I'm most concerned for you with regard to your use of terms in this post!
I most sincerely hope it's a typing error!!!!
downloading infers that you have the program on your pc and wish to DOWNLOAD TO THE TILL
if this is the case, you will have overwritten what is in the till
you then go on to say that you want to store the program for uploading later
you in fact UPLOAD TO THE PC
hopefully the fact that it failed has saved you many hours of reprogramming
the best of luck in your next attempt to UPLOAD the data to your pc.
have you programmed the port on the till to "polling"?
have you set the port on the pc to be com1?
if you get stuck pls post again - but while you are practicing, may i suggest just sending a minor file - such as logo, rather than the entire program
good luck and happy new year
Andy