Basic structure of all ModBus protocols

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Regardless of the variant, all standard ModBus communications follow the same messaging structure. Everything revolves around storing values in registers and then reading them. There are several data types at play here.

For example, discrete inputs can be stored as individual bits. More broadly, input registers store values collected by ModBus-based devices. Multiple discrete devices are stored as a single bit, while analog values or even ASCII characters may be stored in 8-bit registers (least significant bit is 0, most significant bit is 64 bits).

The different types of registers are categorized by the function code used to poll them. A "1" in the function code field is for "read coil" (checks the state of the control relay). "2" means "read discrete input" (check the state of the discrete input). "3" is "Read Holding Registers" (collect data from multipurpose read/write registers). There are many more in the ModBus specification. Registers are requested by their "start address", which is a 16-bit value that uniquely identifies each register in the device. Master requests to the RTU or other device will always refer to the desired registers to report on.