrix 72 for the CO line corresponding to push button 44 and finally xyz = 110 to test button 46 for its corresponding CO line.</li><li>2. Assuming the current value on port 71 xyz = 011 then switches 151, 152, 153 are active and will provide a binary value from 0 through 7 to port 73 on the microprocessor 10. This value corresponds directly to the CO line number associated with push button switch 42.</li></ul>

Any action and information about the network will be transmitted on the IC bus using the value determined on port 73. Thus any station not having access to that particular CO line will not attempt to give CO status indication on LEDs 32, 34, 36. Furthermore, the system need not be a square system i.e. C01 need not be connected to the first tip/ring pair of a station.

The fact that the system need not be square now allows the first appearance on each station to be different - this is important since in the power fail condition the first appearance on the station is the default CO line.

The diodes in switch matrix 72 are the well known solution to "sneak paths" in bi-directional switch matrices.

Figure 6 shows the details of the dial pulse detection circuit 60 shown in Figure 1. In Figure 6 on and off hook conditions are detected by transistor 111 and are input to the microprocessor at port 112, for 1C calls. The base of transistor 111 is connected to a source of positive potential through hookswitch 20 and network 16 when the hookswitch is "on hook". Going "off hook" is detected by transistor 111 to provide a signal to microprocessor 10.

Dial pulses on a CO line call will be collected by the CO but for intercom calls a dial pulse detection circuit is needed. This is accomplished by inserting an additional optical isolator l14 with its optical diode 114' connected between the Rx coil of the transformer 66 and a positive potential as shown in Figure 6. The optical diode is effectively in series with the telephone subset during IC calls and the current pulses generated by the dialler are sensed by the optical diode 114' of isolator 114, causing the phototransistor 114" of isolator 114 to conduct, thereby coupling these pulses to the base of the hook switch detect transistor 111. Thus, the same input port 112 on the microprocessor 10, is used for pulse collection on IC calls and hook switch detection.

The functions of the station cntrol are summarised in the flow chart of Figures 7a and b.

The multiple station intercom telephone system functions under the control of the store programme which resides in the memory of each microprocessor 10 in each station. The stored programme in all stations is the same. In addition to the static memory which holds the stored programme, each station contains dynamic memory which holds system configuration and status.

Previous art of multiple station telephone systems has utilised centralised control and shared memory. In this invention, the control programme resides in each station. The programme controls the processing of calls; visual and audio indicators which correspond to the status of the calls and the transmission reception and translation of messages which indicate changes in the status of the system. Each station maintains complete system status in its own memory.

The initialisation routine clears the dynamic memory, sets up the ports of the microprocessor, initialises system counters to their base value and initialises an interval time clock which resides in each microprocessor. The interval time clock is used to generate a periodic interrupt which is the base counter for the system counters and timing for transmission and reception of messages.

When an interrupt is received from the interval timer, the program will determine if a message is in the process of being received by monitoring the receive port 110. If a message is being received at port 110 the programme will select the appropriate receiving subroutine and execute it. If no message is being received, the programme will examine flags in the dynamic memory to see if a message has to be transmitted. If a message is being transmitted at port 102, the programme will select the appropriate transmit subroutine and execute it. If a station is transmitting a message it also monitors its own message at input port 110 of the microprocessor while it is transmitting. Each of the receive and transmit subroutines receive and transmit a portion of the message during each interrupt period.

After one of the receive or transmit subroutines have been executed, the programme proceeds to update system times in the dynamic memory. These timers are used to cadence acoustic 98 and visual 92 indicators for the station, time the collection of dial pulses from the dialler in the subset 12 and to monitor the inputs from the buttons 40, 42, 44, 46, 48 on each station. Before interrupts are enabled, the control programme will check to see if dial pulses need to be collected by monitoring port 112 of the microprocessor and will execute that routine if called for. When the dial pulse collection task is complete, the programme enable interrupts.

During the time remaining before the next periodic interrupt, the programme executes routines to detect ring from any of the three main exchange lines 81, 83, 85 and set corresponding flags in dynamic memory and read flags which may have been set by call progress routines and the routines which translate messages sent and received. The flags may indicate the need to service the call progress visual and audio indicators. At this time the programme will also cadence the ringing at port 98 and the LED's at port 92 according to the setting of those flags. The control programme will cause the cadencing of LED's and ringing for both the CO and IC lines.

After servicing the call progress indicators, the programme will select the appropriate call progress state and execute that routine. The call progress states use inputs to the microprocessor from the station buttons, 40, 42, 44, 46, 48 the hookswitch 112, the dial pulser 112, 60 and messages received 110 from other subsets. It is the function of the call progress routines to read these status indicators, set flags in dynamic memory, set up messages to be transmitted and alter the pointers to the individual call progress routines. The operation of relays 121, 122, 123, 124 to select central lines, intercom and hold are executed by code in the message translation sections of the transmit and receive routines as well as in the call progress routines. The call progress routines enable the user to make CO calls, IC calls, CO conference and IC conference calls, to transfer calls, put calls on hold and to page. Each call progress routine represents a state of the call progress. A call progress routine may be interrupted by the clock interrupt enabling the programme to execute the interrupt service routine in order to receive and transmit messages.

Figures 8a, 8b and 8c give some of the possible interconnections of multiple stations to form a communication system. Figure 8a shows the simplest interconnection - a square system involving up to 15 stations, 3 CO or PBX lines and 1 Intercom line. In the event of power failure all telephones in this configuration have emergency access to only line 1.

Figure 8b is more complex in nature being a non-square system. In this configuration any 3 of 5 CO or PBX lines are connected to a station. In the event of power failure all external lines may be made accessible by at least one station.

Figure 8c is substantially the same as described in 8a, that is, the system is basically a square system, however, only 2 external lines (CO or PBX) are available to all stations. The remaining 3 external lines are connected to only one station and to the power fail default position of that station. This arrangement gives access to all lines in the event of power failure and simultaneously satisfies the legal requirement in some countries (Italy for example) to have only one user connected to the line when the line is in use.