There are 2 different approaches to using COM ports with a Centro depending on whether you have a Centro-8 or a Centro-C.
Centro-8:
Use the designer software to set the COM port baud rate / parity etc by selecting the Centro-8 device & going to 'Configure Ports'. Then within the Centro-8 pre-defined command module, there are separate command folders for each com port - simply place the appropriate command in those folders. Each COM port has its own command suffix, which is configured along with the baud rate. The command suffix will be added to every command.
Centro-C:
Use the Centro-C test tool to configure the port baud rate / parity etc. Next to the settings you will notice an IP Port setting, this will be 7501 for COM1, 7502 for COM2, etc. In the designer software, create a separate generic device for each COM port, with the appropriate port. Any data sent to that device will be sent out of the associated COM port (which will include the command suffix for that device)