An extra FF added to the outgoing TCP data stream for each incoming FF on the RS232 input

[terry.Haebich]

I have a device attached to an RS232 port of 2901 ISR. The 2901 is utilised to pass the incoming RS232 data stream from the device to the socket of an application running on remote server on the network.

The issue I have is that an extra FF byte is added to the outgoing TCP data stream for each incoming FF byte on the RS232 port. As the incoming RS232 data stream includes a checksum of the total bytes for that particular data stream, when the application compares this checksum with the one it creates for the number of bytes it received, a checksum error results.

The attachment shows an example of what is occuring

Why is the 2901 adding an extra FF byte to it's outgoing data stream for every incoming FF byte on the RS232 data stream?

How can this be disabled?

[terry.Haebich]

When the Cisco 2901 is configured for Natting, the traffic on the RS232 port utilises the Network Virtual Terminal (NVT) functionality of the Telnet Protocol Interpret As Command (IAC) [as per  RFC 854]  i.e. an extra FF byte is added to the TCP/IP message for every incoming  FF data byte to distinguish it from a command byte.

When the Cisco 2901 is configured for Rotary,  the RS232 port does NOT utilise the Network Virtual Terminal (NVT) functionality of the Telnet Protocol i.e. the IAC feature is not implemented and hence, command and data bytes on the serial port pass through "unchanged"