Modbus Addressing

This topic describes the Modbus addressing format as used by the RMC. For other addressing formats, see the Register Map Overview topic.

The Modbus/RTU and Modbus/TCP address format must be used when communicating with the RMC75S via Modbus/RTU and when communicating with the RMC75E or RMC150E via Modbus/TCP.

Address Format

The Modbus addressing assigns a single number to each register.

There are three different ways to refer to Modicon holding registers:

• Holding Register Address (e.g. 400059)
  Holding registers are labeled with the 4 prefix to differentiate them from other types of registers in the Modicon PLCs. Originally, they were called 40 thousand registers and were five-digit numbers starting with a 4. However, as PLC memory grew, an extra digit was added. So, 40059 became 400059.

• Holding Register Offset (e.g. 59)
  Holding Register Offset addresses are 1-based. This is basically the Holding Register Address stripped of the leading 4 and any leading zeros. Therefore, 400059 becomes 59. The RMC Modbus addresses are given in this format. You will need to prepend it with a 4 and any leading zeros.

• Modbus Protocol Address (e.g. 58)
  In the Modbus/RTU and Modbus/TCP protocols, the addresses are encoded using 16 bits with a number between 0 and 65,535. These are 0-based addresses. Therefore, the Modbus protocol address is equal to the Holding Register Offset minus one. This is mainly used internal to devices and is typically not seen by the end-user.

Determining Modbus Holding Register Offset Addresses

To obtain the Modbus holding register offset address of RMC registers, use the Register Maps. The maps provide the Modbus offset for each register. The complete address is the holding register offset prepended with 4 and padded with zeros if necessary. For example, holding register offset 8193 will then become the address 408193 or 48193, depending on the host controller.

In addition, you can use the utility or calculation below to determine the Modbus holding register offset, given the IEC address.

Manual Calculation

The Modbus address is calculated according to the following equation:

Modbus RTU Address = 2 x [(256 x file) + element] + 1

where the RMC IEC addressing format is %MDfile.element

Examples:

%MD12:3 = 2 x [(256 x 12) + 3] + 1 = 6151

%MD9:56 = 2 x [(256 x 9) + 56] + 1 = 4721

Low Address Range

In addition to the standard Modbus register map, which is calculated from the IEC register map as described above, the RMC70 and RMC150 duplicate some Modbus registers to lower address ranges. Some PLCs do not address high enough to access critical data. The lower addresses allow all PLCs to access RMC data.

Modbus Address Sections

For your convenience, the starting registers of each register section are given below:

StartFragment

See Also

Register Map Overview