SDX 420N

APPROVAL No. NS/3841/23/L/601468

The SDX 420N is a self-contained, pre-programmed key telephone systems. The systems have an integral self-test diagnostics routine which continually monitors the system, any resulting reports enabling rapid fault location and repair.

Systems when delivered are in full working order with all standard features and facilities incorporated. On completion of installation a simple self-diagnostic test procedure enables the system to be proved before handover.

A simple change procedure enables features and facilities to be altered following installation (change controller or PC based communication programme) and by the user (using a system manager terminal) during the lifetime of the system.

The main items for use with the systems are the basic FT-1 terminal, the feature and display FT-2, FT-3, FT-4 and FT-5 terminals, the DSS/BLF Unit, and the Operator Console. The System Manager Terminal (an FT-4 terminal with a mask placed over the keys), a VDU with keyboard (VT100 compatible), or the Change Controller enable features and facilities to be changed as required.

The Change Controller and the VDU also provide the maintenance engineer with diagnostic tools. The complete range of features is only available when using a VDU which also aids programming. Alternatively, a personal computer (PC) may be used off-line to program the computer range of facilities.

The SDX 420N system comprises a Central Control Unit (CCU), up to seven Group Control Units (GCU) and up to two Extension Control Units (ECU) per GCU. A system can be sub equipped at installation and expanded to the maximum 420 ports at a later stage.

The Change Controller/VDU connection to the system is via one of two RS232C compatible ports which can also be used to connect an optional, customer supplied printer. This can then be set up to provide a paper printout of call or fault statistics.

A power-fail feature enables one or more standard telephones to be connected to the system, in the event of a power failure these are connected directly to the analogue exchange lines allowing incoming or outgoing calls to be made.

Interconnection between the SDX and the analogue PSTN, Speechband private circuits or host CRA is via BT 239A connectors on the connector block located inside the SDX Control Unit and Extension Control Unit. Alternatively, a separate Test Jack Frame may be used comprising a suitable connection box with BT 237A connection strips, connected within 15 metres of the SDX system.

The apparatus is also approved for use in the Simple Call Routing Mode (SCRM).
Maximum System Capacities

				Control			Each of Two
				Unit (SDX 60)		Extension
				or GCU (SDX42ON)	Control Units

Analogue Exchange Lines			10		10
Private Circuits (DC5/10)		10		10
Private Circuits (AC15)			20		20
Power-fail Telephones			5		5
Key Stations				20		20
2-Wire Instruments			20		20
2 MBit Private Circuits			4		0 (Total 120 Channels)
2 MBit Public (DASS II) Circuits	4		0 (Total 120 Channels)

Maximum Control Unit Module Capacities

Trunk Card				5)		5)
AC15 Card				10)		10)
DC5/DC10 Card				5)		5)
FSC PBA					10)Total 10	10)Total 10
Subscriber Line Card			10)		10)
2-Wire Card (39UPB20001AMW)		5)		5)
2-Wire Card (39UPB20001BMW)		10)		10)
Four Wire Interface Card		9		10
Analogue Signalling Card		4		0

Operator Console Line Card		10		10
DSS/BLF Interface Card			6		6 (System Total 6)
Music on Hold Card			1		0

Digital Trunk Line Cards
(DPNSS or DASSII)			4		0



For each PSTN line on which an incoming call can be connected there shall be at least one key telephone provided, or at least one key and lamp position allocated on a DSS/BLF unit or FT-5 terminal.


The apparatus shall be configured in such a way that each incoming PSTN call shall be presented to at least one terminating key telephone station and, if not answered, shall be presented within the next 60 seconds to every idle terminating station able to accept calls from that exchange line, with at least an audible indication.

SDX 420 Socket Wiring

LJU 2/3A Wire colour MDF termination
2 Blue/White 4
3 Orange/White 2
4 White/Orange 3
5 White/Blue 1

Twisted pair cable must be used and telephone extension leads must not be used.

Card Handle Colours

Handle colour Card Type
Yellow 2cct Subscriber Line Card (SLC - Terminal)
Brown 1cct or 2cct SLT Card (two wire POT or TAMS) 
Blue 2cct LD Trunk Card
Pink 2cct LD/MF switchable trunk card
Orange DC5
Green 2cct MF trunk Card
Black AC15 or DASS/DPNSS Card
Purple DSS Card, supports one key phone port and one DSS port

Two Wire Card information


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Last revised: November 12, 2021