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IIsi onboard ram replacement

JC8080

Well-known member
I think the onboard RAM on my IIsi logic board is failing, so I would like to replace the chips and see if that solves my problems. I found some replacement chips, and looking at the basic specs on the datasheets they look like a match. If someone has more knowledge about old RAM chip compatibility, could you take a look at the two datasheets and let me know if you think they are compatible?

Current RAM: OKI M514256A-80J (datasheet hyperlinked)
Potential replacement: Fujitsu MB81C4256A-70PJ (datasheet hyperlinked) - the datasheet shows multiple form factors, I confirmed the ones I found for sale match the form factor of the originals

Thanks
 

corbinq27

Member
Out of curiosity how did you determine the onboard RAM needs to be replaced. I have a IIsi that doesn’t boot and I’ve been trying to figure out why myself.
 

Forrest

Well-known member
I’ve heard the 1 MB onboard slows the computer down a few %. There is a Ram disk program designed to use the onboard Ram that effectively speeds the computer up.
 

corbinq27

Member
I’ve heard the 1 MB onboard slows the computer down a few %. There is a Ram disk program designed to use the onboard Ram that effectively speeds the computer up.
65scribe’s video on the IIsi on YouTube talks about a similar strategy to deal with this issue (starting around 4:50 in the video)
 

Fizzbinn

Well-known member
I’ve heard the 1 MB onboard slows the computer down a few %. There is a Ram disk program designed to use the onboard Ram that effectively speeds the computer up.

It's not that there is anything wrong with that on board 1MB (Bank A RAM) per se, its that CPU has to share it with the built-in video if you are using it (don't have a PDS or NuBus video card installed driving your monitor).

There is good explanation of the CPU slow down issue with using RAM based built-in video (RBV) on page 25 of the IIsi Hardware Developer Note:

"The RBV and Bank A of DRAM share a separate RAM data bus that can be connected to or disconnected from the CPU data bus by the bus buffers (see “Use of RAM by the Video” in Chapter 3). Data stored in Bank A of system DRAM is used by the RBV to feed a constant stream of video data to the display monitor during the live video portion of each horizontal screen line. The RBV asks the MDU for data as it is needed; the MDU responds by disconnecting the RAM data bus from the CPU data bus and performing an eight-longword DMA (direct-memory access) burst read from RAM while clocking the read data into the RBV FIFO (first in, first out) buffer.​
If a video burst is in progress, a CPU access to RAM Bank A is delayed, effectively slowing down the CPU. This effect is more pronounced for the larger monitors and for more bits per pixel. Note that only accesses to RAM Bank A are affected by video. The optional Bank B of DRAM connects directly to the CPU data bus, and the CPU has full access to this bank at all times, as it does to ROM and the I/O devices."​

Diagram from page 23:

1683932918433.png

If you have a IIsi and use on board video you definitely want to set your disk cache to 768K or even better use this:

IIsi RAM-Muncher INIT
 

Phipli

Well-known member
I think the onboard RAM on my IIsi logic board is failing, so I would like to replace the chips and see if that solves my problems. I found some replacement chips, and looking at the basic specs on the datasheets they look like a match. If someone has more knowledge about old RAM chip compatibility, could you take a look at the two datasheets and let me know if you think they are compatible?

Current RAM: OKI M514256A-80J (datasheet hyperlinked)
Potential replacement: Fujitsu MB81C4256A-70PJ (datasheet hyperlinked) - the datasheet shows multiple form factors, I confirmed the ones I found for sale match the form factor of the originals

Thanks
They look compatible - I haven't checked the refresh requirements because I'm not sure where to find the details of what is compatible.

If the IIsi still works some, have you considered getting a 1MB, 8 chip SIMM with chips with the same pinout and size, then using it with another 3 (they don't need to be identical, just similar enough) to confirm it works in the machine, then transferring them to the board?
 

JC8080

Well-known member
Out of curiosity how did you determine the onboard RAM needs to be replaced. I have a IIsi that doesn’t boot and I’ve been trying to figure out why myself.

I'm not sure it is. The machine worked when I got it, though I only powered it up a couple times. I recapped it and it worked fine for a day, and then it started giving me the death chime after the startup chime. The board was in pretty good shape, no battery bomb, but typical leaking cap corrosion. I found a couple loose pins on the CPU and on the ALS245 chips right next to the RAM (UE5, UF5, UG5, and UH5). I re-flowed the corroded pins on the CPU, and removed and re-soldered the ALS245 chips, checking for broken traces and checking continuity to all the end points from the ALS245s. After that the machine worked again for a few days, but started acting a bit squirrely, and now I'm back to the death chime. The last time it worked before it died again the graphics were odd, definitely some sort of graphics issue. RAM failure is a common cause of the death chime, and the onboard RAM is also used as VRAM, so I'm suspicious of the onboard chips. I figure if I can rule out the onboard RAM and get some soldering practice at the same time, it's a win-win. And maybe it will even fix the machine. 😅
 

JC8080

Well-known member
They look compatible - I haven't checked the refresh requirements because I'm not sure where to find the details of what is compatible.

If the IIsi still works some, have you considered getting a 1MB, 8 chip SIMM with chips with the same pinout and size, then using it with another 3 (they don't need to be identical, just similar enough) to confirm it works in the machine, then transferring them to the board?

That was my thought, but I was given this info by someone on another forum: "A 30-pin SIMM is too narrow in terms of data bus width. It's got eight 1-bit chips so altogether you only get 8 bits, but the IIsi needs 32 bits of RAM data. You can grab the chips off a 72-pin SIMM and that will work." Unfortunately, I don't have any 72-pin SIMMs that would work for this.

Thanks for taking a look at the data sheets.
 

Phipli

Well-known member
That was my thought, but I was given this info by someone on another forum: "A 30-pin SIMM is too narrow in terms of data bus width. It's got eight 1-bit chips so altogether you only get 8 bits, but the IIsi needs 32 bits of RAM data. You can grab the chips off a 72-pin SIMM and that will work." Unfortunately, I don't have any 72-pin SIMMs that would work for this.

Thanks for taking a look at the data sheets.
They're entirely right. I shouldn't post late at night.

The chips (your originals and suggestions) are each 4 bits wide. It might work with a set of 2 chip simms though. I did only wake up 30 seconds ago though.
 

JC8080

Well-known member
They're entirely right. I shouldn't post late at night.

The chips (your originals and suggestions) are each 4 bits wide. It might work with a set of 2 chip simms though. I did only wake up 30 seconds ago though.
Good call, that should work, and let me test the chips first. Now I just need to find 4 matching SIMMs, most people selling them on eBay seem to sell them in pairs only. I thought I had a set laying around but it seems not.
 

JC8080

Well-known member
Any good?
The onboard RAM is 80ns, even though the specs for the machine call for 100ns. I would be a bit nervous replacing the onboard RAM with slower chips in case there is some reason they need to be 80ns, maybe because they are shared with VRAM?
 

JC8080

Well-known member
Or if you want a whole bunch...
There are some good options on eBay, though they do all have quite a few more chips than I need.

A forum member generously offered to send me 4 SIMMs for the cost of shipping. I'll be able to test those in one of my other machine to make sure they are good. Then when I swap them into the IIsi it will either be fixed, or it won't, and I will be able to rule out the onboard RAM as a cause.
 
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