New plaything: A PDP-8/L

I'll follow up later with more details, but I just wanted to share a bit about my latest acquisition, a PDP-8/L:

My 8/L, rebranded for use in an early CNC application

The PDP-8/L was introduced in 1968 by DEC, as a smaller, cheaper, (and less expandable) version of the PDP-8/I.  As you can see, my 8/L has been rebranded by Houdaille Electronics, an early provider of CNC (Computer Numerical Control) machines.  (Normally, 8/L's look more like this.)  I don't know precisely what application this would have been used in, and I don't have a ton of information on Houdaille.  I've been told that they did quite well up until the late 70s building machine tools for automakers and auto parts subcontractors.

The 8/L was fairly popular in the late 60s and early 70s, selling 4000 units or so before it was superseded by the 8/E.  It shipped with 4K of core memory, which was expandable to 8K via a 4K unit in an enclosure the size of the computer itself (the BA08).  While it was possible to expand beyond 8K, it was rarely done and the hardware to do so (the BM8L expansion -- see here for some pictures) is hard to find. 

There were not many peripheral options available for the 8/L; anything beyond a Teletype interface and high-speed paper-tape reader/punch required the BA08 expansion to house the hardware for the peripheral interfaces.

My 8/L is a very basic configuration -- 4K of core and the Teletype interface.  That's it, but that's all you need, really! A Teletype generally came with a paper tape reader/punch, and while slow (110 baud), it was functional and reliable, and allowed for storage, editing, and retrieval of programs and data.

Along with the 8/L, I got a dirty but complete ASR-33 Teletype (pictures to follow at some point), and an incredibly heavy and ugly half-height rack.  I was hoping there'd be something else aside from the CPU unit, but the rest of it is just empty space.  Well, if I ever find a BA08, I'll have some place to put it.

The cards comprising the 8/L CPU and peripheral interfaces

The 8/L's logic is made up of a hundred or so individual circuit boards, called "Flip Chips" in DEC vernacular.  Each of these flip chips is plugged into a wire-wrapped backplane (which is mounted upside-down to make servicing fun.)

The 8/L's wire-wrapped backplane.

And of course, no computer of this vintage would be complete without Das Blinkenlights:

 It's a bit dirty at the moment (and some schmuck stuck some masking tape on the panel and a few of the switches) but with some elbow grease it should clean up nicely.  The lights on this display the contents of the Memory Address, Memory Buffer, and Accumulator registers, and show decoding for the instruction being executed and a few status bits.

My 8/L is serial number 467, which makes it a pretty early unit.  Based on datecodes on the ICs I've looked at, I'd place it somewhere at the beginning of 1969.  So, this is now the oldest digital computer in my collection, huzzah!

I'm still focused on getting the Imlac running, but I'll probably spend some time working on this over the next few months as well.  According to the seller, this unit was working when put into storage a decade ago, and it does look to be in pretty good shape.  I don't foresee any major problems in getting this thing running again.

The 8/L in it's rather homely rack enclosure.

That's all for now -- until next time, make a little birdhouse in your soul!