Modbus Protocol
Modbus is a widespread communication protocol in which industrial devices exchange data using register read-write logic. It runs in RTU form on a serial line and in TCP form over a network. A master asks, the addressed secondary unit answers; secondary units do not speak on their own.
The data model is plain, and that plainness is the reason for its prevalence:
- Coil: a single-bit output that can be read and written, typically a relay or a command bit.
- Discrete input: a read-only single-bit input.
- Holding register: a 16-bit value that can be read and written; setting parameters are held here.
- Input register: a read-only 16-bit measurement value.
In practice the most common problems arise not from the protocol itself but from the ends being configured incompatibly: the two sides being on different speed or parity settings, device addresses colliding, an off-by-one shift in register numbering, and the byte order being interpreted differently for values larger than 16 bits. That is why a device's Modbus map is documented together with the register number, data type, scaling factor and unit. The protocol itself has no encryption or authentication; line security is handled on the network side.
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