Charles in Tennessee was having trouble with his USB to RS232 adapters working with his version 2 SBC-85 CPU board. It was working fine with the native COM port on his computer, but refused to communicate reliably when using a USB adapter. I have used the same board many times on both without issue, so I was afraid did not have any good suggestions other than my experience with USBs occasionally locking up and needing reset.
Fortunately, Charles completely ignored me and dug and dug until he found the problem was in the SBC-85 board, and not the adapter. He reported back to me today his findings and it was a real forehead-slapping moment for me.
As a bit of background, the first version of the SBC-85 CPU was created using Target 3001!. The goal of version 2 was to have universal EPROM sites which required a much tighter layout and KiCAD has features to make that easier, so the schematic and entire PCB layout was recreated using KiCAD. Long story short, I missed the ground on the DB9 for the COM port on the SBC-85.
A quick check of the schematics (below) show it was there on the version 1 board but missing on v2. The shield of the connector is grounded, but not the actual common (pin 5) of the connector.
Unfortunately, I also made this mistake on the SPIO board. No excuse for that one.
Fortunately an easy fix, so take pin 5 over to one of the shield connections which are tied to the ground plane.
