I found a few interesting things by opening up my Radioshack 22-812 DMM:
1) The DMM has a small RS232 circuitboard (daughterboard) seperate from the main circuitboard
2) The daughterboard is powered by the PC's serial port (assuming it can provide enough power)
3) The mainboard transmits to the daughterboard via an LED (optically isolated)
4) The DMM is transmit-only.
With these facts...
