VOGONS


Choosing 386 motherboard and CPU

Topic actions

Reply 20 of 28, by lalakobe

User metadata
Rank Newbie
Rank
Newbie

Thanks everybody for your help! I see now that DX motherboards are indeed rare. Will look for 'classic' mbs that are available (will be assembling 486-era PC some time later, so additional versatility would be an overkill, but thanks for detailed explanation!)

As for CPU/FPU, I have one restriction (kinda like a challenge 😀 ) - it must be the fastest Intel-manufactured CPU and/or FPU, so that narrows down my search even more (no AMD DX-40 motherboards).

Were all 386 motherboards designed with barrel batteries? I found a picture of EVEREX STEP 386 REV. E (presumably) motherboard, and it has only CR-2032 slot attached (possibly later modification by owner?)

Reply 21 of 28, by lalakobe

User metadata
Rank Newbie
Rank
Newbie

I ended up bying M321 rev 2.6 motherboard (https://stason.org/TULARC/pc/motherboards/I/I … -386-M-321.html)

Strange thing, it had no keylock (+power led), turbo switch, turbo led, reset switch and speaker connectors (J10 to J14 on scheme), looks like they were pulled off (why?).

I Installed speaker, power led and reset connectors, and everything works except of power led.

By looking at motherboard images, I saw that mine is missing also some other components - C47 and R38, also located near J10-J14. Can someone, who owns the same MB, tell me C47 and R38 nominals?

I think that R38 might be the ballast resistor for +5v for power led, am I right?

Reply 22 of 28, by Horun

User metadata
Rank l33t
Rank
l33t

R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.
https://www.ultimateretro.net/en/motherboards/3059

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.

Reply 24 of 28, by lalakobe

User metadata
Rank Newbie
Rank
Newbie
Horun wrote on 2022-05-01, 17:09:

R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.

Wattage of R38 should be 0,25 watt or more?

Reply 25 of 28, by Horun

User metadata
Rank l33t
Rank
l33t
lalakobe wrote on 2022-05-01, 18:14:
Horun wrote on 2022-05-01, 17:09:

R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.

Wattage of R38 should be 0,25 watt or more?

yes 0.25 watt should be fine. actual current be about 25mA so about 125mW

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.

Reply 27 of 28, by lalakobe

User metadata
Rank Newbie
Rank
Newbie

I want to upgrade cache to maximum (now mb has 64 kbytes). Two small questions:
1. Is ISSA IS61C256AH-15N memory a good choice?
2. This motherboard has a slot for 16x4 chip (U32) which enables cache write-back. Which mode performs faster on 386s - write-back or default (is it write-through?)?

Reply 28 of 28, by Anonymous Coward

User metadata
Rank l33t
Rank
l33t

ISSI chips should work fine. It would be best to make sure you get real ones. There are some fake ones from China that are also viable, but you need to be prepared to weed out the defective ones. There's about 10-15% failure rate with those.
It's possible that the dirty bit in U32 may have to be upgraded to 64kx4 when upgrading to 256k.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium