OK I fought this battle back in 2003 with my TNC151 which is the same RS232 set up as the 155. This is a copy of what I posted on the Practical Machinist forum when I bottomed out the problem:
The Solution !!!!!!!!!!
Not wanting to be a quiter I put some more time in on this odd problem and got a result.
First a few misconceptions (by me).
A/ What I thought was "TNCRemo" is in fact "TNCServer"
B/ The TNC RS232 interface can exist in three modes - "EXT", "ME", and "FE" selected via the 'MOD' key - I had only ever had it as "EXT" and it should have been "FE"
C/File names & extensions ARE important in block mode transmission. 123.H = program 123 in Heidenhain conversational code and 123.I is program 123 in ISO-Gcode
Setting all these options correctly and using software handshaking (MP222=168) means I'm now a happy bunny loading & saving at 9600.
On the way I've remade the RS232 cables (twice!), had the controller totally to bits, blown loads of swarf out of the cabinet and even some off the TNC151 motherboard, and learned how to reload all the machine parameters into an empty controller.
Many thanks to all that offered help and particularly GISMO who sent me known good code to use as a standard.