Okay, it's been a while but I've been busy learning assembly, adding commands to BASIC and writing my own command interpreter that the system boots into to provide a streamlined assembly-code running area, with the option to boot into BASIC if you want.
I've now started building version 2 of the SBC, based (currently) almost entirely on Grant Searle's CP/M version - schematic below for reference:
The only differences in my design so far are that I'm not using an RS232 interface, I'm using the TTL one which connects to an FT232RL USB/TTL adapter on the board before connecting to a standard USB cable to connect to the computer. The FT232RL adapter provides power to the computer via the USB lead.
I've got the board all wired up and ready for power up, but (holding with tradition so far) it's not working. There's nothing on the serial window. The board is powered, all bits are getting power and the current draw is 0.08-0.10mA, well within expected range.
I've done some initial testing - the clock signal is fine and getting to the Z80 and SIO/2 - but I think I've got an issue with the reset circuit (the bit immediately to the bottom-right of the word 'system' in the image title above. The CPU reset pin is hovering around 0.2V, which is clearly not enough to get it out of the reset state. Below is a close-up of the reset circuit as built on the SBC:
The voltage at the power rail (below the resistors in the above image) is 4.84V. But this drops to 0.2V on the top side of the resistors. The yellow wire that connects to the right pin of the red button (warm reset) connects to the SIO/2 and CPU reset pins, as per Grant's schematic. There's no shorts that I'm aware of (they'd show up on the power usage anyway, wouldn't they?) so I'm a little stumped. My electronics knowledge isn't quite up to debugging this one. I've swapped out the 4K7's for a couple of 1k resistors, but that made no difference... so aside from directly wiring the reset circuit to live to try and get the voltage up to 4.8V, I don't know what else to try.
Any advice?