I have an unconventional question, and I'm really grasping at straws here.
I have a test adapter that has been been reliable in service for several years. It uses five (Kvaser) CAN channels to communicate with 30 devices using six different addresses on each CAN channel. The devices are on a fixture in a thermal chamber controlled by a Watlow F4. The adapter consists of a USB-6501 that controls a home-built 32-channel relay card. There are three two-channel CAN transceivers, and all four devices are plugged into a powered USB hub. The computer's serial port is connected directly to the Watlow F4 controlling my thermal chamber. (The same behavior occurs when I replace the computer with my laptop and a USB-serial converter.)
Last week, the system began to fail to identify most of the devices on the test fixture.
I spent two days banging my head on the station before discovering that the adapter works if I unplug the serial cable. I thought that the Watlow controller had gone bad and had Maintenance replace it. The problem persists.
UUT power is controlled by an SSR triggered by the 6501. Yesterday, I saw the adapter turn on UUT power when I unplugged/plugged the serial cable. It happened twice, in the presence of a witness, but I couldn't cause it to happen a third time. The failure of the adapter to identify all UUTs when I connect the serial cable is repeatable, as is the converse.
I've tried replacing the 6501, our relay cards, Kvaser transceivers... Do you have any ideas?