Comms via Port-3 is giving error despite the configuration is identical to Port-1

The Portserver model is TS 4 MEI.

The serial communications used are ASE4K protocol (propietry of Aker Solutions serial comms to Subsea devices) which similar to Modbus protocol in term off poll/reply. The serial configuration is 9600 baud rate, no parity with stop bit set to 1.

The message transmitted during normal operations are fine. During normal operations, the transmit message is below the maximum message limit which is 256 bytes.
But when file download operations started which required max of 256 bytes to be utilised, the communications just giving an error once in a while. The comms was just perfect on Port-1.

Is there any difference between the Port-1 and Port-3?

The dispswitch setting and cables used are same.

There is no difference between the two ports on these units as 4 ports are controlled by one UART.

Are the configurations identical (on the PortServer and/or RealPort host)?

When the error occurs, you might want to issue the following commands from the root prompt of the PortServer to see if any clues can be found:

#> who
#> display ports
#> info serial:3

Did you mean RealPort host configuration is the Comm Port configured from the Computer Device Manager?