Modbus TCP client
Available from firmware 2024.6
Modbus is a long-living communication standard in industrial automation. For some years now it has been developed communicate via TCP and meeting security needs.
There are different Modbus implementations available, e.g. as a function block in the PLCnext Store. But using a function block, its instantiation is necessary for each Modbus connection and also for each function code call, so in a larger structure that gets confusing for developers soon. There are some other custom implementations, some programmed in high-level languages.
And now there is a Modbus TCP client directly implemented into the Automation Runtime Platform (ARP) of PLCnext Technology, and configurable via the PLCnext Engineer programming software. The implementation depends on the libmodbus open source library, provided by libmodbus.org.
PLCnext Technology does not provide a Modbus TCP server solution itself so far, but there are plans for that in future.
Even an interface for RSC services to use ModBus TCP via high-level language programming has already been integrated but needs some more testing and documentation. Stay tuned for updates...
How to configure a Modbus communication
- Import the PLCnext Engineer Modbus TCP client library:
In the Component panel, right-click on the Libraries folder and select Add Library from the context menu.
- From the PLCnext Engineer directory that opens in the Windows Explorer, pick the Modbus library.
↪ The library is copied into your PLCnext Engineer project. - To assign the Modbus TCP client feature to your device, drag-and-drop that Modbus library from the right-side COMPONENT panel onto the controller in the left-side PLANT panel.
- Click the Modbus TCP node in the PLANT panel and switch to the Modbus Device List tab in the working area.
- Assign the IP addresses of the Modbus TCP gateways or servers you want to communicate to:
The number of Modbus TCP servers depends on the PLCnext Control type running the client:PLCnext Control maximum Modbus servers AXC F 1152 8 AXC F 2152 16 AXC F 3152 16 RFC 4072S 16 BPC 9102S 16 - Click on a node for the Modbus gateway or server in the PLANT view.
- Look at the Settings tab to check and change the default timing parameters for the server connection:
Swap bytes can be activated if necessary Protocol only TCP is available by now Port always 502Connection timeout specifies how long the client waits for a response in this cycle of establishing a connection before a new connection is possible Reconnection interval specifies how long to wait for a new attempt to establish a connection Response timeout specifies how quick the Modbus server should respond to the client's Modbus telegram; otherwise an error is issued to the diagnostic register Trigger rate Cycle time of the telegrams sent to the Modbus server
- Switch to the Registers tab to set up the Modbus function codes (FC) and data types for your application, e.g.:
↪ The function codes are then connected to global variables which you can are also see listed at the bottom of the Data List tab as Process data items. - Once you have set function codes up in the above Registers tab, you can then work with these global variables in your automation programs.
Supported function codes
These are the Modbus function codes (FC) that are supported by the ModBus TCP client:
| FC | Description |
|---|---|
| 1 | Read Coils |
| 2 | Read Discrete Inputs |
| 3 | Read Multiple Registers |
| 4 | Read Inputs Registers |
| 5 | Write Coils |
| 6 | Write Single Register |
| 15 | Force Multiple Registers |
| 16 | Write Multiple Registers |
| 23 | Read/Write Registers |
Features for troubleshooting
Diagnostic registers
The diagnostics register is read out and displayed in the PLCnext Engineer diagnostic view:
Note: The last entry in the above table, CONTROL_DIAG_STATISTIC, has a special feature, in addition to resetting the diagnostic register once: Using a specific bit combination, the user can also activate or deactivate the Modbus connection. This way, specific Modbus devices can be deactivated for troubleshooting.
This feature can also be useful in operation if you want to send a message only once and don't need a cyclic Modbus communication.
Notifications
PLCnext Engineer users will also find the familiar built-in notifications which can contain plain text, such as "connection lost", "wrong Modbus address", or timeout messages.
