Are those buffered modules? 128mb with that layout seems oddball otherwise.

I don't think I've ever seen dip switches on a simm. With them, and especially at that capacity, I'm guessing it's probably some sort of server ram, in which case what they do is anybody's guess.

I'm not sold on them being part of serial-presence-detect as really there is no need to switch anything there

bertrammatrix wrote on 2025-05-09, 15:00:

Are those buffered modules? 128mb with that layout seems oddball otherwise.

No, they are standard 32MB FPM SIMMs, I guess the 128MB is for a set of 4 x 32 that Compaq sold as a kit.

https://en.wikipedia.org/wiki/SIMM says "HP LaserJet 72-pin SIMMs with non-standard presence detect (PD) connections." which made to wonder about PD.

Most of my 72-pin SIMMs have solder jumpers; maybe they do the same thing as the switches you have?

SPD detection, most likely. I know IBM ram likes to be a "special snowflake" and they did their own thing with SPD on simms. it'd bet the farm that's to allow that simm to work in an IBM machine/or a "standard" one.

red-ray wrote on 2025-05-09, 15:20:

bertrammatrix wrote on 2025-05-09, 15:00:

Are those buffered modules? 128mb with that layout seems oddball otherwise.

No, they are standard 32MB FPM SIMMs, I guess the 128MB is for a set of 4 x 32 that Compaq sold as a kit.

https://en.wikipedia.org/wiki/SIMM says "HP LaserJet 72-pin SIMMs with non-standard presence detect (PD) connections." which made to wonder about PD.

That doesn't make sense. I looked closer- the top row of 4 chips has a different part number then the rest. They MUST be buffered/ecc/parity/something like that.

Not to say they won't work for you, they may. Just saying that's more then just "regular FPM"

bertrammatrix wrote on 2025-05-09, 21:16:

That doesn't make sense. I looked closer- the top row of 4 chips has a different part number then the rest. They MUST be buffered/ecc/parity/something like that.

Not to say they won't work for you, they may. Just saying that's more then just "regular FPM"

Indeed, the top row are 4M x 1 chips (for parity), while the other chips are 4M x 4 chips (for payload data). So these modules are possibly standard parity PS/2 SIMMs (8M x 36), except for the switches we assume to be on the presence detect lines (not serial presence detect, tbough). It is quite common to see PS/2 SIMMs with parity.

I have to agree with others, those look like PD switches to me. I've seen them before, many years ago, and just assumed they were for presence detect.

Thank you guys, so the consensus is that they are related to the PD (presence detect). I have been trying to find a datasheet for these SIMMs to no avail so the possible settings remain a mystery.