PDP11 Air Traffic Control system

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Each TNMOC project has either a working group or project team assigned to do the work. Working groups are either managed in association with the CCS (Computer Conservation Society) or solely within the Museum.

In March 2008 LATCC (London Area Traffic Control Centre) kindly donated one of the last remaining PDP11 based air traffic control systems that were housed in West Drayton. Volunteers Ben Trethowan and Peter Vaughan spent over 6 months putting the system back together and at 3.15pm on Saturday, 26th October 2008 the final problem was identified (broken wire in a data cable), fixed and the system became operational once again. The working display can be seen in our workshop area via the large systems room. You can see two short videos of the working system here .

25/11/2009 update by Peter Vaughan

I'm happy to report that the system has been running well for the past month with no sign of the RDI errors reported earlier.

The 'fix' took some time to find but it turned out, after trying many replacement cards and the system still failing, that the final solution was to remove all the cards in both PDP 11/84s, clean all the edge connectors, clean all the backplanes and re-fit all the original cards again. Since this was done, the system has remained fault free on all the times it has been on.

03/10/2009 update from Peter Vaughan

I spent most of Saturday continuing the investigation on why the RDI (Radar Data Interface) fault had returned and was causing the RBEG to reboot more frequently. Reseating the cards in both PDP 11/84s clearly did not fix the problem so it was time to start swapping cards. The first step was to find some boards to swap... this involved going through several large boxes of spares until he found a replacement RDI backplane and full set of boards.

Finding a full set of boards was a major advantage as it identified all the boards responsible for the RDI communication which totalled 1 4 slot DZ11, 4 2 slot proprietory plessey boards and 1 1slot board used to connect the bus to a ribbon cable which connected to the other PDP 11/84 RDI interface boards.

The first board changed was the 1 slot ribbon cable board but the system still failed. Next each of the 2 slot boards were changed with the fault still there.Finally the 4 slot DZ11(a 16 bit parallel interface) which was configured using several wired links and a small 'link' PCB fitted to a pinned chip socket which in turn is fitted to a socket on the board. As the replacement board had a chip fitted in the same place he decided to swap the chip with the pcb link board in case they were different. After about 15 minutes, the RDI fault returned.

While checking the fitting of the DZ11 I happened to noticed the 'link' PCB was very close to the board in the next card slot. On closer inspection with a torch it looked like the PCB was actually touching the next board which may have been shorting either board. As the replacement DZ11 had a chip which was half the hight of the PCB, the chip and PCB were swapped, the system restarted and it remained running for the last hour the museum was open. On Sunday, IRIS was started up and remained working throughout the day so fingers crossed I have found the cause.

28/09/2009 update by Peter Vaughan

I clearly did not touch enough wood last week as the RDI interface fault has returned, worse than before so it looks like we have another hardware issue. This interface uses Plessey proprietary cards so I need to find a replacement somewhere in the many boxes of spares we have for the system.

The main disk has now been copied to a disk image file. Having had a quick look at it, it's not one of the standard file systems either I or my Linux system recognises so most likely a Walton special. Also it looks like the bootstrap is not on the disk but likely to be in Eprom on the replay unit display logic.

19/09/2006 update by Peter Vaughan

It looks like reseating the cards in the PDP11/84 processors has resolved the RDI interface fault. The systems ran without incident for all of Saturday.

My next task is to make a backup copy of the hard disk in the Walton replay unit, just in case we have a hardware failure.

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