On 2/13/2017 4:56 PM, Douglas Crawford via vcf-midatlantic wrote:
On 2/13/2017 4:29 PM, Brian Schenkenberger via vcf-midatlantic wrote:
Douglas Crawford via vcf-midatlantic writes:
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Where do u get them!? The DS1287A was fairly common TOY clock back then. You can replace it with the newer DS12887A. Goggle it, there are ample sources. 12887s yes of course. I thought you meant there were actual replacement 1287A's now available. 12887A's don't work in some applications. They don't work in GRiDs reportedly. Which is the only one I've bothered trying to replace.
Differences summary: 1287 and 1287A (1285 based): 64 bytes total space, i.e. top address is 0x3F; accessing 0x40-0xFF 'mirrors' those accesses to 0x00-0x3F, I believe. no special Y2K handling, byte 0x32 is a plain RAM byte. 12887 and 12887A (12885 based): 128 bytes total space, i.e. top address is 0x7F. accessing 0x80-0xFF 'mirrors' those accesses to 0x00-0x7F, I believe. no special Y2K handling, byte 0x32 is a plain RAM byte. 12C887 and 12C887A (12C885 based): 128 bytes total space, i.e. top address is 0x7F. accessing 0x80-0xFF 'mirrors' those accesses to 0x00-0x7F, I believe. Byte 0x32 (used by MS-DOS on PCs for the 'century' value) the low 6 bits can only ever read as BCD-encoded 0x19 or 0x20 and will update itself properly on dec 31, 1999->jan 1, 2000 rollover. The high 2 bits of 0x32 probably act as ram. Unclear what happens if you try to set the low 6 bits to some value other than those two. The difference between the non-A and A versions has to do with whether the /RCLR pin (pin 21) is bent down and can be plugged into the pcb (DS12x87A), or bent up and hidden inside the potted block (DS12x87). You can convert a ds12x87A to a ds12x87 by snipping off that pin, or bending it up sideways so it doesn't go in the socket. All DS1287 and DS1287A blocks have the same thing inside: a DS1285 chip (often with the part number printed on it!), a battery (CR14xx or CR18xx, i forget the exact type), and a 32.768Khz clock crystal. DS12887 and DS12887A blocks have a DS12885 inside but are otherwise the same as above. DS12C887/DS12C887A have a DS12C885 inside but are otherwise the same as above. If the GRiD is accessing the 0x40-0x7F or 0xC0-0xFF expecting it to read/write from 0x00-0x3f, a ds12885/7/7a won't work (at least not without an added external circuit with a NAND gate between the /AS pin and the A7 pin. If it wants to store something else at location 0x32, a DS12C885/7/7A won't work. In the first case you'll have to re-battery a real 1287 chip, or "make your own module" out of a ds1285 chip by bending up four of the pins (GND pin 16, VBAT pin 20, X1 pin 2, X2 pin 3), connecting a 3v lithium coin cell battery (a cr2032 in a holder works fine, though its a bit large) to VBAT(+) and GND(-), and a 32.768Khz clock crystal to X1 and X2. Then plug the remaining pins into the original socket. If needed, bend /RCLR pin 21 up as well, and don't connect it to anything. -- Jonathan Gevaryahu jgevaryahu@gmail.com jgevaryahu@hotmail.com