Like every other directserial connection. If that doesn't work for you then you can try out my nullmodem patch (either here in the patches section or on my homepage). It's directserial is improved and it can also simulate a nullmodem connection over network.

The program I'm using is a programmer. The unit in question goes into program mode but I'm unable to send and data to it, any ideas. Hal your patch did improve things slightly before your patch I couldn't even get program mode

Does the Dosbox status window say anything unusual?

Yes it says Serial2 Errors occurred framing 0 parity 0 overrun (various no's as it retries) and break 0. Hope this means something to you I've no idea

Tried to make Dosbox go faster? (cycles, dynamic core)

Anything over 8000 resets the unit I'm trying to program

Try this file
http://home.arcor.de/h-a-l-9000/test/dosbox_sertest2.zip

and set the serial port like this
serial1=directserial realport:comx rxdelay:20
maybe vary the 20...