While creating the schematic for the SBC-85 CPU board I sort of went too far when connecting the Addresses to the 8155 address equality detector. The idea of the equality detector for the 8155 is to have it select the 8155 whenever the addresses are within a narrow range from 0x4000 through 0x47FF. CPUv1.0 has address lines from A15 down to A8 connected to the decoder and compared to all 0s except A14 which is compared to 1. This would enable the 8155 everywhere from 0x4000 through 0x40FF, i.e., just its memory locations. But when it comes to addressing the Command Status Register, the ports, or the timer ports they are above 0x40FF and the equality detector would have turned off the CE* up there.
Remember that the 8085 puts the port number out as the address (along with taking the IO/M* line high). It is important to remember that the 8085 mirrors the port number on the high address byte, so the highest address for the 8155 is not 0x4045 but 0x4545.
Back to the modifications….
We need to stop the equality detector from comparing addresses A8, A9, and A10 if we want to use the 8155 ports. To do this, on the bottom of the board each of the traces to pins U10.2, U10.4, and U10.15 need to be cut. Cut them with a utility knife and then confirm with a continuity check that they are no longer connected. Then with a short piece of jumper wire these three pins need to be taken to ground which, conveniently, is the next adjacent pin for each of the three.
Hopefully the photograph sufficiently shows the detail.
thanks
craig
