pdp-8/L

Serial number 1127 963




The pdp-8/L, first produced in 1968, was another of DECs most successful machines - until the pdp-8/e came along and set new standards for 'success'.

I actually have two of these; just a few days ago (7/2003) I aquired a second one - it works, and comes with a tabletop paper-tape reader, but I haven't laid eyes on it yet. I'll update this page when I have more information.

The 8/L was concieved as a smaller, cheaper version of the 8/I - it all fits in a single 10 1/2" rackmount chassis, similar to a pdp-8e in size. Like the 8/I, it is constructed of seperate M-series logic modules. Like the pdp-11/20, the chassis is laid out with the backplane at the top, and the modules in effect 'hanging down' in the box - it's designed to pull out of the rack and pivot 90 for servicing.

My 'old' 8/L came from the Mersey Tunnels in Liverpool, UK. I rescued it, along with a couple of complete pdp-11/34 systems, and the remains of a pdp-8/I, in 1992. The 11/34s had been running the tollbooth systems; the 8/L was in charge of monitoring the tunnel ventilation system, and was in a series of racks with a bunch of non-DEC custom instrumentation hardware - I only took the 8/L.




The above image shows the system fairly soon after I removed it. The paper tape unit and rack are not from the same source; they just provided a handy place to mount the 8/L. It did however come with one unusual feature - an expansion box! This is below the 8/L, and is connected to it by a series of thin ribbon cables. The chassis of the expansion box is identical to the 8/L, except for the lack of a front panel.


A more recent photograph shows the front panel in more detail:




Less complete than an 8/I panel, it still retains the lights for instructions and major states, but only has three rows of lights: MA, MB, and AC. There's no indication for PC or MQ.




With the top over off, the backplane is visible - told you the boards hang down!




A bunch of M-series modules. The green G-series modules are associated with the memory circuits, the corestack itself (4K) is the item with the bundles of red & white wires coming out.

The system was & is in beautiful physical condition - the only blemish is the cracks (visible in the above picture) in the front panel bezel - unusually, on the 8/L it's plastic, not metal. The machine worked beautifully for a couple of years, then died horribly - I powered it up one day, and it was locked solid - no front panel switches had any effect. I've done a spot of troubleshooting and one thing I've noticed is that it seems to be missing some volts... I'd need to pull the manuals to check the details, but if I recall correctly, one of the voltages needed by memory was something like 20V. Rather than have a 20V regulated power supply, the 20V required is derived from two other voltages, IIRC - so the '20V' memory supply isn't 0V - 20V, it's -5V - +15V, or somesuch - and the +15 was missing... haven't got to the bottom of it yet. Will do at some point.



There's not very much on the 8/L out there...
http://highgate.comm.sfu.ca/pdp8/pdp8l-docs/8lmodloc.tif is a useful module location diagram
http://webpages.charter.net/thecomputercollection/pdp8-12/ Jay Jaeger has one...
http://www.digidome.nl/digital.htm ...so does a Dutch museum