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.

Finding Modbus Addresses for RMC Registers

When configuring the host controller (such as a PLC or HMI) to communicate with an RMC, you will need to enter the Modbus addresses of the RMC's registers that you wish to read from or write to.

Ways to find RMC addresses:

1. In RMCTools editors:
   Some register addresses are displayed in RMCTools, such as in the Reg # column in Axis Tools, Indirect Data Map, and the Variable Table. To view those addresses in various formats, in the Reg# column, right-click the address and choose Address Formats.

2. Use the Address Maps in RMCTools to browse all the register addresses for any addressing method.

3. Use the Register Map help topic to browse all the register addresses for any addressing method.

Modbus Address Sections

The Modbus address space for the RMC's is divided in sections as listed below. See each section for addresses of individual items within each section.

RMC75

RMC150

RMC200

The RMC200 Modbus addressing includes the pre-defined addresses as listed below. The remaining Modbus addresses may be configured in the Address Maps.

1 The maximum axis number n is 101 for the CPU40 and 47 for the CPU20L.

Address Calculation Utility (RMC75 and RMC150)

For the RMC75 and RMC150, the standard Modbus addresses can be calculated from the IEC addresses as described here, with the exception of the first four items in the table.

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

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