I'm trying to get a feel for what it looked like when you designed a computer to have a lot of memory chips stuffed into it. To that end, I found this: https://commons.wikimedia.org/wiki/File:VAX_11_780_16mb_memory_board.jpg

If I'm counting correctly, that's 256 RAM chips on the visible side of the board. Assuming the use of 256kbit RAM chips, 256 x 256 x 1024 / 8 = 8Mbytes on the visible side, so if it's double-sided, that would be correct.

What are the physical dimensions of the board? Height, width, and depth in terms of how far apart they are spaced in the machine?

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    There are more than 256 chips. 8 submodules, 40 chips per submodule so total 320 memory chips of 256k bits. – Justme Oct 29 '20 at 19:47
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    @another-dave I don't know what that board is and how much of the memory is used. All I see from the photo is that the memory chips are TMS 4256FML. Datasheet says they are 256 kilobit chips. – Justme Oct 29 '20 at 20:03
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    @justme - I found a print set for a board using 256K RAMs, but it nets out at 4MB per board, 156 chips (see my answer) – another-dave Oct 29 '20 at 22:49
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    I posted a link to this question in a Facebook group named Digital Equipment Corporation Alumni. Some field service engineers who maintained the 780 will probably see it. A few of them probably have one of these boards in their garage (yuk yuk). – Walter Mitty Oct 30 '20 at 10:11
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    I've had no luck finding mention of 780 boards of more than 4MB. I wonder if it could be a 3rd-party board? Apart from that, maybe it's really for Venus (the 8600). Venus was an SBI machine like the 780. – another-dave Oct 31 '20 at 1:44

In answering this question, I found that there were multiple generations of 11/780 memory board, and so sizes, capacity, etc., are quite hard to pin down. I still have not found a board larger than 4MB.

I did get an answer for physical board size, though not for slot width (board separation).

One MS780 memory controller can take 16 memory array boards. For the original release of the 11/780, the boards were the M8210 or M8211 (32Kbyte/8Kbyte capacity, respectively), leading to a maximum of 512 KB per controller.

The system supported up to 4 controllers, thus a max of 2MB.

So, for the 'get a feel' part, that's up to 64 memory boards. If we assume the boards are spaced at least 1" apart (I could not find slot-size info), that's over 5 feet of memory alone.

ECC was used, 8 check bits per 64 data bits.

The board size us 12.5 x 15.5 x 0.75 in; I assume the 'depth' includes the tallest chips.

The board numbers came from the MS780 memory system tech manual, see figure 1-1 on page 1-3.

Here's a link to an M8211 catalog entry in the Computer History Museum. When they say '64K module' they apparently mean 'bits'.

Later memory boards increased the board capacity and thus the system capacity. My 1980 "VAX Hardware Handbook" (includes the 780 and 750) has the following:

  • Per chip 16K bits
  • Per board 256KB
  • Per controller 4MB
  • Per system 16MB

The physical board size won't have changed for these though.

This 1984 print set for the MS780H memory controller talks about 256Kbit MOS devices, which squares with the OP's chip identification.

The memory board appears to be an M8374, a 4MB board (which nets out to 144 chips including 1 check bit per byte; the print set say there are 156 RAM chips).

The system might only support 2 controllers at this point though. It's not clear to me what is authoritative.

  • Great link, but ... err ... didn't rwallace ask for dimensions? <DuckAndRun /> – Raffzahn Oct 29 '20 at 18:55
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    @Raffzahn - Last time I studied physics, 12.5 x 15.5 x 0.75 in were considered to be dimensions. Though I confess that we didn't measure things in "inches" even back when I was in school. For the board sizes, I did at least convert the fractions to decimal values. – another-dave Oct 29 '20 at 19:37
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    I did know that you might be over 30, but didn't expect you to be that old, still being trained in medieval units:))) – Raffzahn Oct 29 '20 at 19:44
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    I grew up using imperial units in the real world, SI units in school, but now I'm living in a medieval country, and the ancient scrolls for that memory board use inches. – another-dave Oct 29 '20 at 19:45

To give you a sense of the inter-board spacing in these types of systems, it is useful to consider the main constraints. These include airflow for cooling, the physical space between the connectors on the backplane and component height.

  • The gap for airflow purposes was such that most folk could slide their fingertips between boards, but only at the expense of scratches caused by the sharper bits of the through-hole pins. After component height, this means ~10 mm.

  • The connectors on this generation of system were simple slide in affairs, much like a contemporary PCIe slot. Later generations (e.g. VAX BI backplane) had a much more elaborate cranked/clamped system. Signal integrity along the backplane demands the spacing between the connectors be minimised, but the physical constraints typically limit the pitch to ~20 mm.

  • To get a sense of the size of the components, those two large silver cylindrical capacitors by the legend card are ~0.5in in diameter (say 10-12 mm). Double sided board were rare because of assembly difficulties, but not unheard of.

So, the net result is an inter-board spacing of around 20 mm.

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    The depth of the original M8211 board was given as three quarters of an inch in the tech manual, which is a hair over 19mm. 20mm might be a little tight. OT: I once destroyed a DMR-11 removing it from an 11/24 (never let a programmer inside the machine!) because the tall ROMs got ripped off by wire spikes on the next board. – another-dave Oct 31 '20 at 13:57
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    Good point, but your story kinda suggests that I was close :-) – Shaheed Haque Oct 31 '20 at 18:22

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