Thanks Rich. I'm not sending an EOF as the data I'm sending isn't a file per se, the protocol is defined such that the requests are bound by 0x02 [Start] and 0x03 [End].

It's not as if the batch file is getting stuck writing to COM, it does complete with a "1 file(s) copied" echo, but the output file is zero length.

Thanks for the pointer on the /B flag, I didn't know about that. I will see where that takes me, cheers.

Alex