You need RPM to program it and here is the cable:
Programming Cable for GE/Ericsson Jaguar 700P and M/A-Com / Harris P7100 / P7200 / P5100 / P5200 series Portable RadiosConnects directly to PC serial port, no special programmer/adapter required! High-quality, high-performance replacement for the GE/Ericsson TQ3392 (RPM1132472/23) cable and...
rfguys.com
You'll also need the RS232-USB interface and need to install the driver for it.
To program the P7100 connect the programming cable to the interface and plug in the USB. Connect the other end of cable to the radio UDC and make sure it's held to the radio securely.
Open RPM on your computer. Power on your radio THIS WAY: Press and hold down the 3 side buttons as you turn it on.
The screen should be blocked out. That's program mode. Now verify COM settings by going to "device manager" and double click Ports (COM & LPT). It will give a number for the ATEN USB to Serial Bridge, for example COM2).
Then in RPM under Tools make sure the Comm settings is set to the same number, in this case COM2.
You should now be able to read the radio.
Save the original personality in case something goes wrong and you need to go back to it.
Write down the radios Feature Data string before you start too. If you can't view it on display you'll need to open RM which comes with RPM.
If you have ProGrammer you can back up the Radio Code and DSP code and save to file.
This works for the 7100 even if it was programmed by RPM and not ProGrammer.
You'll need to find a dealer to get RPM.