Success!!!
Removed the keyboard/Timer/Interrupt sections, no change.
Looking at the "A" register, and programmed with with 0AAH, it reads back as 2AH. I then noticed that the clock was advancing erratically, eg 1 second forward every 20-30 seconds.
Swapped the xtal for the 4rd time, and all started working!
My first two xtal's were Abracom AB38T-32.768KHZ from Mouser, the third was a LP49 type from Ebay (Pletronics LP490071-32.768M. The last xtal was pulled from a kitchen timer.
I don't want to jinx myself and substitute back one of the Abracom xtals to see if they were really defective - or if Frank's programming hint fixed the problem. It's sure nice to see it working.
Thanks to all for the help!
- Gary
On Wednesday, July 16, 2014 2:37:14 PM UTC-4, gek...@gmail.com wrote:
I'm pulling out my hair trying to access the DS12885 RTC on the MSDOS Support PCB.
The rest of the board (timer, interrupt and EPROM) appears to be functioning properly. I have not tried the mouse/keyboard section yet.
I can see low pulses on Pins 14/17 as expected. I'm using the "E" command from the 8088 board for testing time/date in V10.3 of John's monitor software.
When I set the time the seconds and day will program, but the hours/minutes/year/month remain gibberish. I also get errors when entering the "X" MSDOS menu - either that the RTC is stuck or that invalid BCD data exists.
I've swapped TTL and DS12885. I've also changed the 32.768khz xtal. When I scope pins 2/3 on the DS12885 (XTAL) I can see somewhat noisy pulses.
Would the DS12885 at least store the date if the xtal isn't oscillating properly? It looks like the RTC shares most of the same address/data circuitry with the other sections. The 8088 has a 8 mhz clock, could speed be an issue?
Any thoughts or hints appreciated!
- Gary