X10 programming from PC

["Geordie" <bill-gibson_remove_@xxxxxxxxxxxxx>]

Hi,

I'm trying to control an X10 system through a CM12U (similar to CM11) via
the serial port, using my own software.

I know the software is correctly talking to the serial port (because I tried
it with a modem), but I'm having difficulty communicating with the CM12.
What I'm sending is 0x04 (Header) then 0x5E (to address unit G2) but I can't
get a Checksum back from the CM12.

Any thoughts?  One thing I noticed in the "Interface Communication Protocol"
document at the end of section 3.1 is "This format is typical of all
transmissions between the PC and the interface with the difference being in
the first transmission from the PC." - but I couldn't find anything about
__what__ is different about the first transmission.

TIA,
Geordie