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mkemp
June 18th, 2016, 06:07 AM
After succesfully getting an 896k 5160 XT working I cast an eye at my XT-286. "How cool would it be to replace the 256k SIMMs with 1m or 4m," I thought.

It turrns out that the 256k SIMMs aren't JDEC standard and the machine won't start with anything else. "What would be involved in gimmicking together an adapter," I thought.

After some grovelling over the circuit diagram in the Tech Ref I found that the SIMM sockets only have addressing lines for 256k. So, another Bright Idea collapses wnen encountering Reality.

Darn.

Chromedome45
June 18th, 2016, 06:33 AM
Going to have to use expansion cards to get extra memory.

ibmapc
June 18th, 2016, 08:22 AM
Going to have to use expansion cards to get extra memory.
Yes, but the memory on the expansion card will be slowed with 1 wait state while the sims are at zero wait state. It would be great to be able to put more memory on the system board.

yuhong
June 18th, 2016, 09:03 AM
Yea, seems like they used SIMMs merely to save space on the board. The IBM XT-286 did not use a chipset. At the time I think one of the first PC chipsets was NEAT by C&T, and it was commonly used on boards of similar size (later called "Baby AT").

Anonymous Coward
June 18th, 2016, 02:52 PM
Are you certain that all memory placed on the ISA bus is going to be 1 ws? Isn't there a pin on the ISA bus for 0WS? I think I own an expansion card that claims support for it.

ibmapc
June 18th, 2016, 06:41 PM
Are you certain that all memory placed on the ISA bus is going to be 1 ws? Isn't there a pin on the ISA bus for 0WS? I think I own an expansion card that claims support for it.The following is taken from Upgrading and Repairing PCs linked on Modem7's MinusZeroDegrees. Of course, this info could be inaccurate. I wonder what the Tech Reff say's.

31733

yuhong
June 18th, 2016, 09:54 PM
I wonder if three chip 256K SIMMs (based on 1 megabit chips) work.

Update: A 74LS590 is only a 8-bit counter, but it looks like they used a 74ALS74 and the RCO output of the 74LS590 to extend the refresh counter to 9-bit.

Anonymous Coward
June 19th, 2016, 08:52 PM
The following is taken from Upgrading and Repairing PCs linked on Modem7's MinusZeroDegrees. Of course, this info could be inaccurate. I wonder what the Tech Reff say's.

31733

It might be the case for IBM memory expansion cards. The BocaRAM AT/Plus manual mentions zero wait state operation: ftp://psm.pulawy.pl/pub/drivers/bocaresearch/Drivers&Software/Archived/File_area_20/BRATPLUS.TXT

yuhong
June 20th, 2016, 05:54 PM
Actually, looking at the schematics, there is a connection to the A9 pin in the SIMM socket. I wonder what this is used for.

per
June 20th, 2016, 06:55 PM
Actually, looking at the schematics, there is a connection to the A9 pin in the SIMM socket. I wonder what this is used for.

That problably depends on the content of U90 on page 5. This small PROM decides the mapping of the various memory-mapped devices on the motherboard.

Technically it might just be possible to use 1MB SIMM modules if you replace U90 with a custom version, much like the 5160 HIMEM PROM mod.