How to Use Modbus RTU Communication with MicroLogix 1400 PLCs

You need to connect the Modbus RTU device to the PLC’s serial port, configure it, and then write a Modbus RTU communication program using the RSLogix 500 software in order to use Modbus RTU communication with MicroLogix 1400 PLCs. To ensure dependability and stability, communication must be tested prior to system deployment.

Configure the MicroLogix 1400 PLC:

You must set the PLC’s communication parameters to correspond with the Modbus RTU device’s parameters in order to configure the Rockwell Automation MicroLogix 1400 PLC for Modbus RTU communication. Either the keypad on the front of the PLC or the RSLogix 500 software can be used for this. Depending on the software version and the device you are connecting to, the exact steps may vary, but typically you must set the following parameters.

  • Baud rate: the rate in bits per second (bps) at which data is sent over a communication line.
  • Data bits: the number of bits each message character is represented by.
  • Parity: a mechanism for checking for errors that are introduced during transmission.
  • Stop bits: the number of bits used to indicate a character’s end.

Depending on your particular application, you might also need to configure other settings, such as flow control or protocol-specific settings. The documentation for the Modbus RTU device and the PLC should be consulted for detailed instructions on configuring these parameters.

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Connecting the MicroLogix 1400 PLC to the Modbus RTU device:

Use an RS-232/RS-485 converter or an RS-232/RS-422 converter to connect the Modbus RTU device to the serial port of the PLC.

Configure the Modbus RTU device:

A Modbus RTU device must have its communication parameters set to correspond with those of a MicroLogix 1400 PLC in order to be configured for communication with the PLC. Setting the following parameters is typical for doing this:

  • Address: A device’s on-network communication device’s specific address. The address needs to be configured to the PLC’s specifications.
  • Baud rate: The amount of data transmitted over the communication line per second, expressed in bits (bps).
  • Data bits: The amount of data bits used to represent each message character.
  • Parity is an error-checking system that locates mistakes made during transmission.
  • Stop bits: The quantity of bits used to indicate a character’s end.

Depending on your particular application, you might also need to configure other settings, such as flow control or protocol-specific settings. For detailed instructions on configuring these parameters, refer to the device’s user manual.

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Check: Rockwell Automation 1766-L32BXB MicroLogix 1400 PLC

Configure the communication parameters in the PLC:

The RSLogix 500 software can be used to set the communication parameters for the MicroLogix 1400 PLC’s serial port.

 Follow these general steps:

1. Open the MicroLogix 1400 PLC project in RSLogix 500 by starting a new one or by opening an existing one.

2. Expand the “Controller” folder in the project tree, then double-click the MicroLogix 1400 controller to bring up the “Controller Properties” window.

3. The “Serial Configuration” window can be accessed by selecting the “Channel Configuration” tab, followed by the “Serial Port Configuration” button.

Configure the following communication parameters in the “Serial Configuration” window to match the Modbus RTU device’s settings. Program the Modbus RTU communication.

  • Baud rate: the rate in bits per second (bps) at which data is sent over a communication line.
  • Data bits: The number of bits each message’s character is represented by.
  • Parity: a mechanism for checking for errors that are introduced during transmission.
  • Stop bits: the number of bits used to indicate a character’s end.
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Test the communication:

You should test the communication to make sure it is functioning properly after configuring the PLC and Modbus RTU device’s communication parameters.

  1. Use the proper cable to connect the Modbus RTU device to the MicroLogix 1400 PLC’s serial port.
  2. Turn on the MicroLogix 1400 PLC and the Modbus RTU device.
  3. Start RSLogix 500, then transfer the file to the MicroLogix 1400 PLC.
  4. In RSLogix 500, open the “Channel Monitor” and configure it to watch the serial port to which the Modbus RTU device is connected.
  5. Check that the Modbus RTU device is responding correctly by sending it a few test commands from the PLC.
  6. Check that the data is being transmitted and received correctly by monitoring the data traffic between the PLC and the device in the “Channel Monitor” window.
  7. Check the PLC and device configuration settings to make sure they are accurate if there are any communication errors or problems.
  8. After making any necessary modifications, retest the communication to ensure that it is operating properly.
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