hi
Your earlier posting about Modbus spec mentioned 10 ms scan time. A scan time of 10 ms would be dificult to achieve if data from I/O cards is collected on modbus protocol by the PLC CPU.
It also makes more sense to acquire data from I/O cards parallely than serially because of the high requirement on scan time.
I ve worked on ABB PLC and have noticed a 8-bit data bus for data acquisition from I/O cards.

I think the same Modbus serial port may be used for modem communication too. Modbus communication can be achieved over Modem link too.

Regards

Kunal