Connecting Multiple Modbus Slaves to a Single Modbus Master I/O Server

Issue Details

Solution

When configuring a Modbus I/O Server in LabVIEW, each Master server is configured to communicate with a single COM port for Modbus Serial or a single slave IP address for Modbus Ethernet.

• For Modbus Serial, up to 247 slave devices can be connected on a single serial network. It is important that each slave device have a unique Unit ID.
• For Modbus Ethernet, a Modbus Master I/O Server can only connect to a single slave device. To communicate with multiple Modbus Ethernet slave devices, you must define a separate Master I/O Server for each device.

When configuring a Serial Modbus Master to communicate with multiple Modbus Slaves, you can control the target of any read/write operations completed by the Serial Modbus Master. This is done by changing the UnitID Property of the Master. You can change it via the Set Unit ID VI or a Property Node by selecting the UnitID Property.

Additional Information

In the LabVIEW Datalogging and Supervisory Control Module Programming Reference Manual, examples are available on how to configure communication between a Serial Modbus Master and multiple Modbus Slaves.

To configure Serial communication with multiple Slaves, you can use one of the following options:

• Create multiple Serial Modbus Master instances. This is done by forking the wire to create multiple branches and configuring the UnitID and/or other properties for each branch separately.
  • This method is depicted in the example image above.
  • Make sure that all Master instances are properly managed and closed at the end of execution.
• Create a single Serial Modbus Master instance.
  • You can configure the instance's UnitID and/or other properties, complete read/write operations for a given slave (e.g. Slave with UnitID = 1), then change the Unit ID to complete read/write operations that target a different slave (e.g. Slave with UnitID = 2).
  • This method allows you to dynamically assign any required properties.