Modbus Data Model

Modbus defines its data model based on a series of tables of four primary types, which are called register maps. Modbus functions operate on register map registers to monitor, configure, and control module I/O. You should refer to the register map for your device to gain a better understanding of its operation.

Modbus registers are organized into reference types identified by the leading number of the reference address:

• Read/write discrete outputs or coils—A 0x reference address is used to drive output data to a digital output channel.
• Read discrete inputs—The on/off status of a 1x reference address is controlled by the corresponding digital input channel.
• Read input registers—A 3x reference register contains a 16-bit number received from an external source—for example, an analog signal.
• Read/write output or holding registers—A 4x register is used to store 16-bits of numerical data (binary or decimal), or to send the data from the CPU to an output channel.

For more information, see Modbus Overview.