If you get nulls in front of the .HEX file on the receiving machine, use
[H] as a final parameter on the receiver command line. LOAD and UNLOAD only
work on the same DRIVE as the .COM or .HEX, so this sets an effective limit to
the size of the file you can handle – roughly 25% of your drive capacity, as
the .HEX file is 2.81 times the size of the original .COM file. UNLOAD only
works on .COM files, so if your file is of another type, then REN it to a .COM
type, remembering to change from a .COM type at the receiver.
What happens during transmission is that the transmitter sends a
character, then waits for the receiver to transmit the character back again.
The receiver gets a character and retransmits it immediately. If either
machine is busy, then the acknowledgement does not occur, but the unread
transmitted character is held in the UART buffer until read.
I hope that this very simple method of transmission is as much use to
readers as it has already been to me – 400k transmitted, with no errors!
LISTING 1 ---- UNLOAD.HEX