Maximum Simultaneous Requests settings

When using ARTI3 communication (UDP), if you configure the Maximum and Maximum per station fields under the Simultaneous Requests section with a value greater than 4 for each field, the CODESYS Communication Driver cannot connect to more than 4 devices.

The real maximum simultaneous requests settings cannot exceed 4 since there is a limitation in the CODESYS protocol library PLCHandler.

Each device attempts to create 1 UDP port. If the number of UDP ports created from all devices is greater than the driver's maximum simultaneous request, then it stops creating new UDP ports.

Regardless of how many devices there are, when the maximum simultaneous requests are less than or equal to 4, the driver distributes and shares those ports to all connected devices. Each device has multiple connections to the PLC (based on the maximum simultaneous requests settings), and also depends on the device's unused available channels/connection limit.

If the maximum simultaneous requests are greater than 4, the attempts of the device to open more ports fail, and the Communication Driver logic assigns an exclusive port per device. This is the reason why the driver cannot connect to more than 4 devices as all ports are being held exclusively per device if the maximum simultaneous requests are set to 4.

In conclusion, the CODESYS Communication Driver has the following limitations in functionality:

• Port sharing: The driver opens a new port for each device upto 4 UDP ports and shares them as long as maximum request setting is lesser than 4.

• Exclusive port assignment to each device: The driver does not share the ports assigned to each device with any other device once the maximum number of UDP ports are opened.

To avoid exclusive port assignment to each device, maximum simultaneous requests and maximum simultaneous requests per station should be limited to 4 each under 1 channel.