ITS No. 3 (Proteus)

The Internal Telephone System No. 3 had a sales name of PROTEUS 24 (24 ports) and PROTEUS 64 (64 ports).

Information on this product can be found in TI's B4 N0011 Description and Facilities, C3 M4010 Installation and Customers Apparatus Guide Notes - Proteus 24 and 64

The system is space switched based on a Central Switching Unit (CSU) which utilises "Complementary Metal Oxide Semi Conductor (CMOS)" solid state technology. The system is available in two sizes, 24 and 64 extensions and the CSU for both sizes measures 472 x 276 x 353 mm (1822 x 11 x 14 in), and weighs 7 Kg (19 lb. approximately). The CSU is free standing and is suitable for handling by one man. Growth of the system is achieved by the provision of additional Apparatus Slide-in Units (ASUs).

Extensions are wired from the CSU using single pairs of wires and numbered in an octal manner. Any British Telecom standard 10 pps telephone may be used and most existing Plan arrangements can be provided at the extension terminals. Generally, battery type Press Button Telephones should not be provided, but if they are, power unit charging must be adopted. Line charging is not suitable as Proteus is a 30 volt system and the telephone Charge Switching Unit is intended for 50 volt working therefore it will not switch on to charge the telephone battery.

The system is mains powered and completely self-contained. No external source is required for ringing or tone supplies.

In the event of mains failure all calls are lost and there is no provision for standby power. However, the system automatically restarts when power is restored.

Maintenance of the common circuit elements of the Central Switching Unit is appropriate to the Technical Officer (TO) grade, while extension telephones, wiring and line interface units of the CSU are appropriate to the Technician 2A grade.

To minimise the number of double visits, clear instructions should be available in RSCs (Repair Service Control) enabling the appropriate grade of staff to be directed to deal with particular reports, e.g. TO for system off or general difficulties common to a large number of telephones, T2A for single telephone faults. When the nature of a fault is uncertain a T2A may be sent.

The aim is to restore customer service as soon as possible. The complex Technology used and the high density of the components does not make the system suitable for on-site repair. Routine maintenance is not required and there are no inbuilt maintenance and diagnostic tests. Maintenance is corrective only using a logical process to identify the faulty unit, and changing it out for a working spare. It is possible to provide maintenance spares kits to cater for the majority of situations taking into account that Technician IIA deal with telephones, interface units, and cabling in between; with Technical Officers dealing with common control elements of the system.

No special tools are required for maintenance, but an extractor is provided on the back panel adjacent to the spare set of fuses to aid in the easy removal of the ASUs.

Modular construction techniques are employed enabling disruption to service arising from CSU failure to be cleared quickly by interchange of units. Guidance in the location of faults is given in Appendix 1 and the Logic Faulting Chart at the end of this Instruction. When faults are not cleared by applying these procedures the complete CSU may be changed.

Maintenance of the distribution wiring and terminal apparatus is as normally practiced at PBX installations. The outputs of the Power and Ringing supplies are not earthed and testing on the system should be across the pairs of wires and not line and earth.

Anti Static Precautions
As electrostatic sensitive devices are used in the CSU the following precautions should be taken:-

(i) Discharge any body charge by occasionally touching earth before removing any ASU.

(ii) Handle all ASUs by the handles provided.

(iii) Do not touch the circuit track or components.

(iv) Place all ASUs in Anti Static Bags inside the original packing before transporting.

Access and Removal of Units
Access to the front of the CSU is gained by slackening the captive fixings positioned at the corners of the front panel.
The CSU houses the following equipment.

NOTE - The Power Supply Unit develops dangerous voltages

(i) Power Supply Unit (Black Handle) Capable of supplying a fully equipped system. Removal of the Power Supply Unit (PSU) and access to the back wiring is via the back panel.
To remove the back panel slacken all captive screws around the outside edge. With the panel removed the mains connector to the PSU can be unplugged and the single fixing screw retaining the PSU removed. The PSU can then be withdrawn from the front.

When removing the PSU care should be taken to secure a firm grip on the handle provided as the heavy mains transformer is fitted to the rear of the unit.

(ii) Line Interface Unit (Blue Handle) One required for each eight extensions provided. Consists of a mother/daughter board which houses the basic 8 x 8 switching array and the line interface for eight extensions. Each extension circuit is provided with a small switch on the ASU. For normal operation the switch should be in the forward position towards the front of the ASU. If an extension has not been provided the switch position is towards the rear of the ASU resulting in the return of Number Unobtainable (NU) tone if called.

