Modbus data appear in different memory areas in a Modbus device. The four most common areas are coils, digital inputs, input registers and holding registers. These are also referred to as 0X, 1X, 3X and 4X areas respectively.
Modbus defines addressing in two ways: PLC addressing starts from address 1 and regular Modbus data addressing starts from 0. For example, a holding register at PLC address 234 can be referred to either as 4X register 234 or as 40234. The regular Modbus address, that is the PLC address decremented by one, is shown when analyzing the Modbus traffic on the physical network.
Note: Modbus addresses in this documentation follow the PLC addressing scheme. Slave unit address range is 1 – 255 and the data point address is 1 – 65535.
See also the Modbus protocol standard documentation that can be found for free at www.modbus.org.