Configuring the Processor as a Modbus Slave

The following steps show you how to access the serial port configuration screen for the 171 CBU 78090, 171 CBU 98090 and 171 CBU 98091 processors in Control Expert. Use the Control Expert configuration screens to configure a processor for serial link operation in Modbus slave mode.

NOTE: You can use L9_XMIT or XXMIT function blocks to configure a serial link to operate in either Modbus master mode or character mode.

During run-time, these function blocks overwrite the serial port configuration settings in Control Expert. After these function blocks have finished executing, the serial link retains the settings written to the serial port by these function blocks. To restore the original serial port settings, cycle power to the processor or download the original configuration to the processor.

Accessing the Serial Link

To access the serial link, follow these steps:

Step

Action

1

In the project browser, navigate to and open Project > Configuration > Momentum Bus > <Processor> :

2

Double-click the Serial Port node to open the Configuration screen:
NOTE:

  • The 171 CBU 78090 configurable RS232/RS485 serial port uses Channel 0.

  • The RS485 fixed serial port uses at Channel 1.