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Can RTC replace original motherboard battery?

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musicforlife

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Is it possible to replace motherboard battery with ISA card containing real-time clock? Can it retain CMOS memory or does it work only for clock?
 
Most if not all of those ISA cards with clocks were meant for XT class machines which didn't have CMOS to store settings. The settings were configured on those Moterboards with dip switches and jumpers. Therefore, none of the ISA card clocks had CMOS that I know of. What mother board are you using that needs a battery?
 
ibmapc said:
Most if not all of those ISA cards with clocks were meant for XT class machines which didn't have CMOS to store settings. The settings were configured on those Moterboards with dip switches and jumpers. Therefore, none of the ISA card clocks had CMOS that I know of. What mother board are you using that needs a battery?
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286 computer with 3.6v 60mAh battery. The battery was bulky enough to prevent one isa socket being used hence I hoped to find solution with less space and easily replaceable when needed like coin battery. Sure, I could just solder wires instead and attach the battery elsewhere.

I could only find something like this but it's quite expensive replacement for a cheap battery. Besides I have no idea whether this would actually work or not: https://www.ebay.co.uk/itm/Gilbarco-T19140-G4-G-Site-Gas-Pump-ISA-CMOS-Memory-Board/151755693703
 
I don't think that will work. I'm a bit confused, the battery prevents one ISA socket from being used, and the solution is to use 1 ISA socket to host the battery? Perhaps I've misunderstood.
 
mR_Slug said:
I don't think that will work. I'm a bit confused, the battery prevents one ISA socket from being used, and the solution is to use 1 ISA socket to host the battery? Perhaps I've misunderstood.
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Sorry for confusion. The main point is that I would prefer a replacement battery which wouldn't need another soldering job later in future but I guess there are no coin battery or alkaline equivalents? Battery on isa board for cmos was just a nice idea but I guess such thing doesn't exist. The bulkiness of the battery was only a side note but I am not running out of isa sockets either so it's not an issue.
 
I buy 3.6 volt cordless phone batteries. Solder in long wires and put the battery well away from anything that could be damaged by a leaking battery. The ones I’ve used are 600 mAh instead of the 60 mAh of the original so it should keep things going for a long time between uses of the computer. I have done this to 2 computers so far with no problems.
 
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An RTC contains RAM, a crystal, and a battery. Your motherboard likely already has the RAM and the crystal necessary for counting time. Therefore, what you need is a suiting battery.

I use rechargeable batteries that total up to the necessary voltage when the system's specifications say that they can be rechargeable. When in doubt, always make sure to add a diode if you plan to use regular batteries!
 
musicforlife said:
Is it possible to replace motherboard battery with ISA card containing real-time clock? Can it retain CMOS memory or does it work only for clock?
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Simple answer - probably not. The easiest way to fix is probably unsoldering existing 3.6V battery, and soldering 3*AA battery holder in series with a diode instead.

More complicated answer: The clock in an ISA RTC card might work if you install the right clock driver. But it won't be functionally equivalent to on-board RTC, and it can't be used to store AT configuration.

musicforlife said:
Besides I have no idea whether this would actually work or not: https://www.ebay.co.uk/itm/Gilbarco-T19140-G4-G-Site-Gas-Pump-ISA-CMOS-Memory-Board/151755693703
Click to expand...

This looks like a battery backed up SRAM card. Not an RTC.
 
There were some PC XT ISA clock cards using the MC14686, complete with battery, so in theory, it might be possible. I'm not sure that standard 286 ISA decodes I/O addresses below 100H--and the 286 "CMOS" uses 70h and 71h My memory is rusty on this one. I'm pretty sure that none of the ISA cards that I have in mind were interrupt capable.
 
Chuck(G) said:
There were some PC XT ISA clock cards using the MC14686, complete with battery, so in theory, it might be possible. I'm not sure that standard 286 ISA decodes I/O addresses below 100H--and the 286 "CMOS" uses 70h and 71h My memory is rusty on this one. I'm pretty sure that none of the ISA cards that I have in mind were interrupt capable.
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The data signals of on-board peripherals in an AT, including the RTC, are connected to the ISA bus through a transceiver (U113 on attached schematic). When reading an I/O port in 0h - 0FFh range, the direction of that transceiver set to send the data to the ISA. Therefore placing any I/O devices in that address range on the ISA bus and reading them would create a bus contention. (It would work for I/O writes though, e.g. to the common POST port - 80h).
Also the IRQ8 used by the RTC is not available on the ISA bus.

at014.jpg
 
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Well, that pretty much answers that question. In particular, some cells in the RTC chip memory are used by the POST routines in a 286, including the "why did I shut down?" byte. That can create big problems in some cases.
 
The charging circuit in a vintage PC isn't of the "smart" nature, and Li-ion rechargeables require that. I'd stick with a lithium cell or two and a blocking diode.
 
You can buy the exact replacement for the original battery, so another option would be to solder in wires and fasten the battery in a safe location. If I did that, I would use shrink tubing over the connections at the battery to make sure nothing shorts it out. Also, the cordless phone batteries I mentioned in an earlier post are mostly Nickel-Cadmium (Ni-CD) or Ni-MH. I don't think either of those require a smart charger.
 
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