A switching system has to perform many functions including switching, receiving signals from terminals and other switching centres, transmit signals to terminals and other switching centres and control system to operate switches. The control system used to perform logical operations, store information, provide an interface between the control and the switching and information elements. All the above functions can be provided by simple relays. With the advant of latest electronic components and equipments the use of the relays are reduced, but still majority of the telephone terminals throughout the word are still controlled by relay based systems.

Conventional relays are simple devices used for switching. To perform switching relay acts as a crosspoint. This crosspoint is latched by electrical, magnetic or mechanical means. Reed relay is a common electro mechanical switch used in modern switching equipments. In the following section, the reed relay device, its uses in telecommunication switching systems are described.

4.8.1. Reed Relays

Reed relay offers several advantages over conventional relays. One of the main feature is the speed. The fastest switching reed relay is the 9800 series, with a typical actuate time of 100 microseconds. Release time is approximately 50 microseconds. Actual time is defined as the period from coil energization until the contact is closed and has stopped bouncing. Reed relay structure achieves a degree of mechnical simplification and operation with a minimum of moving parts. Hence this structure have increased mechanical efficiency and a significant increase in operating speed over the corresponding properties of the conventional telephone relay.

Fig. 4.20 shows the normally open, single contact pair, dry reed relay. A number of such glass enclosed pairs may be included within the same control winding or coil and linked by the coil flux to operate in a manner similar to the multicontact, reed arrangement.

Fig. 4.20. Basic reed relay arrangement.

In basic reed relay, the contacts are plated on the ends of overlapping centilevered magnetic strips or reeds which serve as contact springs. The reeds, contacts and air gap are enclosed in a glass envelop to obtain the benefits of a relatively corrosion free atmosphere for contact operation. The enclosed reeds are wound by the control winding for the magnetic flux. The reeds are magnetized by the flux and are pulled together to close the air gap at the plated contacts. This closes the external circuit path connected to the activated reeds. When a demagnetising current is applied to one or the other of the coils, the contacts open.

Several varieties of the reed relay were developed. Mercury wetted reed relay and dry reed relay are quite popular. The operate and release values (speed) for reed relay operation depend on the structural details of relay such as the gapwidth and overlap, reed dimensions, electrical and magnetic properties of reed relay material.

An alternative to electrical latching is a magnetic latching reed relay called a Ferreed. In this structure, there is one permanent magnet and one magnet whose direction of magnetisation can be reversed by means of current pulse through its windings. When both the magnets are magnetised in the same direction, the contacts operate. The disadvantage is the large current requirement.

Reed relays serve in many different applications requiring low and stable contact resistance, low capacitance, high insulation resistance, long life and small size. They are fitted with coaxial shielding for high frequency applications and process control equipments. Also their low cost and versatility makes them suitable for many security and general purpose applications.

In telecommunication switching systems, relays make a convenient interface elements, used as logic and memory elements for combinational circuits. Relays can be used to provide timing elements by connecting capacitors in parallel with their coil. In can also be used as a translators.

4.8.2. Reed Relay Systems

TEX 2 is a reed relay exchange system (200 to 4000 lines) in common use in united kingdom. It uses 5 x 5 and 5 x 4 crosspoints. Each crosspoint consists of a 4 reed inserted in a single unit. The TEX 2 is of minimal cost but too inflexible for the introduction of linked numbering within numbering plan areas.

TXE 4 is a reed relay system brought into service in 1976. The capacity of the system is 2000 to 40000 connections or 5000 erlangs. It uses a single general purpose switching network.

TXE 4 achieves high levels of system availability. Fig. 4.21 shows a general view of TXE 4. The actual switching circuits composed of many hundreds of reed relays and various add on units to interface with existing signalling systems.

Junctor controller

Outgoing trunk junctor

Incoming trunk junctor

Line unit

Special services

Register/ Sender

Line scanner

General purpose switching network



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