In spring 2019 I found on ebay a 486 motherboard with condition: NOS - new old stock to an acceptable price. I could not withstand... During the shipping time I already realized I have bought the most notorious 486 board: a PcChips M912 with fake cache (chips labeled with Write Back 61C328AH-15) which means that only empty plastic housing are placed on the board instead of real cache chips.
I found some comments in the web - the chips are also not connected. My board has revision V1.7 and to all chips go real pcb tracks. It can be placed up to 1MB cache memory (8 pieces of 128kx8 SRAM e.g. ISSI IS61C1024, Winbond W241024AK or UMC UM611024AK). So, I de-soldered the fake cache, opened one part and found - only plastic...
IC-sockets with 32 pins and narrow pin distance (3/10" or 7.62mm) aren't available anymore today. I took two sockets with 16 pins each and placed it in series. Next time I will take the simpler version of sockets with double spring contact because put in a chip with 32 pins into a socket with small round holes needs really a lot of patience... The sockets with spring contacts offer the chip pins much more guidance.
For first tests I bought ISSI IS61C1024AK-10N chips in China but I can't find a datasheet from ISSI with this speed rating... I assume these chips are relabeled fakes but up to now 19 of 20 pieces work. I tested the parts with my EEPROM-programmer TL866 II plus (only the lower 512kB are possible as part Winbond W24512) and the whole 486 system with the floppy version of memtest86+ Version 2.0 successfully. (This version of memtest86+ runs well on my 486 machines, version 4 not.)
Performance
With real cache chips the board delivers a fantastic performance:
All measurement results are documented here.
Facts
| Manufacturer: | PcChips |
| Year of manufacture: | December 1995 |
| Bios-String: | 40-P301-001437-00101111-072595-DGREEN-H |
| Bios: | AMI (64kBytes) |
| Chipset: | UMC UM8498F (46/1995) HYS UMC UMC8496F (46/1995) AT |
| Manual: | You can found it e.g.on elhvb.com but JP28 doesn't correspond with the silk print on the board for P24D processors. I corrected this here. |
Summary
Strong points:
- Very good performance with real cache chips
- Supports nearly all 486 processors
Weak points:
- Only two 72pins memory slots
- The action with the fake cache was one of the reasons for the decline of PcChips :-(
falk.richter*at*yandex.com
Last update: May 22, 2020