Modifying Motorola RSS and CPS Out of Band

Modifying Programming Software - Benefits and Pitfalls

People often hex-edit their programming software. The two major reasons are to allow entry of amateur frequencies and to increase the number of available channels/modes. For example, GTX software only allow 10 conventional modes, but the hardware is capable of at least 15.

The modifications often take place in a Model Definition File, a file with the MDF extension. These files are usually small and version-dependent, so you can not take an MDF file from R01 software and expect it to work with R04 software. In addition, the program reads the MDF file and adds all the byte values together, a process called "generating a checksum". It then compares that total with a known value stored in the program; if the two don't match, the software will complain and usually not let you proceed any further. This revelation often doesn't happen until after you've read the radio's code plug, and the program tries to find the model number in the MDF file but won't due to the checksum mis-match.

Some changes may have to be made in other files. For example, SPECTRA1.EXE is where the frequency limitations are maintained. This data is also controlled by a checksum, so once again it's important that if you alter bytes in one area, you make corresponding changes in another, less important area. Text messages are prime candidates for fixing checksums.