!Resolved! DS12887 Replacement Module not saving settings after power cycle. \ VOGONS

!Resolved! DS12887 Replacement Module not saving settings after power cycle.

Topic actions

First post, by Chadti99

Posted on 2021-07-10, 20:18

Chadti99 Offline

Rank Oldbie

Rank: Oldbie
Posts: 1071
Joined: 2020-07-28, 11:01

See the 2nd post but the fix was disconnecting pin21.

I’ve used one of these before in a Biostar486 board and it worked great. Now I’ve tried it on another brand of 486 board(GA-486AM 2.2) and it will save my settings after a soft reset but not after a power cycle. Is the part defective or possibly something else going on? Was thinking maybe there was a cmos reset jumper closed but I can’t find one on the board. I’ve swapped for a known good battery as well.

Last edited by Chadti99 on 2021-07-10, 22:01. Edited 1 time in total.

Reply 1 of 3, by Horun

Posted on 2021-07-10, 21:12

Horun Offline

Rank l33t++

Rank: l33t++
Posts: 6910
Joined: 2019-09-19, 02:19
Location: USA

Which exact Dallas chip was the original ?
There are different versions of the DS12887 including DS12887A, DS12C887, DS12C887A and they are not exactly the same.
114 Bytes of General-Purpose, Battery-Backed RAM DS12887, DS12887A (113 Bytes in the DS12C887 and DS12C887A)
RAM Clear Function DS12885, DS12887A, and DS12C887A. My guess is that Pin 21 on your board is grounded causing the CMOS to clear when powered down:
"RCLR must be forced to an input logic 0 (ground) during battery-backup mode when VCC is not applied."
Pull the new RTC and gently bend pin 21 out a bit so it is not inserted into the socket and see what happens.

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. Stuff: https://archive.org/details/@horun

Reply 2 of 3, by Chadti99

Posted on 2021-07-10, 22:00

Chadti99 Offline

Rank Oldbie

Rank: Oldbie
Posts: 1071
Joined: 2020-07-28, 11:01

Horun wrote on 2021-07-10, 21:12:
Which exact Dallas chip was the original ? There are different versions of the DS12887 including DS12887A, DS12C887, DS12C887A […]
Show full quote

Which exact Dallas chip was the original ?
There are different versions of the DS12887 including DS12887A, DS12C887, DS12C887A and they are not exactly the same.
114 Bytes of General-Purpose, Battery-Backed RAM DS12887, DS12887A (113 Bytes in the DS12C887 and DS12C887A)
RAM Clear Function DS12885, DS12887A, and DS12C887A. My guess is that Pin 21 on your board is grounded causing the CMOS to clear when powered down:
"RCLR must be forced to an input logic 0 (ground) during battery-backup mode when VCC is not applied."
Pull the new RTC and gently bend pin 21 out a bit so it is not inserted into the socket and see what happens.

My man! You are a life saver. It was using DS12887 and looking at the original part, pin21 wasn’t connected. I bent pin 21 back as instructed and we’re in business, thanks so much!

Reply 3 of 3, by Horun

Posted on 2021-07-10, 22:47

Horun Offline

Rank l33t++

Rank: l33t++
Posts: 6910
Joined: 2019-09-19, 02:19
Location: USA

Great ! Glad it was something simple.

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. Stuff: https://archive.org/details/@horun

Go to top of page Go to top of page

Back to General Old Hardware