Front and Rear General Views
Title: Essex Electro-mechanical Circuit Switch
Catalogue number: CL-unset
Dimensions: unset
Weight: unset
Brief description: Steel chassis containing relays, uniselectors and a power supply that is clearly a programmable circuit switching matrix.
Markings: Dymo tape: 'Designed and constructed by Telecommunications Group Electrical Engineering Science Department University of Essex'.
Front and Rear General Views
The origin and precise purpose of this item is unconfirmed, but it is clearly a 12-by-6 circuit switch implemented using uniselectors. Only four out of the 6 ports on the smaller axis are populated.
It is possible that this was the port expander used for the Titan terminal multiplexor ...
Top Section Front and Rear Views
The top section of the chassis was punched for six ports but only four are populated. An additional relay has been mounted in one of the spare port slots.
For each port, a pair of terminal blocks serve to connect to the outside world.
Row of six Switching Elements Front and Rear Views
The central part has 12 similar units, arranged in two rows of six, each with a uniselector and three relays. Each of the 12 units implements a port for the larger axis. Each port has a uniselector and a controller consisting of 3 relays and a 100 uF capacitor with a few diodes and resistors.
The uniselectors use five wipers. Six positions are wired but only four are bonded out to the top channel units. Four of the uniselector wipers carry the data connections, perhaps conveying an RS232 current loop in each direction, forming a full-duplex system. A fifth wiper is commoned over all six positions and presumably carries a 'connection-in-use' signal that is disconnected when the uniselector is not selecting any of the six possible ports.
Power Supply and Title Legend
The very bottom section houses the power supply with mains inlet socket, two fuses, a neon indicator, an MES indicator and test sockets.
Rear wiring connections for the 12-way axis (two wires per port)
Although there are four wires per channel through the uniselectors, only two wires per channel are connected to the 12-way terminal blocks at the bottom. These are marked R and S, presumably for Rx and Tx. The detailed wiring could be traced out to explain this.
Rear bottom wiring
The Design of Circuits using Electromechanical Devices by RA Brooker,
University Mathematical Laboratory,
Cambridge.
1952 J. Sci. Instrum. 29 45 (http://iopscience.iop.org/0950-7671/29/2/303)
Even in Cambridge, Electromechanical circuits were commonly used in support of auxiliary equipment for computers, although not for the department's famous computers themselves.
END. (C) 2025 University of Cambridge Computer Laboratory (DJG).