Building a BA08 for a pdp8/L

Chris Zach cz at
Fri Jan 21 21:09:48 CST 2022

Hi Paul!

That is really interesting. Did you need to recreate the delay lines or 
any of that circuitry, or was it just wire the NAND gates into the chip 
address matrix and ignore all the various TP1-TP4 states?

Looking at it I could probably build the basic circuitry on a large 
breadboard, I've certainly got the chips from fixing all those M series 
boards. That plus a pair of 32k*8 static ram chips from my 386 cache 
days and I should be set....

Now where did that big breadboard go?


On 1/20/2022 3:51 AM, pbirkel at wrote:
> Not a direct answer Chris, but back in the mid-70's I built a BM8 surrogate the hard way (we were an impoverished neurophysiology lab so cutting corners wherever reasonable) using banks of 2102L and S100 RAM PCBs.  For density I piggy-backed the 2102's to get 8Kbyte boards, then 3 boards gave 16Kword, so 6 RAM boards were sufficient to fill out the memory space.  The controller was a 1:1 implementation of the DEC schematic, which (if memory serves) just fit onto a single S100 prototyping board.  Cabling from the 8/L was simple ribbon with alternating grounds.  Chassis was ad hoc using a S100 backplane suitably reinterpreted plus a pair of 8" fans. Added suitable additional DF/IF switches and lamps to the existing filler-panel and then attached that to the chassis-front so that it all looked pretty good.  It worked for many years without incident, although AFAIK only core memory was used for execution; the SRAM was used only for data -- and every added word was taken advantage of :-}.
> So there's an existence proof that what you propose isn't unreasonable.  I'd probably have preferred to have a nice backplane and applicable modules to WW, but point-to-point wiring on the prototyping board worked.  Given my skill-set at the time it was a rather risky venture, but the ROI was excellent.  Evidently the DEC design was forgiving enough that my recreation worked on the first try.
> If you decide to pursue the FPGA (or something) approach I'd be interested in doing the same with my current 8/L.  If you go that route, might as well build out all three bits of DF/IF control even if there may not be any accompanying front-panel.
> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Chris Zach via cctalk
> Sent: Wednesday, January 19, 2022 11:08 PM
> To: CCTalk mailing list <cctalk at>
> Subject: Building a BA08 for a pdp8/L
> So now that my pdp8/L is up and running (it now has a serial port and runs FOCAL69 quite well) I'm thinking about the next step, which is of course more memory.
> This requires a BA08 or BM8/L or something expansion box but to be honest I have enough spare flip chips and such from the wrecked 8/I to build about 3 core memory systems. So given that the schematics for the
> BA08 are online, they look pretty darn simple, I have the parts, and I have the parts does anyone know if it's possible to get a flip chip backplane to work on and wire up to emulate a BA08?
> It looks like they just used the data break interface lines to hook up to the processor. Everything's there, Memory address bus, memory data bus, and the various signals for jumps and the like that could allow one to decode and implement the extra instructions needed.
> Hm. Might just be easier to build it with an FPGA or something as it's mostly linking up simple gates and the whole core memory section could be removed by a 4k*12 memory array. Anyone ever done this?
> C

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