When nothing arrives
Almost every RS485 fault is one of six, and each has a symptom that points at it: TX to TX, A and B crossed, a direction pin never set, the wrong board for the microcontroller, no ground wire, or too many 120 Ω resistors. Start from the symptom.
Symptom first
A meter goes a long way
Three measurements find most of these without a scope:
| Measure | With | Expect |
|---|---|---|
| A to B, at any terminal | power off | about 60 Ω on a bus, 120 Ω on a lone board |
| 5V or 3V3 to GND, on each board | power on | the board's own voltage |
| DE to GND, on a listening board | power on | 0 V: listening |
A listening board's DE should read 0 V; if it reads the supply, that board is holding the bus and nobody else can be heard. If every DE is right and nothing moves while the other side sends, look at the talker: its direction pin, its RX wire, and whether its bytes reach A and B at all.
Change one thing at a time
Go back to two boards on a short wire, with both 120 Ω fitted and a sketch that prints what it hears, and get that working first. Then add length, then boards, then your own protocol. A fault that appears at one step belongs to that step.
When it does not work
Test the suspect board against one you trust: run the two-board ESP32 build from this book with the suspect board as board B. If it hears the good board, the fault is in the other microcontroller's wiring or sketch, not the board.
That board has no supply: the LED sits across it through 5.1 kΩ. Check the wire to its 5V or 3V3 hole and its GND.
Each new board brought its own 120 Ω. Measure across A and B with the power off: under about 50 Ω means too many are fitted. Take the resistor off every board that is not at an end of the cable.
The listener's UART caught a stray edge on the idle bus and was part-way through a false byte when the real message began. Have the talker raise its direction pin a moment before sending, and discard anything before the start of a message.
The reference page: both boards, their specifications and downloads.
RS485 transceiver boards →Edit this page — content/books/rs485/when-nothing-arrives.mdx
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