The T28H-Relay does not support MODBUS so you can't use it with the Wonderware driver. There is not much we can do about it except to ask you to switch to a T100MD+ PLC.
For M-series PLC such as T100MD+ or T100MX+ you can change the PLC baud rate to other values such as 300,600, 1200,2400,4800,9600,19200, 38400. You can even change the communication format to 7 or 8 data bits, 1 or 2 stop bits and odd, even or no parity. It is very flexible. The PLC can also be used as MODBUS master as it can send out MODBUS ASCII or RTU message to other MODBUS slave.