This is the Modem Cartridge sample program from the PX-4/HX-40 
Operating System Reference Manual, II-549 for the Modem 
Cartridge. It compiles and runs, but I have no Modem to test 
it with. I am not sure it will, as my DMM cartridge does not 
return the expected Device code. The Modem could have the same
problem. 

The program is assembled and linked with Microsoft M80 and L80.
Below are the commands used.

fjkraan,20100416


m80:
=MODEM/L	- compile MODEM.MAC with object file MODEM.REL and listing MODEM.PRN

l80:
/P:100		- set program starting point for CP/M programs
MODEM		- load MODEM.REL
MODEM/N/E	- output MODEM.COM