Following on from mention of slow speed scanning in
this thread, I have heard mention of scanning before in control rooms - I wonder if perhaps somebody could possibly give a little more info on the term?
I'm thinking scanning relates to the information coming into control rooms and shown on the line diagrams; information such as train locations, numbers & destinations. Would that be right?
Colin, that is half right.
Basically scanning is a method of mutiplexing information in order to use less line plant (wires) than would otherwise be required.
If the control site is nearby the cost of the cable is small but if it is very far away the cost may be considerable. For a few one wire circuits the difference in cost for the difference in distance may not be significant but if say 200 one wire circuits are required the difference in cost between say 100 metres of 100 pair cable and 30 kilometres of the same cable is considerable. If the size of the cable can be reduced and yet still carry the same information then the cost benefit is enough to pay for the equipment required at each end to achieve it and generally the whole is cheaper to install.
LUL has several forms of 'scanning', slow speed, high speed and S2 for example (there are others) but they all do the same thing.
Imagine 5 wires labelled A,B,C,D,E and a common return wire running from a control room to a remote site. Each carries a signal from the control room, say voltage or no voltage, which is monitored at the remote end and used to operate or release a relay connected to the common return wire.
Now consider replacing the wires A,B,C,D & E with a single wire, let us call it S for scan between the control room and the remote site. The five relays each need to operate and release according to the voltages present on the A,B,C,D & E wires at the control room using only the 'S' wire and the common return between the two sites. To achieve this the 'S' wire is connected firstly to the 'A' wire at each end, then the 'B', then the 'C' etc in turn as a single cycle and then the cycle is repeated infinitely.
In order for the relays at the remote site to follow the control voltages when the appropriate individual wire is not actually being scanned the remote end has a 'stick' unit which holds the received signal (voltage or no voltage) until the next cycle.
Of course the two ends have to be synchronised to ensure that the 'S' wire is connected to the 'A' wire at end each simultaneously etc and the Master scanner takes care of it.
This is in very simple terms the basis of slow speed scanning and it does indeed work on groups of 5 circuits as described. There are literally hundreds of scanned one wire circuits grouped in this way and each is known as a 'function'.
Slow speed scanning uses 1950s/60s discrete components and you'll know it if you are in a room where it is in use, you'll hear the charactersitic ting, ting, ting, ting, ting sound of the magnetic amplifiers in timer as A, B, C, D, E functions are being 'read' on the read out panels with the appropriate lights flashing slowly on and off.
High speed scanning is a different animal working on similar principles arranged in 108 channel (function) units as systems or shelves and is loosely based upon the Pulse Code Modulation (PCM)
system which is the basic building block of all modern communications transmission systems. It can be regarded as 1960s and 1970s early digital technology using even fewer wires. This is the system built into GPO 62 type shelves and known as 'dripping blood' scanning as the 18 red LEDs on the front of each card illuminate and extinguish in turn as the 108 functions are scanned.
Next came S2 which is 1980s digital technology. I can't say much about it as I never worked on it but it is just another method of multiplexing functions, the principle is similar, cram lots of data into a cable.
{The ultimate in multiplexing really is the internet which in basic terms is doing exactly what slow speed scanning does although very much faster, with millions of functions and using at the point of use just a single pair of wires i.e. your home phone local loop (DSL internet and telephone voice muliplexed)}
What is scanning actually used for ? At a control room it carries the signal operators actions (clearing signals, controlling PMs, TD etc) to the remote sites and receives information from remote sites such as signal indications, track occupancy, alarms etc).
Scanning was LT's word for a non safety signalling transmission system!
. Another system is FDM which is Frequency Division Multiplex(ing). This works on a similar principle except that rather than the use of the wires being divided in time they are shared by different frequencies each one carrying out a unique function.
, plus the two 1980/1990 'Green' and 'Red' books and the reprint of 'Route Control Systems' that is the combined edition: it covers PM working and PM working by computer.
