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leonardoelias300
April 15th, 2017, 02:22 PM
Hello, I recently purchase a Microtec MF88, it is a brazilian XT clone. This was very dirty and damaged. I understand a bit of electronics and soldering and I fix the PSU and some bad ICs and it now start and boot from floppy.
The problem is the ram memory, and I don't know any compatible IC to replace the bad ones... Since it was made on brazil, it dont have schematics and no datasheets... the ram chips says 'SDM54256E-80' on top and 'SID U030' on botton... any clue?37830
It also have 3 41256 ram chips...

mR_Slug
April 16th, 2017, 06:52 AM
I would guess the SDM54256E-80 are 256K by 4 bit 80ns. There are 6 so:

(256k by 4) x 6 = 256k by 24. Or (256K by 8) x 3. or (256KB) x 3. Or 768KB.

The three 41256 (256K by 1 bit) give the parity.

Never seen a memory config like this. Its on a separate card so maybe they just used what chips they could.

You can get 256KB 30 pin SIMMs that have 2x(256k by 4) plus 1x(256k by 1) for parity. So I would guess its wired like those are. I don't know how many different pin configurations there are for DIP 256k by 4, but i would suspect there is a common one. You can determine the pinout somewhat, VCC and GND are easy. The Address 0, 1, 2 etc should be tied to the address 0, 1, 2 etc of the corresponding 256K by 1 bit chips. So you should be able to verify the pinout at least partially.

Another option,if it is possible is to set the motherboard for 256KB or 512KB, and leave the last bank empty. Which chips correspond to the last bank, and how to set the memory size with no jumpers, may require trial and
error.

leonardoelias300
April 16th, 2017, 10:25 AM
thanks... I'll try to fix it, make some tests before to check it and ill post the results... ill try to wire one of this memory in a breadboard to check the pinout...

leonardoelias300
April 23rd, 2017, 05:52 PM
thanks mR_Slug for the tips and modem7 for the pdf files! I fix it and it is running very well.... I clean all the parts, swap the old ega card and now it have a vga card working good in the 8bit slots... also I bought a XT-IDE card and will change the old mfm hard disk... I have found some boards on my grandpa's garage... will check it later... 37945
37946

leonardoelias300
April 23rd, 2017, 05:57 PM
its showing 736kb on post... 8088 running at 10mhz... Im using a 16bit isa controller to have 3.5 floppy disks and 5.25 at same time... I build an Arduino PS2 to XT adaptor... I found schematics here on this forum... I like this computers easy to fix... Im planning use it to control my home using Qbasic + serial + arduino...