On Mon, Jan 18, 2016 at 10:13 AM, tony duell <a...@p850ug1.demon.co.uk> wrote:
> IF the PC100-A is like the PC100-B (which I have full schematics for)... > > > All LEDs on, means, "replace the system board". > > > > I'm not sure, he wanted to hear that ;-) > > And it's not even right!. You can have all LEDs on with a perfectly good > system board. A fault in the power-OK circuit in the PSU could cause it! > > OK. As has been suggested the LEDs are turned on when the hardware is > reset (they are simply connected between the outputs of TTL latches and > +5V, > said latches are reset to 0 by the hardware reset signal) and then > controlled by > the processors. So all on suggests the processors are not running. Half of > the > LEDs are controlled by the 8088, the other half by the Z80. The 8088 is > reset by > the hardware reset signal, one of its output ports controls the Z80 reset > line. > Yes, the 8088 is expected to load Z80 program and take it out of reset. The only interrupt wired to the z80 is the software interrupt from the 8088. > So it is likely that the 8088 (at least) is not running. I would start > there. Check its > reset and clock inputs, does it seem to be accessing memory, etc. Check its address and data bus too... Warner
