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
and when communicating with the RMC75E
The Modbus addressing assigns a single number
to each register.
There are three different ways to refer to Modicon
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
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.
The Modbus address is calculated according to
the following equation:
Modbus RTU Address
= 2 x [(256 x file) + element]
where the RMC IEC
addressing format is %MDfile.element
%MD12:3 = 2 x [(256 x 12) + 3] + 1 =
%MD9:56 = 2 x [(256 x 9) + 56] + 1 =
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.
(RMC70 and RMC150)
- Current Values
Modbus Address Sections
For your convenience, the starting registers
of each register section are given below:
Duplicate: Indirect Data
Duplicate: Indirect Data Map
Duplicate: Variables - Current Values
Axis Status Registers
Axis Parameter Registers
Command Area - Small (RMC70)
Command Area (RMC150)
Indirect Data Map
Event Log Configuration
Variables - Current Values (RMC150)
Variables - Initial Values (RMC150)
Dynamic Plot Upload Area
Static Plot Upload Area
Variables - Current Values (RMC70)
Variables - Initial Values (RMC70)
(c) 2005-2012 by Delta Computer Systems, Inc.