'Anti-static' precautions

The printed circuit board (p.c.b) holds devices which can be damaged by the discharge of static electricity, resulting in severely shortened life expectancy or failure. These devices are the integrated circuits mounted on the p.c.b. Care should be taken to avoid touching these devices or adjacent circuitry.

Securing the Central Control Unit (CCU) to the wall

The CCU is designed for fixing to a vertical wall (it must not be operated in a horizontal position). A three meter mains cable is fitted with a 13 Amp plug, so the CCU must be fitted within safe reach of a suitable 240 Volt a.c. mains socket.

Do not connect the CCU to the mains supply until all wiring has been completed and the CCU is securely fixed to the wall, with the cover in place.

First remove the cover by loosening the 4 captive screws recessed into the corners of the lid. Visually check the CCU to ensure no damage has occurred in transit.

Drill four holes into the wall, to take No 8 X 25 mm counter sunk screws. The top holes should be 250 mm apart and similarly the bottom holes. The distance between each top hole and the bottom hole vertically below should be 246 mm. If fixing to a masonry or stud-partition wall, suitable wall plugs must be used. If fixing to a wood surface it may only be necessary to drill pilot holes.

Screw 2 No. 8 X 25 mm screws into the top holes until approximately 15 mm protrudes. Hang the CCU onto these 2 screws using the key hole slots in the base. Put two more screws into the bottom two holes and tighten until secure. Now tighten the top screws until they too are secure. Check that the CCU is now firmly fixed to the wall.

Attaching the wiring to the CCU
The functional earth should now be attached to the earth terminal. Pass the earth cable through one of the cable entry holes in the

lower edge of the CCU and run it towards the earth . Strip 5 mm off the insulation, and insert the bare end into the earth terminal. This screw terminal is located at the bottom right hand edge of the p.c.b to the left of the exchange line terminals. The earth cable to be used is specified later in this section. Ensure that this earth cable is securely terminated.

The exchange lines should now be run in through the cable entry holes in the lower edge of the CCU and terminated on the Insulation Displacement Terminals (IDT's) marked EXCH 1 and EXCH 2 at the bottom right hand edge of the pcb. If only one exchange line is to be connected, it must be connected to the IDT marked EXCH 1 and switch 4 must be set to 'ON'.

The wires from the ten extensions must now be run in through the cable entry holes in the bottom edge of the CCU. Ensure that sufficient cable is available to enable re-termination if necessary.

The two wire from each extension must be terminated on the associated extension IDT marked from right to left as 21, 22, etc to 29 and 20. If all ten extensions are not being equipped, the preferred arrangement is to equip adjacent extension positions, starting at extension 21.

A system equipped with only one exchange line must always have extension 21 equipped. A system equipped with 2 exchange lines must always have both extension 21 and extension 22 equipped.

Note: Only equipment complying with BS6301 and BS6317 may be connected to these extension ports.

Setting the option switches
Four dual in-line (dil) switches are located on the top left hand side of the p.c.b. to the right of the mains transformer. These switches are used to select the required incoming call ringing sequence and to select one or two exchange line operation.

Switch 4 is nearest the top edge of the p.c.b. A switch is in the ON position when the protruding plastic used for operating the switch is pushed towards the left hand side of the p.c.b.

If one exchange line is supplied, set switch 4 (top switch) to the ON position (Push switch to the left).

The following table should be used to set the desired incoming call ringing sequence. The sequence ends when the call is answered.

Incoming call ringing sequence

Options SW1 SW2 SW3 Incoming Call Ringing Sequence
1 OFF OFF OFF Incoming calls only ring extension 21. This option must only be selected if only one exchange is fitted.


2 ON OFF OFF Incoming calls ring extension 21 for 20 seconds and then ring extension 22 for 20 seconds. This sequence is then repeated.


3 ON ON OFF Incoming calls first ring extension 21 for 20 seconds, then ring extension 22 for 20 seconds, and finally ring extension 23 for 20 seconds before repeating this sequence from extension 21.


4 ON ON ON The incoming call sequence is the same as for option 3 above (ON ON OFF) but the sequence does not repeat itself ringing 23 for 20 seconds. Instead, 23 continues to ring until the call is answered or the caller abandons the call.  This option must only be selected if a central bell is equipped at extension 23.


Confirm that the switches within the CCU have been set to select the incoming ringing sequence required by the customer and to provide for the number of exchange lines fitted.

