durgadas311
Veteran Member
Also note, if the EPROM is truly broken (as the dumps appear to show), you have little hope of getting anything to light up (without replacing the EPROM with a complete image).
Sure. It would be good to find out if the keypad uses RS-232 to communicate or something else. I haven't seen a photo of the connections. The computer RS-232 cables seem to terminate at a telephone modular plug and a telephone junction block (or maybe just a female modular housing). Knowing this helps with interpreting the ROM code.I feel pretty confident the keypad and computer go together. But I think the keypad was used before the computer came along with something else. I'd power it up but without the computer being functional it's pointless.
So far for the 74LS139:Discovering more about the system, but 2-D photos aren't really working well. Need to be able to examine it in 3-D.
The four loose wires are for power, where black is ground, red is +5V, brown is -12V and blue is +12V. The unlabeled push button next to RESET is for NMI. The four toggle switches are connected to the A/B modem control inputs on the SIO - looks like a "cheap" way to implement configuration switches. I must not be able to properly trace the wires for the SIO/RS-232 port, as they don't make sense. But, the twin-telephone-cables are definitely RS-232 for the SIO.
It appears that the display device operates at I/O ports 0x80-0xbf, however I see no code in the ROM that accesses this port range (not surprising, since half is missing).
I'm having trouble tracing the I/O decoder lines for the PIOs, SIO, and CTC. Maybe you can trace where the 74LS139 pins 9, 10, 11, and 12 go. They should be going to the Z80-CTC pin 16, PIOs pin 4, and SIO pin 33. Also, I can't tell how the address lines are connected to the PIOs, SIO, and CTC. A0 and A1 should be connected, in some order, to PIOs pins 5,6; SIO pins 32,34; and CTC pins 18,19.
I did confirm that the left-most ROM socket is for address 0000. Interesting, the CPU leaves A13, A14, A15 unconnected. Clearly, it was never intended to use more than 8K of ROM/RAM.
The two empty 14-pin sockets in the upper right are some sort of I/O expansion, it would seem. They provide buffered access to A0-A7 and D0-D7. I can't tell what control signals are presented there.
The system seems to operate at 2.5MHz.
The 50pin IDC connector is definitely connected to the PIOs, but it seems like more of a GPIO than parallel printer port. It even could be for a EPROM programmer, but that's just a guess.