(iii) Common Control Unit (Red Handle) One per system. Provides timing and control of the system. Generates all tones required, i.e. dial, busy, NU and ring. Eight small switches are provided enabling individual Call Connect Circuits to be busied if required for testing purposes. The switch number corresponds to the Call Connect Circuit number. The switch positions are towards the rear of the ASU for normal operation and forward for busy. A small LED is also included which flashes at regular intervals when the system is working correctly.

(iv) Call Connect Unit (Yellow Handle) Two connect circuits are provided on each unit and provide the necessary connecting circuits required for calls. Two small LED's are provided to indicate circuit busy.

NOTE - The first ASU position provides Call Connect Circuits 1 and 5, the second provides 2 and 6 etc along the four positions of the CSU.
Removal of ASUs requires the use of the Extractor.

When a Proteus system is installed it is essential that a supply of maintenance replacement items are available. For up to 10 systems one complete set of spares listed below should be held and the percentage holdings of individual items adjusted according to trend as the number of systems in use increases.
Description	                                        BT ITEM CODE
Case 246A   Includes 24 way backplane}         		435995
Case 246B    Includes 64 way backplane         		435996
Power Unit 155A                                		314364
Common Control Card ASU/1A1/FDH/16891            	418051
Dual Call Connect Card ASU/1Al/FDH/16900       		418052
Line Extension Cards ASU/1Al/FDH/16894         		418053
Cable Connection 14A                            	435997

3 Internal
Issue 1, Apr 1982




The Internal Telephone System No. 3 (ITS 3) started life in 1977 as a Factories Department student project to look into the feasibility of a British Telecom produced internal telephone system to extend the existing internal telephone system (ITS 2). It will be marketed as PROTEUS.

PROTEUS will only accept Loop Disconnect Pluses and is compatible with telephone instruments employing this type of signalling. Battery driven key-pads should not generally be used.

PROTEUS may be provided with a maximum capacity of 24 or 64 extensions. The system will be known as PROTEUS 24 and PROTEUS 64. PROTEUS 24 is the same physical size as PROTEUS 64 but is only wired to a maximum of 24 extensions.

The system is based on a Central Switching Unit (CSU) which utilises CMOS solid state technology. The CSU for both system sizes measures 472 mm wide 276 mm high 353 mm deep. The 24 way system weighs 25 Kgs and the 64 way system weighs 26 Kgs fully equipped. The CSU is free-standing (on a desk or table supplied by the customer). Growth to each system is achieved by the provision of additional Apparatus Slide-in Units, (ASU's) see paragraph 3. UP to three cords (depending upon the number of extensions provided) connect the CSU to a Box Connection. Extensions are radially wired from the Box Connections using a single pair of wires. The numbering range due to technical reasons is as follows:-

00-07, 10-17, 20-27, 30-37, 40-47, 50-57, 60-67, and 70-77

PROTEUS is a single wire switched system making extensive use of CMOS logic circuitry for control and switching functions. Line balance conditions are achieved by unbalanced to balanced line transformers. The system is based on the use of single time shared data highways for each of the required system conditions; such as phone busy or number unobtainable. Conditions on any one highway associated with a particular line condition depend on the address present on the telephone address bus and the current status of that particular line.

The crosspoints used in PROTEUS are CMOS analogue switches namely the CD 4051 which is a single 8-channel multiplexer-demultiplexer i.e. an 8 x 1 switch.

These are arranged to form the basic switching array of an 8 x 8 matrix. Thus any extension can feed onto any one of the 8 available interconnect circuits. Expansion to the maximum system size of 64 extension is achieved by adding additional 8 x 8 arrays.

The modules/ASUs used in PROTEUS are as follows:-

(a) Power Supply Unit (PSU) -One required for a system. Mains Powered, capable of supplying a fully equipped system.

(b) Line Extension Module - One required for each 8 extensions provided. Consists of a mother/ daughter ASU which houses the basic 8 x 8 switching array and the line interface for 8 extensions.

(c) Common Control ASU One per system. Provides timing and control of system. Generates all tones required i.e. dial, busy, NU, and ring.

(d) Call Connect Circuit One per 2 connect circuits required. (Dialler) ASU Connects calls required onto connect circuit. Contains numbers dialled register.

