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Shadow Lord
December 22nd, 2013, 09:28 PM
We all know these guys are a pain to replace. I've seen the articles on shaving down the chip and soldering on to leads but I was wondering any reason a dead one couldn't be replaced with a ISA clock card? I.E. If I have the extra ISA slots couldn't I just plop in a RTC ISA clock card and be done? Am I missing something on this that would prevent it from working?

Along the same lines: in a modern system the coin battery provides the juice to keep the CMOS settings and the RTC settings. In these older boards how were the CMOS settings maintained? Did they use the battery in the Dallas chip? Thanks.

Chuck(G)
December 22nd, 2013, 09:57 PM
I used to have a short ISA card with a MC146818 CMOS/Timer, coin cell, and support circuitry on it, but it was for an XT. In other words, just like a PC AT CMOS/timer setup. And that was fine for an XT.

But a BIOS for an AT or better is relying on the 146818 or equivalent being at I/O ports 70 and 71 (hex). That's where the basic configuration as well as the time of day is stored--and the decoding for those addresses is on the motherboard ahead of any ISA slots. So you have the problem of getting past the BIOS diagnostics...

Shadow Lord
December 22nd, 2013, 11:09 PM
I used to have a short ISA card with a MC146818 CMOS/Timer, coin cell, and support circuitry on it, but it was for an XT. In other words, just like a PC AT CMOS/timer setup. And that was fine for an XT.

But a BIOS for an AT or better is relying on the 146818 or equivalent being at I/O ports 70 and 71 (hex). That's where the basic configuration as well as the time of day is stored--and the decoding for those addresses is on the motherboard ahead of any ISA slots. So you have the problem of getting past the BIOS diagnostics...

Thanks Chuck. That explains why everyone goes through the trouble of replacing the battery on the chip. I assume the chip has been long out of production so even the ones on eBay have old batteries, right?

g4ugm
December 23rd, 2013, 06:08 AM
Depends on the type. Several are listed by local suppliers...

Shadow Lord
December 23rd, 2013, 06:45 AM
Depends on the type. Several are listed by local suppliers...

So the chips are still being made in some form? I have to pull out my old Tyan boards and see if it uses a RTC that is still being produced. Thanks.

SomeGuy
December 23rd, 2013, 09:01 AM
Just keep in mind some people might try and sell you an old pull. Most of these have date codes printed on them, so you will want something made in the last few years.

Agent Orange
December 23rd, 2013, 09:31 AM
If you're looking for a 'standard' Dallas RTC, Jameco has them: http://www.jameco.com/1/1/25731-ds12887-ds1287-real-time-clock.html

cr1901
December 23rd, 2013, 11:08 AM
I used to have a short ISA card with a MC146818 CMOS/Timer, coin cell, and support circuitry on it, but it was for an XT. In other words, just like a PC AT CMOS/timer setup. And that was fine for an XT.

But a BIOS for an AT or better is relying on the 146818 or equivalent being at I/O ports 70 and 71 (hex). That's where the basic configuration as well as the time of day is stored--and the decoding for those addresses is on the motherboard ahead of any ISA slots. So you have the problem of getting past the BIOS diagnostics...

Why would what you stated in bold make a difference if OP wants to replace the clock module with an equivalent ISA card? If the clock module itself is removed then shouldn't ports 0x70 and 0x71 on the motherboard go nowhere- and all devices save for a potential ISA card at the same address will ignore the address bus when those addresses are requested?

Re: OP's original question, you can also perform a Dallas clock module mod: http://www.mcamafia.de/mcapage0/dsrework.htm

You CAN get large numbers of old pulls for cheap on Ebay... in my experience, most of them are dead but at least two or three of them still have life in them :D.

Chuck(G)
December 23rd, 2013, 11:59 AM
Why would what you stated in bold make a difference if OP wants to replace the clock module with an equivalent ISA card? If the clock module itself is removed then shouldn't ports 0x70 and 0x71 on the motherboard go nowhere- and all devices save for a potential ISA card at the same address will ignore the address bus when those addresses are requested?

A lot depends on how the motherboard is set up. Many simply decode anything under 100H as being local and the ISA bus never sees the signals. But the BIOS will stubbornly read 70 and 71H and complain with every boot.

MikeS
December 23rd, 2013, 12:13 PM
A lot depends on how the motherboard is set up. Many simply decode anything under 100H as being local and the ISA bus never sees the signals. But the BIOS will stubbornly read 70 and 71H and complain with every boot.Almost looks as though not everyone's aware that the CMOS BIOS settings are stored in the Dallas module and I've never seen an ISA RTC card that also provides CMOS memory (which wouldn't work anyway as Chuck explains).

Shadow Lord
December 23rd, 2013, 12:25 PM
Almost looks as though not everyone's aware that the CMOS BIOS settings are stored in the Dallas module and I've never seen an ISA RTC card that also provides CMOS memory (which wouldn't work anyway as Chuck explains).

Which answers the second question in my original post, thanks.

MikeS
December 23rd, 2013, 12:49 PM
Which answers the second question in my original post, thanks.Yeah, the RTC and multi-IO w/RTC cards were mainly intended for PCs and XTs (and clones) that used DIP switches for configuration instead of CMOS memory and did not come with an RTC.