5150 Post Code 601

[modem7]

Guys, I need an opinion. If lyonadmiral manually pulls the pin 7 on U23 (the output of the gate buffering SW-1) high, would that screw anything up ? In theory, this should let the PPI read the correct value during boot. patscc

By default, the circuitry is such that the keyboard shift register (U24) is enabled and SW1 gate (U23) is disabled.
Thus reads of port A on the 8255 get keyboard data.
If all that is done is to manually pull U23-7 high or low, then you'll be 'fighting' with whatever is coming out of U24-13.
U24 needs to be disabled first.

The way I would do it is:

1. Run BASIC code of OUT &H61,&HCC (crude way of: disable U24 / enable U23). Note that 5150 can no longer read keystrokes.
2. With voltmeter, verify that U23-1 (enable/disable U23) is low.
3. SW1-1 to ON.
4. With voltmeter, verify that U23-13 is low.
5. With voltmeter, verify that U23-7 is low.
6. SW1-1 to OFF.
7. With voltmeter, verify that U23-13 is high.
8. With voltmeter, verify that U23-7 is high.
9. Restart 5150.

That checks the switch 1 on SW1 and checks the corresponding line in U23.

 

[patscc]

I didn't realize he could get into BASIC. Anyways, isn't the LS322 on pin 13 wired as an input ? My idea was basically yours, except that since OP currently doesn't seem to have a multimeter, it would be the manual pull-up and reboot approach.
patscc

 

[modem7]

Anyways, isn't the LS322 on pin 13 wired as an input ?

On a 74322, pin 13 is tri-state, however, because pin 2 is wired high in the 5150, that means that pin 13 will either be output or high-impedance.
The state of pin 8 will determine whether output or high-impedance.

Pin 13 is the LSB of the scan code from the keyboard, read by the 8255.