PDP11 backplane boards for DIY hardware - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 12, 2016, 07:44:19 pm
content: Hi! So I've spent the last couple of months playing around with my PDP 11/23, ordered a couple of boards for it (everything is in backplane card edge-cards), and that pretty neat. Currently it has just the bare basics: CPU card, 64Kword RAM, quad serial interface, and a floppy/harddrive controller.It would be great to be able to develop some hardware for it myself, partially for fun, partially because Ebay gets expensive fast when you have to import from the US to EU.So I was thinking about what would be the best way to make your own plugin cards?-designing the boards and getting them made would obviously be the most professional, but for boards that large, and with the edge connector routing that gets pretty expensive.-buying ready made prototype boards is (usually, almost exactly) as pricey as buying a vintage board that does what i'd wanna make.-home etching would almost certainly be cheapest, but cutting the slots for the card-edge connectors would be tough (could be done with a drill and hacksaw, but might not be pretty)-maybe one could get small boards that had just the card edge connector end, and then just bolt on some protoboard with a bracket.What approach would you go with? Did anyone create DIY pdp11 hardware themselves? I'm not sure, it's not really a hobbyists machine.Attached is a picture of one of the cards, for size/shape reference. It's roughly A5 paper sized.

author: edavid
author_rank: Super Contributor
timestamp: July 12, 2016, 07:49:15 pm
content: I think it would be a lot easier to get a DRV11 family parallel interface and connect to that. QBus is a hassle.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2016, 08:46:57 pm
content: It depends how messy you want to get...A G7273 Unibus 'NPG and Bus Grant continuity' board is a double height board (same size as your photo) with fully populated gold edge connector fingers with just 5 pairs linked (easy to cut without damage to the fingers). You could maybe mount a daughter board to one. There's enough length to tack a thin wire to each finger. The equivalent Qbus card (M9047) only has the relevant looped fingers populated (cost saving).Obviously it would be hassle as you would have to hard-wire to the daughter board, including drilling to get to the fingers on the reverse side, but would get you the mechanical form-factor that you want.Just a thought anyway.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 12, 2016, 09:06:13 pm
content: Quote from: Gyro on July 12, 2016, 08:46:57 pmI have actually thought about that. Either a bus grant card or some other card that was beyond saving.Problem is bus grant cards are still rather pricey (and I'd prefer not do damage cards that works for its intended use).Also, the tolerances between cards is very very tight. Even just a socketed DIP chip is enough to almost make the board not fit, so in that case (to save the handles) you'd had to cut out a square of the board and mount your protoboard in that.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2016, 09:33:07 pm
content: The only other option I can think of is a Qbus backplane extender. Qbus was extended on a pair of 40way ribbon cables (on 2x20way 0.1" IDC connectors), you could use this to change to your own physical standard. You would need to be careful of bus termination obviously. It really doesn't sound like a cheap option either unless you're planning on building lots of extra hardware.If you are going to trash a card there isn't anything of less intrinsic value than a bus grant jumper (aside from purchase price obviously). I agree, headroom would be tight unless you can leave a vacant slot above.Edit: DEC did make prototyping cards, but that would restrict you to hardwiring everything... and in DIP. Very hard to find too.