When the exchange is idle the busy line from the line interface is continuously monitored until a phone busy condition is present.

When this busy condition occurs the crosspoint switch monitor line is interrogated to determine whether the particular extension has already been allocated a speech path, if not, a call connect circuit with its associated transmission path is allocated by the common control card to that line and dial tone is returned to the caller.

When the caller dials, the number dialled is entered into a register in the call connect circuit via the common Loop/Dis highway. The call connect circuit then monitors the busy and number unobtainable lines during the address of the required number, and if either are active returns the appropriate tone to the caller, if not then the ring current generator is enabled and the called number rings. If the ringing telephone is answered the ring current generator is cleared, the speech path enabled and the connect circuit cleared ready for conference dialling.


5.1 Intercommunications

5.1.1 All extensions will be able to dial every other extension on the system.

5.1.2 All two-way calls are secure against intrusion.

5.2 Conference

5.2.1 This facility enables up to 6 extensions to be linked by the same speech channel making it possible for all to be able to speak and listen.

This facility is available to all extensions.

5.2.3 It is possible to "call in" or "drop out" conferees without disrupting the call.

5.2.4 The facility remains under the control of the conference convenor

5.2.5 If any extension called into conference is not answered within 12 seconds the connection to that particular station is forced released. Equally if the called extension is engaged the connection is forced released after 4 seconds.

5.2.6 The conference calls are secure against intrusion.

5.3 Incoming Calls Barred

5.3.1 Any extension can be connected to this facility and any extension calling a "Barred" extension receives NU tone.

5.4 First Part Release

5.4.1 All extensions whether they are in two-way conversation, or are called into conference have the First Party Release facility.

All 700 type telephones
Press-Button Keyphone. Battery driven key-pads should not generally be used
Press-button Trimphone
Compact or Jubilee Telephones, and any Special Range Telephone (subject to the limitations of that SRT)
All LSTs including DORIC-HARMONY. Plans 1, 1A, and 4
Any add-on terminal equipment, e.g. Handsets 4 and 7 Speakerset etc.

Basic Equipment All systems will comprise the following basic equipment:-

  • Case 246A 24 way (fully wired equipment cubicle) (ITEM CODE 435995)
  • Case 246B 64 way (fully wired equipment cubicle) (ITEM CODE 435996)
  • Power Unit No. 155A (ITEM CODE 314364) - To be requisition separately :- Plugs for Socket Outlets No. 103.
  • Cable Connection 14A (ITEM CODE 435997) - Up to three cords may be required depending upon the number of extensions to be provided (see paragraph 8.2).
  • Call Connect Circuit ASU/1Al/FDH 16900 (ITEM CODE 418052)
  • Line Extension Circuit ASU/1Al/FDH 16894 (ITEM CODE 418052.)
  • Common Control ASU/1A1/FDH 16891 (ITEM CODE 418054)

Provision of ASUs Call Connect and Line Interface Circuits are provided in accordance with the Advice Notes issued.

The ASU's necessary to meet these requirements are shown below:-

Line Extension cct - system max = 8 - circuits per card = 8 (item code 418053)

Call Connect cct - system max = 4 - circuits per card = 2 (item code 418052)

  • 2 Conn ccts up to 16 Extns
  • 4 Conn ccts up to 32 Extns
  • 6 Conn ccts up to 48 Extns
  • 8 Conn ccts up to 64 Extns


8.1 A set of diagrams will have to be requisitioned and left on site. They should be ordered in accordance with TI K5 A040l.

The diagrams are as follows:

FDH 16900 Call Connect Circuit
FDH 16894 Line Extension Circuit
FDH 16891 Common Control Circuit

8.2 The CSU should be desk or table mounted, preferably in an office environment. Service access to the front of the unit should be unrestricted. The mains lead and the line cords are fed from the rear of the unit and will have a free length of 3 metres, measured from the rear of the unit. It is therefore essential that the Box Connection to which the cords will be terminated is located within this limit. Care should be taken that these cords do not cause a safety hazard. The customer must provide a SWITCHED 13 amp socket outlet in a position conveniently close to the equipment for the SOLE use of PROTEUS.

8.3 The line cords will be terminated at one end to a plug 226. Although the line cord is described as a cord it is in fact a cable of 3.5 metres in length, the extra 0.5 metres of cable is to allow the cable to be stripped and terminated on a suitable box connection (see wiring chart).

