Limitations

Because of the limitations on MODBUS protocol, I/O extension addressing is limited to 247. Maximum number of devices that can be connected to the bus simultaneously is limited to 64.

The master device initiates the communication and sends the commands to the slave devices to read or write to their registers. A slave device cannot initiate communication with the master device. The master device needs to poll the inputs on each device  in order to update the input status. The update cycle time is therefore increased with the number of devices connected to the net.

The total number of digital input/outputs per device is limited to 16, and analog input/output is limited to 8. This means that digital inputs/outputs can be extended up to 1024 and analog input/outputs can be extended up to 512 when the maximum number of slave devices are connected to the net.

Despite the fact that the use of I/O extension is designed to be similar to use of the native input/output of the RTCU and with the only difference being in the I/O configuration in the project tree, some devices need to be set a defined value instead of the value that would be used to set a native analog output. In such cases, please refer to the technical manual of the device to see which value to use to get an expected value at the output.

Because of limitations on some modules, the maximum baud rate that can be selected is set to 57,600 bps, even if the serial port natively supports baud rates up to 115200 bps.

When I/O extension is used, the configured port cannot be used as an ordinary RS-485/RS-232 port.