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LUL Programme Machine

This page last modified on 11 October 2010

This page contains a photograph and explanation of (part of) a Programme Machine, as used on London Underground for storing timetable information and passing it on to the signalling system. The technology is old-fashioned and crude, but robust and effective.

Introduction

The Programme Machine was the brainchild of Robert Dell OBE who, jointly with the British Transport Commission, filed for its patent in June 1960⁽¹⁾. Dell worked for the London Passenger Transport Board and its successors between the ages of 16 and 70, and was Signal Engineer responsible for the entire department from 1941. He was instrumental in developing the systems which would allow the Victoria Line to be driven and signalled automatically, and fares collected by machinery.

Programme Machines are used to automate the signalling of trains in the correct sequence through a controlled area, with reference to the working timetable and to the passage of real time.

The machine contains a roll of plastic in which holes have been punched that encode timetable information; these data are read by electrical contacts.

Two types of Programme Machine are used; there is the Sequence Machine, which contains one row for each signalling instruction; and the Timing Machine, which contains one row for each half-minute. The Sequence Machine “steps forward” (its roll advances by one row) with the passage of trains, whilst the Timing Machine steps forward with the passage of time. The machine actually illustrated on this page is therefore a Sequence Machine. There are still Sequence Machines (and just a few Timing Machines) in use on certain parts of London Underground in 2010.

Description

Above: A programme-machine roll in its carrier.

This particular programme-machine roll and carrier came originally from Edgware, on the Northern Line; it would have been used in the Interlocking Machine Room (IMR). Latterly it has been kept at the Northern Line’s (soon-to-be former) signalling centre for training and demonstration purposes.

It consists of a roll of Melinex plastic, which is spooled from the upper drum to the lower drum as train-movements occur; each row of holes/blanks seen represents one piece of timetable information for Edgware’s signalling layout.

Each row (or register) contains 30 columns, or places where a hole might be found, and for the purposes of this page I am referring to each of these columns as a track. See the second photo (below) for more details.

A lamp (called the exciter lamp) is fitted within the machine and focuses light onto a photocell in front of the roll, through the holes in the opaque section in the middle between tracks 15 and 16. As the roll is advanced to the next register, the light falling on the photocell indicates that the registration of the roll is correct and the roll stops advancing.

Every track is read by an electrical reader-mechanism comprising a sprung wheel finger which does or doesn’t make contact with a metal plate behind the roll, according to whether or not a hole is present in the track on the row being read. The reading takes place at the point where a metal strip with holes in it is seen to be laid right across the apparatus, part-way between the two spools.

A hole in the Melinex denotes a binary “1”, no hole denotes “0”.

Careful inspection of this photo reveals some pencil handwriting on the Melinex at the level of the upper spool. It says, “024 10.59.30”. Even more careful inspection, with an understanding of the remainder of the material on this page, allows one to realise that the writing actually refers (somewhat unhelpfully as it happens) to the row just beneath the upper edge of the perspex, i.e. the fifth row down from the writing. I am sure that the writing was intended to be visible immediately above the perspex when the row it describes was lined up with the reader-holes below.

10 hours can be described as 8 + (blank) + 2 + (blank).

59½ minutes is equal to 32 + 16 + 8 + (blank) + 2 + 1 + ½.

Therefore, we are looking for a row which begins like this (O means hole, dot means no hole):

O.O.OOO.OOO

I hope you can make out this row, just beneath the upper lip of the perspex, and would agree that we are looking at the correct row. Sure enough, the train number 024 is given by:

...O.O..

That is 20 + 4 as one would expect (see below for why!). The row in question requests the signalling to clear the route from platform 3 to the southbound line, for a train whose destination is described by “OOO..”. We can also see how the next row (above) has 8 + 2 + 1 for the Hours and very little in the minutes area, indicating elevenses-time.

Details of the punched-hole coding

Below is a zoomed-in portion of the original photo, showing the tracks more closely; their descriptions are written on a piece of perspex fixed to the machine for the benefit of trainees. The top (or left-hand-most, in the original photo) track is numbered 1. The tracks are identified as follows:

1. 8 ===\ 2. 4 \ hours 3. 2 / 4. 1 ===/ 5. 32 ----\ TIME COINCIDENCE 6. 16 \ 7. 8 \ 8. 4 > minutes 9. 2 / 10. 1 / 11. half --/ 12. PLAT 3 TO SB 13. PLAT 2 TO SB 14. PLAT 1 TO SB 15. PLAT 1 TO SB EX NR 16 SIDING [photocell-operated: beginning of service] [photocell-operated: row "in register"] 16. END OF SERVICE 17. 18. A -----\ 19. B \ 20. C > T.D. CODE (Train Description code) 21. D / 22. E -----/ 23. 200 ===\ 24. 100 \ 25. 40 \ 26. 20 \ TRAIN NUMBER (in binary-coded octal) 27. 10 / 28. 4 / 29. 2 / 30. 1 =====/

You will see that the train numbers are described using 1, 2 and 4 for the units; 40, 20 and 10 for the tens; and just 200 and 100 for the hundreds. Therefore the highest train number that this system could cope with would be 377, and no train number could contain an eight or nine. This is indeed the case on London Underground, except that some lines (e.g. Metropolitan) have their train numbers in the 400 series by the (presumably fake) addition of 400 to every number. This might explain why, when a train is due to depart from Wembley Park north sidings, the train number displayed on the theatre-type indicator (to invite the driver of the correct train to “plunge” and request signal-clearance) has a value 400 lower than it should.

End of service

The End of service track exists in order to indicate to the electro-mechanical computer (comprising this machine and a Time Coincidence unit) that the end of a day’s service has been reached. A special dummy instruction for Train 376 at 0300hrs, together with a hole in track number 16, will result in the Programme Machine rewinding the roll to the beginning. The beginning is itself marked by a hole in the left-hand side of the opque strip between tracks 15 and 16, and detected by a second photocell.

Most sites have one roll which contains information for an entire week, Monday through to Sunday, so the Time Coincidence unit (which knows what day of the week it is) will demand a rewind on the nights of Monday, Tuesday, Wednesday, Thursday and Sunday; and the system will run unattended indefinitely. However, some early sites on the Northern Line (probably Camden Town, Kennington and Edgware) have separate rolls for weekdays and for weekends, requiring that a human visits the IMR on Friday and Sunday nights to physically change the rolls over; at these sites a rewind will always be required where the T376/hole-16 combination exists because the end of the roll will have been reached. Intervention will also be necessary at any site where special timetables are being introduced or withdrawn (e.g. around weekend engineering works).

Train Description Codes

Train Description Codes describe the destination and route of trains. I have a list of them for Finchley Central, which forms the basis of the rest of this page.

I assume that the more distant and straightforward destinations would have the same codes throughout the line, but the more local codes (e.g. Mill Hill ex Highgate depot) would have different meanings at other locations. “E” appears not to be used, both from my list and from the lack of any hole in track 22 of the sequence roll.

You will see how the ingenious system uses the same codes to mean different things in northbound and southbound directions. I haven’t noticed any particular patterns in the logic shown below, except that “D” mostly (but not always!) seems to mean NOT via Bank.