I have had a Commodore PET for the last seven years. Recently I purchased an IBM PC. Is there any way I can use the PET programs on the PC without having to buy two modems?
Calvin E. Phillips
Even the purchase of two modems won't help, unless you plan to use one of the computers as a remote terminal while you run the programs on the other.
There are too many differences between the PET and IBM PC to expect the programs to be interchangeable. Of course, if the program in question is written in very plain BASIC, it is possible that it will run on either machine without modification.
As you mention, one popular technique for exchanging programs between two different brands of computers is to use a pair of modems to upload a program from one computer to the other, then modify each line as necessary to produce a working version. Of course, this only works for BASIC programs or other programs that you can easily edit, which excludes most commercial software.