Colin Administrator The Man to ask about past threads! member is offline
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Scanning « Thread Started on Nov 4, 2009, 8:57am »
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?
Joined: Mar 2007 Gender: Male Posts: 3,107 Location: darllen yng nhgaergaint
Re: Scanning « Reply #1 on Nov 4, 2009, 9:24am »
Yes. Unless I've got it completely wrong using the Picc. example. There are various terms such as Site Extra Train, First Signal, Last Signal and various keybord commands out to a nodal computer network - ie the controlled interlockings remote from the Regulating Room.
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!
Cheers for that explanation railtechnician - I'm not as daft as I look then!
I can certainly see now exactly why 'they' use the term scanning.
No problem, I always try to be helpful.
Daft? Certainly not, at the end of the day what defeats us most often when trying to get our heads around any "mystery" is the terminology. This forum is a great example of a place where we could baffle everyone with scientific, technical and slang words. Every industry has them but most of the time there is no need to hide behind them.
Cheers for that explanation railtechnician - I'm not as daft as I look then!
I can certainly see now exactly why 'they' use the term scanning.
No problem, I always try to be helpful.
Daft? Certainly not, at the end of the day what defeats us most often when trying to get our heads around any "mystery" is the terminology. This forum is a great example of a place where we could baffle everyone with scientific, technical and slang words. Every industry has them but most of the time there is no need to hide behind them.
OK then I'll be the difficult one
In main line practice such a scanning system is called TDM. That's short for Time Division Multiplex(ing). It works exactly as described on the tin above . 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.
Tut. Tut. I'd got all me 'Green Books' down to write about FDM and TDM then thought better of it and just put 'Yes' instead!
Okay, while we are at TLAs (three letter abbreviations) let's just explain also that TDM and FDM are two ways of doing the same thing both originally from the world of the telephone industry where FDM came first and was popularly known as carrier, basically a system in which 12 VF (voice frequency) channels were grouped as 4kHz bandwith slots between 60kHz and 108kHz, groups of groups could then be further multiplexed to create systems of up to 2048 channels on a single coaxial cable. It was used in trunk and international telephone networks before the days of satellites and fibre optics. TDM is just a catchall name for transmission systems that view a number of channels sequentially in a cycle and send the data down a pair of wires or as pulses of light over a fibre. Examples of TDM systems are LT's Scanning (although the high speed scanning also uses carriers at different frequencies to discriminate between different systems on the same route), telephone service providers PCM transmission systems and digital telephone exchange internal connections, the display bus in a calculator, memory and video buses in computers etc. Dot matrix of course employs TDM as does the Picc line East End Signalling Computer X25 transmission network. I'm quite sure that most lines have plenty of TDM around, it really is very common these days throughout industry, where there is automation there will be TDM !
Green books? Would they be the IRSE publications or something else?
Joined: Mar 2007 Gender: Male Posts: 3,107 Location: darllen yng nhgaergaint
Re: Scanning « Reply #8 on Nov 6, 2009, 8:30am »
Green Books: Yes, they are the IRSE textbooks, I've got two sets and nearly every edition , 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.
Green Books: Yes, they are the IRSE textbooks, I've got two sets and nearly every edition , 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.
Aha, I haven't read the green books since the staff library at 55, Broadway disappeared years ago but I do have the two large IRSE Green and Red books which are of course very much Network Rail oriented.
Of course I still have all my notes from the Tech2S course and those that I continued to write up during my years as a signal maintenance lineman. These of course knock the text books into touch in many ways, though the text books are of course indispensible as reference sources.
Joined: Mar 2005 Gender: Male Posts: 1,346 Location: margate kent
Re: Scanning « Reply #10 on Nov 6, 2009, 11:25am »
TDM is currently in operation from Barking to Bromley By Bow in which it replaced the Thin Wire Control cable which had got destroyed by a fire in 2000 as far as im aware its not used anywhere else it is realible but when it detects a fault it just shows every track down and every signal red (fail safe) this basically shuts the east end of the line down as the signalman has no idea where trains are
General Electric, Delphin1024 (Bromley and East Ham) and Delphin512 (Plaistow)
A single platform for a signal data transmission, Delphin1024 is a compact, stand-alone hardware platform for signaling data transmission and remote control of interlocking from a central panel. Each digital input/output module has 64 bits for a maximum capacity of 1,024 I/Os, brought together into a serial data stream of a higher rate for transmission to a satellite site. This allows data transfer from one place to another over one physical link, saving costs.
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. 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.
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