Thanks Dan for the information on Byte 3. I had figured from the code I have that it was something to do with that, but now I actually understand :-)
My progress so far:
I have an interface between my HP41 and my PC (my PC pretends to be the IR printer)
I can send ascii or binary info to the PC
I am currently preparing the barcode on the HP41 and sending it (as a byte stream) to the PC
The PC can successfully print usable barcode
BUT (damn!) For program barcode I'm getting checksum errors. I have not been really confident of the code for creating these. If anyone can tell me how it's done I'll be almost finished.
(well, after that is the direct PC to HP41 interface via the wand (but without printed barcode...)
Is anything ever finished? The HP41 is very slow at preparing the barcode. Eventually I'll just send the binary code for the program to the PC and let it do all of the barcode creation.