Check once again that all wiring is secure, the switches have been correctly set and the installation looks tidy and correct.

Replace the CCU cover and gently tighten the 4 retaining screws until the cover is secure.

The mains power should not be connected at this stage.

Lift off the handset of extension 21 and confirm that exchange dial tone can be heard. If a second exchange line is fitted, lift off the handset of extension 22 and confirm that exchange dial tone can be heard.

Connect the system to the mains and turn on the supply.

Lift off the handset of each fitted extension and confirm that internal dial tone can be heard.

Make a call on each of the exchange lines. Check that transmission is functioning.

Make an internal call to each fitted extension. Confirm that all bells ring satisfactorily. Answer the call and confirm that transmission is functioning.

Arrange to have an incoming call an each line. Confirm that the incoming call ringing sequence is the option selected by the switch settings. Answer the call and confirm that transmission is functioning.

Earth connection
A functional earth must be provided for the system, and must not be derived from the systems mains earth through the 13 Amp plug.

This functional earth must originate from a proven building earth point and be terminated within the CCU on the screw terminal provided on the printed circuit board. The earth cable should be insulated, and the conductor should be copper with a minimum cross sectional area of 1.5 mm. The resistance of this earth should not exceed 4 ohms.

Extension wiring
The two wire extension wiring is connected within the CCU or IDT. The extension end of each cable is terminated at a master line jack on terminals 2 and 5. The master line jack enables plug and socket connection of terminal equipment.

The loop resistance of each extension circuit must not exceed 200 ohms. For example, if 0.5mm sq CSA copper cable is used, the maximum distance an extension can be from the CCU is approximately 1190 meters.

External extensions
The system can support extensions with a maximum wiring loop resistance of 200 ohms.

If the external extension has any overhead wiring it is recommended that local lightening protection is provided in the extension wiring.

Connection losses within the CCU
When planning the installation of a system, the loss within the CCU on an extension to exchange line connection must be considered, to ensure that the installation conforms with the local exchange planning requirements.

The connection losses are:

  • Extension to exchange line connection, nominally 1.5 dB at 800 Hz.
  • Extension to extension connection, nominally 7 dB at 800 Hz.

Single exchange line installation
The system can be equipped with a single exchange line:

The exchange line must be terminated on the IDT marked EXCH 1.

Option switch 4 must be set to ON.

The system will always select exchange line 1 when the exchange line access digit '9' is dialled.

If the user attempts to access the unequipped exchange line 2 by dialling the access digits 32, the system will return engaged tone.

As the system is only equipped with one exchange line the following are permitted:

The incoming call ringing sequence option 1 can be selected.

'Night service' can be set-up to announce incoming calls at any equipped extension position. The extension position need not be equipped with a central bell.

Physical specification
CCU dimensions: 385mm x 280mm x 80mm
CCU weight: 3.5 Kg

Power arrangements

Power supply: 240 Volts, 192 Volts minimum, 264 Volts at 50 Hz.
Power consumption on idle, all extensions on hook, nominally 16 Watts.
Power consumption, all extensions off hook: 45 Watts nominally.

The ringing generator
The ringing generator of the System has the nominal capacity to support a maximum of four high impedance bells at each of the extension positions, inclusive of the bell within the extension telephone. The actual capacity depends on the bell types and mix used with each installation. If the ringing generator is overloaded with bells they may not sound at an acceptable level, they may ring intermittently or they may not ring at all. However, no damage will be done to the system.

Central bell
A central bell is a centrally sited bell to which all calls can be routed if not previously answered. The central bell is fitted to one of the extensions terminals, usually extension 23, in place of other terminal equipment. A central bell is therefore always free to ring. If there are two incoming exchange line calls both ringing at the central bell, the bell will continue to ring until both calls have been answered.

The bell must be high impedance.

The provision of a central bell effectively reduces the system to an extension capacity of nine.

Subsidiary Connection (Piggybacking)
The system can be used as a subsidiary off another private branch exchange (PABX).

Due to the transmission loss through the system being greater than zero it is a List B system.

The system cannot obtain the host PABX's recall, so only limited facilities of the host PABX will be available.

The use of the system 'repeat last number' facility is not recommended if the host PABX requires an exchange line access digit pause. The system is unable to insert such a pause and digits could be lost if the facility is used.

The exchange lines to which the system has access through the host PABX must be of the required quality to satisfy the local exchange planning regulations. This is to compensate for the loss within the CCU and within the host PABX.

