Offhand sounds like you are not addressing the onboard ROM correctly. Did you say it works at 4MHz but not higher on this board. If so flakey EPROM. Should really use EEPROMS anyway. Faster, easier to program etc. Check pin 19 of U15A pulses low all the time (must do so if ROM addresses are selected) along with MEM_RD.
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of norwe...@aol.com
Having trouble getting the Z80 CPU card going. Trying to run at 4MHz with a Z80B CPU (known good, runs in another CPU card at 4 MHz just fine). Using a 27C32B EPROM (supposed to be easily fast enough to function at 4MHz). No wait states selected (all dip switches off). 24-pin EPROM is installed in the 28-pin socket with 4 vacant pins visible at the notch end of the chip (I hope that's right). With an EPROM full of HALT's (76h), at power up OE of the EPROM wiggles for a time, and then goes (and stays) high. Same for pin 18 (/HALT) of the CPU. Chassis reset does NOT do anything at this point. The ROM LED comes on at power up, but then goes off after a short while (maybe a minute). Reset does not change the status of the LED -- only a power cycle does.
first and then wiggles as the EPROM is executed.
Clocks from 4 MHz and 2 MHz oscillators seem fine.
Any ideas about what's wrong?