At the rear of the CSU there are three 226 sockets which are numbered right to left PL1, PL2 and PL3. Each socket is allocated a number of extensions and are wired to number from top to bottom.

PL1 is allocated extensions 00-07, 10-17, 20-27. PL2 is allocated extensions 30-37, 40-47, 50-57. PL3 is allocated extensions 60-67, 70-77.

Each plug connected to the cord will have to be signwritten PL1, PL2 or PL3 and plugged into the appropriate socket.

8.4 Remove the front and rear panels from the CSU and insert the power module from the front securing it with the single screw from the back. The power cord will be terminated to a Plugs for Socket Outlets 103 fused at 3 amps.

8.5 The ASUs can now be inserted into their appropriate positions (see poster diagram on reverse of front cover). (See also annex A.)

Before inserting the ASUs care should be taken to ensure the busying switches for the Connect Circuit units, the (Connect Circuit busy switches will be found on the Common Control unit) and the incoming calls barred switches on the Line Interface units are in the normal position. Fig 1 shows a bank of switches as mounted on a Line Interface ASU. Switch 1 and 2 are operated, 3-8 are in the normal position. (Note the busying switches on the Call Connect circuits are opposite in operation to the Line Interface circuits e.g. to busy a Call Connect circuit operate the switch to the operated -position

The Line Interface ASUs are inserted into card guide numbers 36-81. They are identified by the Blue extraction handles on the card. One Line Interface ASU caters for 8 extensions. The maximum number of extensions is 64.

The Connect Circuit ASUs are inserted into card guide numbers 21. 24, 27 and 30. They are identified by the Yellow extraction handles on the card. One Connect Circuit unit caters for 2 circuits. The maximum number of connect circuits is 8.

The Common Control ASU is inserted into card guide number 33. It is identified by the Red extraction handle on the card. There is one Common Control unit for the system.

As Electrostatic Sensitive Devices i.e. CMOS are used in the CSU, the normal precaution should be taken by personnel to discharge any body charge by occasionally touching earth. Handle all ASUs by the handle provided and avoid bodily contact with circuit track and components wherever possible and anti static bags should be used when transporting ASU's.

NOTE: It is hazardous for personnel to be earthed while working on live equipment, so on no account should staff be connected to earth, unless the approved high resistance to earth wrist-straps are employed.


10.1 When the power is switched on the LED on the common control unit should flash to indicate it is functioning correctly.

10.2 Using d test telephone on an extension which is terminated at the distribution point, seize each connect circuit in turn by taking the test telephone 'off hook' and ascertain that dial tone is received. The first connect circuit to be seized is the top circuit on the first ASU in card guide number 21. On seizure the LED associated with the circuit will glow and dial tone is received. If the test is satisfactory replace the test telephone and busy the circuit by operating Switch SA1 which is mounted on the Common Control unit.

To test the next connect circuit lift test telephone 'off hook' and this should seize the top circuit on the second ASU and the associated LED will glow. If the test is satisfactory replace telephone and busy the circuit by operating switch SA3.

Test the remaining fitted connect circuits in sequence in the same manner. The connect circuits are seized in the following order:- 1, 3, 5, 7, 2, 4, 6 and 8.

Switches SA1-SA8 will busy the Connect Circuits. On completion of testing restore all the busying switches to normal.

10.3 Using the test telephone and another telephone on an extension generate a call on each of the connect circuits, busying each connect circuit in turn as in paragraph 10.2.

10.4 Using the test telephone generate a call to a busy extension on each connect circuit to ascertain busy tone is received.

10.5 Using the test telephone generate a call to a spare extension on each connect circuit to ascertain NU is received.

10.6 Using the test telephone generate a call to another extension and ascertain the 'First Party Release' feature on each connect circuit.

10.7 Using the test telephone and various other extensions set up a conference up to the maximum of 6 extensions. The extension convening the conference dials the extension of the first conferee and establishes the connection, requests the conferee to hold whilst the next conferee is dialled, no dial tone is received. This procedure is then continued for further conferees.

Whilst in the conference mode, call an engaged extension and ascertain that busy tone "times out" after approx. 4 seconds. Call a free extension but do not answer the call and ascertain that the ringing tone "times out" after approx. 12 seconds. A conference and the subsequent tests should be carried out on each connect circuit.


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