GEC
REX 20-E

200 line
Reed Electronic
PABX

REX 20-E is a medium-capacity PABX which uses a reed-relay switching matrix with electronic common control.

Up to 48 junctions (each of which can be allocated to public-exchange or tie lines as desired) and 200 extensions may be provided, although REX 20-E may be installed with as few as two junctions and 25 extensions and later extended as the need arises by the addition of plug-connected units.

The REX 20-E package incorporates an attractive desk-top switchboard with a separate cabinet for the switching equipment.  A main terminating frame (MTF) and a mains-derived power supply unit are built into the cabinet.  A useful range of features is included as standard, and many optional facilities are available to enable the system to be tailored to suit individual requirements.

The simplicity of the system and the long life of the reed relays and solid-state components combine to make the PABX inherently reliable.  If a malfunction does occur however, swing-out gates and built-in aids to fault location make inspection and servicing of the equipment easy.

STANDARD FEATURES
Outward calls either direct outward dialling (DOD) or Mere permitted via the switchboard operator.
Operator's switchboard with:
Cordless key-per-trunk operation.
Pushbutton dialling to all types of exchange.
Call-progress indication.
Call split.
Camp on.
Trunk offer.
Operator load control of DOD.
Direct extension-to-extension dialling.
Incoming calls via the operator.
Distinctive ringing for external calls.
Enquiry and transfer.
Automatic first-party release on all calls.
Operator recall.
Night service with full PABX facilities.
Mains fail switching.
Switchboard testing at equipment cabinet.

OPTIONAL FEATURES
Direct outward dialling (DOD) restriction for any number of extensions according to class of service allocation.
Demand working, with or without calling-line identification at the switchboard.
Tie lines.
Standby power - battery or generator.
Absent extension transfer.
Extension metering.
Executive right-of-way (EROW).
Three way conference on exchange line calls.
Ring back when free.
Dictation access.
Staff location access.
Group hunting.
MF signalling to CCITT recommendations.

Switching equipment
Reed-relay matrices with fully electronic common control.

Capacity Maximum

Signalling
Loop-disconnect pulses, 7 to 12 IPS; break pulse ratio 60 - 72% or DTMF to CCITT recommendations.

Ringing
Nominal 25 Hz, 70-100v RMS; 0.4 s on, 0.2 s off, 0.4 s on, 2 s off (from public exchange); 1 s on, 2s off (inter extension).

Ring tone
400 Hz + 450 Hz; 0.4 s on, 0.2 s off, 0.4 s on, 2 s off.

Busy tone
400 Hz; 0.36 s on, 0.36 s off (special busy tone: 0.1 s on, 1.4 s off).

Insertion loss
Better than: 2 dB over the band 300 to 500 Hz, 1.5 dB up to 800 Hz, 1.2 dB up to 1000 Hz,
1 dB over the band 1000 to 3400 Hz.

Crosstalk coupling
Better than -75 dB over the band 300 to 3400 Hz.

Group delay
74.5 us over the band 600 to 3000 Hz.

Extension line loop limit
1000 ohms, including the telephone instrument.

Minimum leakage resistance
20000 ohms (extension line); 30000 ohms (public-exchange line).

Primary power requirements
110V and 200 - 240V, 50 - 60 Hz; typical busy-hour consumption for a 24 + 100 line system, 750 VA maximum.

Construction
Free standing cabinet housing a main terminating frame (MTF) on which are hinged one or two equipment mounting 'gates'.  Gate 1: 25 extension circuits, up to 6 local links, and up to 10 junctions.  Gate 2: equipment for up to 75 additional extension circuits, up to 6 additional local links, up to 10 additional junctions, with route restriction facility.  Fixed gate (single sided mounted to the rear of gate l): 4 additional junctions and any required optional equipment.  Capacity is extended above 100 extension circuits and 24 junctions by the addition of a second cabinet.

Approximate dimensions and weight
MTF and Gate 1: 204 kg (450 lb) fully equipped.
MTF and Gates 1 and 2: 340 kg (750 lb) fully equipped.

All gates 2000mm (79in) high and 1215mm (48 in) wide.

Total equipment-cabinet floor space required (including maintenance access):
1900 mm (75in) x 1540 mm (60in). 

1976 The general Electric Company

Last revised: August 01, 2025