Top trace:  Sync (IRQ7)
Middle trace: Balanced RS485 signal
Bottom trace: I/O Board Rx line (logic level)

Before fix:
000 - 4 boards no term
001 - 1 board no term
002 - 1 board, 150 ohm term
003 - 4 boards, 150 ohm term

After fix:
004 - 115200 baud
005 - 57600 baud (Note how we caught it when Rx didn't go back down)
006 - 57600 baud, more normal case

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007 - Stepping on RS485 line when switching relays

008 - Fixed!  Transaction at 57600 baud.  Approx 6.2 ms total.
009 - Transaction at 115200.  Approx 5.1 ms total.