modem7
10k Member
By default, the circuitry is such that the keyboard shift register (U24) is enabled and SW1 gate (U23) is disabled.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
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.