It is not recommended that on a two exchange line system one of the exchange lines is connected to an extension position on a host PABX and the other exchange line is connected to the local exchange.

Before connecting to the host PABX, consult the approved maintainer for the host PABX.

Ringing cadence
One of two ringing cadences can be supplied to an extension, to indicate to the user the origin of a call. An external call is announced by two short rings followed by a two second silent period. An internal call is announced with a cadence of a single ring and a two second silent period. 

Caller Freq Cadence
Internal 25Hz 1.00 sec on - 2.00 sec of f
External 25Hz 0.40 sec on - 0.20 sec off
0.40 sec on - 2.00 sec off

Tones used by the system

The system provides a tone of a single frequency and this tone is cadenced to produce the required indications.

Tone  Freq  Cadence
Dial tone 425 Hz continuous
Busy tone 425 Hz 0.38 sec on - 0.38 sec off
Ringing tone 425 Hz 1.00 sec on - 2.00 sec off
Alert tone 425 Hz 0.10 sec on - 2.50 sec off
Diversion dial tone  425 Hz 0.75 sec on - 0.75 sec off


Control Circuit
The system is controlled by an interrupt driven RCA 1802A microprocessor. The programme store is an 8K 2764 EPROM and the data store is a 2K RAM. An I/0 capability of 16 input and 24 output ports enables the processor to determine the state of various elements in the circuit, e.g. extension off-hock, incoming call on exchange line etc, and can in turn send out signals in response.

Switching Matrix
The switching matrix consists of semi-conductor or crosspoint switches. These switches enable the various call connections to be established e.g. extension to extension, extension to exchange, call conference etc. The various system tones are provided by cadencing the relevant connections between the extension and internal tone generator path.

Power Supply
Circuit diagram No. C6024

The power supplies for the various circuit elements are derived from the mains 240V a.c. supply. A linear mode design is used. The power supply outputs and functions are as follows:

-48 Volts d.c. Extension d.c. feed.

-43 Volts d.c. To enable the extension ringing relays to be controlled by the logic voltage level.

+5 Volts d.c. Logic voltage level.

+12 Volts d.c. To power the crosspoints.

5 Volts 100Hz pulse. To provide the 10 MSec interrupt signal to the processor.

50 Volts 25Hz. To provide ringing to the extensions.

Extension Line Circuit
Circuit diagram No. C 6027

In the following description all component designations will be those associated with extension 21.

1. The -48 Volts d.c. feed for the extensions is fed to each leg of the line through a 750 ohm resistance and a winding of the inductor Ll.

2. R61 and C11 provide proper impedance conditions on the line during speech transmission.

3. The ringing voltage is connected to the extension by operating relay RL1.

4. Off-hook detection when the extension circuit was previously idle or when ringing is being supplied to the extension is provided by circuitry based on the potential divider principle.

5. C21 and C31 isolate the switching matrix from the signalling voltages used on the line. ZD1 and ZD11 protect the switching matrix from voltage surges which may occur during connection and disconnection of the ringing voltage or during decadic pulsing from the extension.

Exchange Line Circuit
Circuit diagram No. C 6028.

The component designations used in the description below are those associated with exchange line 1.

The mains fail connection relay RL11 is continuously operated while power is connected. If the power supply fails, RL11 connects exchange line 1 to extension 21.

1. Incoming ringing is detected by an opto-coupler, software analysis validates the signal. To answer an incoming call, relays RL13 and RL15 are operated, this connects the exchange line to the transmission bridge.

2. A current sink for the d.c. current flowing in the line is provided by TR19 and TR21. TR23 provides a feedback element.

This gives the current sink a high to ac signals in the line.

3. The rectifier bridge BR1 ensures that the polarity of the voltage at the current sink is correct regardless of the polarity on the exchange line.

4. A gas discharge tube GDT1 is connected across the line for protection against atmospherically induced voltage surges. Additional protection is provided by R123 and R125 and varistors VR1 and VR3. ZD21 and ZD23 protect the switch matrix and ZD25 protects the current sink.

5. Loop disconnect signalling pulses are sent to line by releasing RL15 and pulsing RL13.

6. An exchange line may be put on 'hold' by releasing RL13 while holding RL15.

7. The exchange line returns to the 'idle' state when RF13 and RL15 are released.


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Last revised: February 27, 2021