Thanks all,

Bug 4165 ( http://chirp.danplanet.com/issues/4165 ) sounds a lot like my problem and explains all the behaviour I have observed. At some point I will try prodding the DTR line and see if that is what puts the radio into transmit which would pretty much confirm it. Only thing is that I didn't think the DTR line was connected in my cable, though it could still be an issue of default states of an uninitialised port (with this USB-serial device and driver anyway). What is interesting is that the PTT /RXD line is grounded for PTT so sending a logic-zero is inherently equivalent to pressing PTT, I guess it's just normally of too short duration for the radio to respond to it. That would mean that if the PC-TXD-->Radio-RXD line defaults to low when the port isn't open (as opposed to high OR tristate) then it is exactly equivalent to holding PTT. Or have I missed something here?

It's useful to know that RTSYSTEMS and Wouxon cables behave. I have the guts of the first one I bought, I could use that to make a new cable up. Has anyone used the version which level-shifts RS232 levels with two transistors rather than using a TTL-level USB-serial board? This kind of thing: http://www.hamradio.in/circuits/images/baofeng/programmer.jpg

Cheers
Richard