Here's how it happened.
I had finished up with the dial indicator, and reinstalled the hot end. I had to do this three times because of issues with the silicon tape I use to insulate it. On the third reinstallation, the RAMBo went wonky. I turned it on and the LCDs never initialized. I poked and prodded things, and a few minutes later the LCD display came back. But (a) the LCD says the thermistors are bad - prints "def" for the current temperature, (b) Repetier Host-Mac can not open the USB port.
What have I tried?
- Restarted the Mac.
- Checked both thermistors' resistance at the RAMBo connectors. Both read ~100K.
- Looked for the USB port in the Arduino IDE. It is not there.
- Verified that an Arduino Uno works on that USB port and cable. (Flashed firmware and did serial comms.)
- Tried a different USB cable in the same USB port. Repetier Host-Mac nor Arduino IDE can open the port.
- Verified that the Arduino Uno works on the new cable.
- Unplugged the hot end (thermistor and resistors).
What else can I check before I replace the RAMBo?