ASCI u68 (SystemX)

On 2015-Oct-16, at 7:30 PM, Brad wrote: IC26 6875 clock generator IC22 68B00 processor . . obviously IC7 TBP18S22 PROM 256*8 probably for address mapping of the RAM / ROM / IO. IC19 2716 EPROM 2K*8 (or 2708 1K*8, note jumpers) firmware/monitor ROM IC18 2716 EPROM 2K*8 (or 2708 1K*8, note jumpers) optional ROM IC16 6514 SRAM 1024*4 IC17 6514 SRAM 1024*4 1 KBytes of RAM IC30 6821 IO 2*8+4 IC31 6821 IO 2*8+4 IO to the keypad & LED display. It looks like the hex-to-7-seg decoding is done in software, over on the kbd/display PCB is a CA3082 7*NPN-tran, presumably used as LED drivers. One of the chips is masked by the photoflash. IC29 6821 IO 2*8+4 Goes to connector P1, just GPIO pins to play with. IC15 6850 UART IC13 1489 RS232 receiver IC28 75150 line driver IC25 4702 bit-rate generator Comprise a serial line interface. Not clear how the bit-rate is set, I'm guessing the rate selection pins of the 4702 are fed by IC14 74175 latch, fed in turn from IO pins on IC30, so the baud rate would be be under programmatic control. IC27 555 timer Not clear what that's for, might be an oscillator/timer for kbd/display scanning, feeding into IC30 or 31. The remaining chips are bus drivers and a little glue logic. -- It's pretty straightforward for what it is. Date codes of 1981, but the support for 2708 and 2716 EPROMs suggest the design is a few years earlier. As IC30, the bad 6821, is involved in the keyboard/display you might try pulling it and replacing it with IC29. If you want to be particular, pull it and check for bad voltages and current limits on the IO pins before plugging in a sub. The expectation if it's running properly is that you'd press a key for a command followed by hex address or data, e.g. EXAMine, enter the address XXXX, displayed as entered, memory contents displayed on entering 4th address digit.