TimWolf wrote on 2022-07-15, 20:34:

Trying to upgrade vram on a PB motherboard. It uses 40pin zip ram. Manufacturer calls for this model https://www.datasheets360.com/part/detail/mt4 … 07739297131202/ but I can't find it. I ordered NEC D4204170V-70 but the motherboard does not detect it. Do any of you data sheet gurus see something I don't? Is this not a direct replacement?

First, you seem to have misspelled the NEC type number. It seems to be µPD424170V without the extra zero. (It's common for NEC parts to omit the µP prefix, so I wouldn't count that as mistake). There are two issues with your alternate part:

• We are dealing with 16-bit RAM here. All common 16-bit RAM chips support write cycles in which only 8 bits are updated, but there are two common signalling protocols: Either you have a CAS line per byte (that is CASL for the low byte and CASH for the high byte) and only the bytes for which CAS is enabled get written when the WE (write enable) signal goes active, or you have just a single CAS line, but two write enable lines. The MT4C16257Z is a dual-CAS chips, whereas the µPD424170 is a dual-WE chip.
• The MT4C16257Z is organized as 512 rows of 512 columns, whereas the µPD424170 is organized as 1024 rows of 256 columns. The different organization requires different addressing logic on the main board, and support of the extra A9 pin that is not present on the MT41C16257Z.

Every one of these differences will (most likely) cause the chip to not work, so your replacement is indeed not close enough to the original part. Actually, there is a dual-WE Micron chip, the MT4C16256, which would match the NEC chip. Also, there is a NEC chip organized as 512x512, which is the NEC µPD424270. So the MT4C16256 could be replaced by the µPD424270, but your substitution is wrong.

Clearly I know nothing. Just want some more vram. So is this one going to work in place of MT4C16257Z-7S ?

https://www.datasheetarchive.com/pdf/download … &term=MT4C16256

Thanks

TimWolf wrote on 2022-07-15, 21:38:

Clearly I know nothing. Just want some more vram. So is this one going to work in place of MT4C16257Z-7S ?

https://www.datasheetarchive.com/pdf/download … &term=MT4C16256

Thanks

The data sheet you link is a combined data sheet for the MT4C16256, MT4C16257, MT4C16258 and MT4C16259. Only the MT4C16257 that your vendor requires is a good fit in your board. You need to make sure you find chips with 2 CAS and 1 WE signal (some datasheets call it "2 CAS / 1 WE", other datasheets call it "dual CAS", generally, you have two CAS-like pins, and only one WE-like pin). You furthermore need to make sure you find chips that don't have an A9 pin. Alas, the ZIP-40 form factor seems to be quite rare, and searching for data sheets by thos features using google didn't quickly turn up good results, but it seems the Samsung KM41C16256AZ is a viable replacement. Note that Samsung's "256" chips matches Microns "257" chip and Samsung's "257" chip matches Micron's "256" chip. So you can't just rely on numbers, but you need to compare datasheet details.