Two layers, one long line

Why one wire fails on a long run

A UART output is a voltage on one wire, and the receiver compares that wire with its own local ground. Over a long run the two grounds differ: a motor starting can lift one end's ground away from the other's, and the wire picks up noise along its length.

A 3.3 V line whose receiver ground has slid by a couple of volts can put a 1 across the receiver's threshold, which sits at about half the supply, and it reads as a 0. The wire carried the 1 correctly. The measurement was wrong.

Job one: the wire

RS-485 is an electrical standard. It says what the voltages on a pair of wires are, and nothing about what they mean. It has no message format and no addresses. Its fix is to measure one wire against its partner, not against ground, so a shift that hits both wires cancels. The next two lessons are that idea.

That allows about a kilometre at slow speeds, or megabits over much shorter cable, never both at once.

Job two: the message

Modbus RTU is a message protocol. It says who is addressed, what is asked and how the answer looks: an address, a function, data and a check value. It leaves the volts to RS-485, which its serial specification names for the electrical side. Modicon introduced Modbus in 1979.

Keep the two apart. A silent bus can have a wiring fault, a message fault or both, and the symptom does not say which.

Where the picture bends

The bench is a drawing. A real bus is not paper passed along a bench. It is voltage changing on two conductors, thousands of times a second, and the gauge is a stand-in for a voltage. The RS-485 book covers the real parts, RS-485 and Modbus on the ESP32 the code, and UART and baud rates the single wire this lesson starts from.

Common mistakes

• Calling Modbus a kind of wire. It is a message format. The same frame can travel over other links, and RS-485 can carry other messages.
• Expecting RS-485 to give you addresses. It has none. Who answers is decided by the message on top.
• Quoting a kilometre without a baud rate. The distance falls as the speed rises, and about a kilometre is a slow-speed figure.