Replacement for HM65256 ram

PCB problems and fixes
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ar2r
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Replacement for HM65256 ram

Post by ar2r » November 29th, 2018, 10:20 am

I am looking to by few HM65256 srams (can be found on e.g. outrun)

Can anyone recommend a replacement that can be bought from known electronic sites for reasonable price?

I have already got that wrong twice ...

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leonardoliveira
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Re: Replacement for HM65256 ram

Post by leonardoliveira » November 30th, 2018, 12:06 am

65256 isn't SRAM. It's *PSEUDO* SRAM. In fact it's DRAM with SRAM pinout. It requires a refresh pulse to operate properly.

Depending on the function it's performing on the circuit a standard SRAM (62256) might not function properly in place of the 65256.

If you can't source 65256s you can try 62256s (those are SRAMs). If they not function in place of the 65256 you can source them on Mega Drive consoles, for example. The Mega Drive will actually function fine with 62256s in the place of the original 65256. So you can keep the Mega Drive working fine by replacing the PSRAMs with regular SRAMs which are easier to obtain.
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keropi
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Re: Replacement for HM65256 ram

Post by keropi » November 30th, 2018, 6:52 am

Utsource has them: https://www.utsource.net/itm/p/912298.html
If speed rating is ok grab a few more than needed just in case some are doa.

ar2r
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Re: Replacement for HM65256 ram

Post by ar2r » December 1st, 2018, 11:57 am

Thank you both for the explanation and the link

According to sega manuals those two should be interchangeable but have different size

HM65256 (300mm)
uPD42832 (600mm)

I have found replacement for uPD42832 but this size will not work for me ( https://www.mouser.co.uk/ProductDetail/ ... 256A-70PCN )

I will give utsource a go but it would be good to find more sustainable source ...

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leonardoliveira
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Re: Replacement for HM65256 ram

Post by leonardoliveira » December 1st, 2018, 11:40 pm

Breaking down the chips names:
HM65256-15
HM (Hitachi Memory) 65 (PSRAM) 256 (256 kilobits aka 32 kilobytes) -10 (150 ns)

uPD (greek micro character PD) 42 (NEC naming for SRAM devices) 8 (8bit) 32 (size in kilo words. since each word is 8bits it's kilobytes)

Since 256Kbit is 32KB the devices have identical size.

Basically uPD42832 is NEC's part for JEDEC type 62256 SRAM.
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Kold666
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Re: Replacement for HM65256 ram

Post by Kold666 » January 3rd, 2019, 6:22 pm

They are used also on namco system 1 pcbs and they are not replaceable with normal SRAMS

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leonardoliveira
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Re: Replacement for HM65256 ram

Post by leonardoliveira » January 4th, 2019, 4:13 am

Kold666 wrote:
January 3rd, 2019, 6:22 MP
They are used also on namco system 1 pcbs and they are not replaceable with normal SRAMS
It actually *depends* on what for/how they're being used. For example, if the data and address lines are multiplexed on the bus, the SRAM will not work as it does not latch the addresses like PSRAMs do.
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