PDP-8/A 128K memory - M8417 or M8418?

Ethan Dicks ethan.dicks at gmail.com
Wed May 11 22:12:11 CDT 2016

On Wed, May 11, 2016 at 10:20 PM, Jack Rubin <j at ckrubin.us> wrote:
> I've been looking for a 128K MOS memory board for my PDP-8/A for a while. I finally got one,  but it turned out to be an M8418.

Huh... I'm not sure I've seen that version.

> The docs I've seen (bitsavers EK-MS8CD-TM-001, 1980 + printsets) talk about an M8417 with 4k DRAM chips (MS8-C) and an M8417 with 16k DRAM chips (MS8-D),

Right... I remember the MS8DJ in particular as the full part number
for the 128K MOS board.

> ...but apparently at some point the 16k versions became known as the M8418. The card I received has 96 Fujitsu MB8116E 16k DRAMs, arranged in two 8 x 12 chip arrays. The actual circuit board says PDP MOS MEMORY - M8417 - 50 12701B on the back but the metal card ejector edge is stamped M8418 JC. Chip dates on the board put it at 1980 production.

OK... then that sounds like a later change to help easily distinguish
the 16K/32K boards from the 128K boards, rather than a stamped "D" or
"C" on one of the handle sections.

> None of my literature has the M8418 p/n but most of it may be too old. The M8418 part number (with JC suffix to indicate Fujitsu RAM) does show up in the 1988 Options Module List.

Well then it sounds like a real thing that lasted long enough to make the list.

> I know several list members have similar boards. How are they marked? Can anyone point to more info on the M8418?

I haven't stared at mine in many years, but I'm fairly certain I have
an M8417 with "DJ" stamped on the other part of the handle.  I would
have gotten it from a 3rd-party reseller like Newman Computer Exchange
in the late 1980s (I got the PDP-8/a in 1984 but wouldn't have upped
it to 128K right away - I was buying a lot of used PDP-8 gear between
1985 and 1987).


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