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Is a MK48T87B functionally compatible with a 1287 RTC?

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First post, by pico1180

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I recently acquired a 486 board with what I assumed was a standard 1287 RTC. I didn't bother reading the letter as I just assumed it was your standard Dallas RTC I have seen on so many other boards.

It was of course dead, but hard soldered to the board. I desoldered it, installed a socket and popped in a 1287 Dallas RTC, but it didn't POST. The desoldering went well so I didn't suspect anything went wrong.

I went on eBay to find an MK48T87B and found some close matches. But those didn't work either.

Should a Dallas 1287 RTC be a functional replacement for this MK48T87B? Or am I chasing my tail?

The first pic is the Timekeeper I am trying to replace.

The second pic is the one I used as a replacement, but didn't work. A Dallas 1287 RTC doesn't work either.

Reply 1 of 1, by Ryccardo

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According to the datasheet it's a "DROP-IN REPLACEMENT FOR IBM AT COMPUTER CLOCK/CALENDAR" so the basic functionality (I/O mapped SRAM with the first bytes shared with the RTC) is the same 😀

(This doesn't mean there may not be edge cases - the basic functionality of the DS12887 is also supposed to match the DS1287 but more than one BIOS doesn't like the difference)

But from your post it isn't clear whether the computer partially worked with the old chip? If yes, does it return to that condition if you insert the old chip in the new socket?

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