plessey1.jpg (6468 bytes)PLESSEY

CDSS 120

The CDSS 120 is one of a family of digital switching systems manufactured by Plessey. It offers the full range of facilities and management controls available to the CDSS family, and provides for the connection of up to 120 extensions, 32 trunks (28 of which may be Public Exchange trunks) and two operator’s consoles.

The system employs time-division multiplex (TDM) techniques for switching, pulse code modulation (PCM) for speech and signalling, and modern microprocessor technology for fast, reliable control. Traffic-handling capability is greater than 0.21 erlang per extension

The equipment is modular, highly standardized and uses plug-in design techniques to facilitate installation and maintenance. The compact metal equipment cabinet is finished in textured stove enamel, and a ‘smoked’ polycarbonate door can be provided as an optional extra.

The desk top operator’s console is a mirror-finish polycarbonate moulding incorporating touch sensitive keys for ease of operation, with light emitting diodes (LEDs) and a liquid-crystal display (LCD) unit for comprehensive information display.

Extension telephones can be conventional press button MF instruments, or rotary dial type with reduced feature access.

General Specification

Microprocessor control
Solid-state TDM switching
PCM transmission

Extensions: 120 (max) in incremental steps of 8 Trunks: 32 (max) within overall limit of up to 28 exchange trunks (in incremental steps of 4) and up to 12 inter-PBX trunks

Press button: CCITT recommended MF4
Rotary-dial: l0p/s 2:1 break/make ratio

Extension lines
Wiring: 2-wire + local earth connection
Loop resistance: 1350 ohms (max), inc telephone
Insulation resistance: 50k ohms (mm)
Line voltage: 50V
Line current: 25-35mA (constant current feed)
Ringing: 75V, RMS 25Hz

4 extensions, or
3 extensions + 1 Trunk (Exchange or Private Wire)
4 simultaneous conferences (max)

Extension hunting groups
8 (typically) 15 extensions per group

Extension pick-up groups
8 (typically) 15 extensions per group

Exchange lines
Loop resistance: 1200 ohms (max)
Insulation resistance: 50k ohms (mm)
Signalling: Earth or loop calling

Route barring
8 categories

Inter- PBX lines

Short codes
System: 20 codes (max)

Power fail extensions
25% of trunks

Operator’s console
Desk top, 2 (max)
Height: 130mm
Width: 470mm
Depth: 250mm
Weight: 7kg
Keys: Touch sensitive
Visual display: LEDs and alphameric LCD characters
Cabling distance: 1 km

Equipment cabinet
Height: 835mm
Width: 557mm
Depth: 595mm
Weight: 30kg (fully equipped)

32 simultaneous conversations
Traffic: 0.21 erlang/extension (approx)
Interface: V24 @ 300, 1200 or 4800 baud (2 off for CIL and Maintenance Teletype)
Transmission bandwidth: 300-3400Hz

Input: 230V± 10% or 115V± 10%, 50-60Hz, or alternatively -50V DC, nominal

Environmental limits
Ambient temperature: 5° - 35°C long term and 00 - 45°C short term
Relative humidity: 20 - 80% long term and up to 90% short term (non-condensing)


Taken from Plessey Publication No. P8304/4 1983

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Last revised: October 10, 2019