Westermo RM-115S User Manual

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115S Serial I/O Module

User Manual

man_115S_1.14.docx

Page 23

3.2 MODBUS Slave Configuration

If configured as a MODBUS Slave the module will need to have a unique Slave address,
see section 2.1.2 “Address Switches” above for details on how this is done.

The Address selected will be the address that the MODBUS Master needs to poll to
gather data.

The module also has a number of internal registers that hold the I/O values, status, etc.
These register are polled by the MODBUS Master using standard commands that
Read/Write values to or from the device.

When the Modbus Master Polls the Slave is does so using a Modbus Command and an
I/O location. This Modbus command will varies depending on whether they are Inputs /
Outputs and Analog / Digital values
Example

• To read a Digital Input on the module an “02: Input Status” command is

used and the Modbus address will be in the range 10001 – 19999

• To read an Analog input on the module an “04: Input Register” Command

is used and the Modbus address will be in the range 30001 – 39999.

• To write to a Digital output (Turn it on) you need to use a “01: Coil

Status” Command and the Modbus address location will be 0001 – 9999

• To Write to an Analog output you will need to use an “03: Holding

See Appendix A “Modbus address map” for the I/O registers locations.

Note: all Analog values are 16 bit unsigned integers.

We recommend using Wintech’s Modscan and Modsim which are low cost Modbus
Master and Modbus Slave simulators to help diagnose any Read/Write addressing issues.