Appendix C. Theory of Operation
C.1 Theory of Operation
The COM220 modem is used to transmit data over bandwidth-limited channels such as telephone lines by modulating audio tones. The COM220 uses various modulation schemes including FSK (Frequency Shift Keying), TCM (Trellis Coded Modulation), QAM (Quadrature Amplitude Modulation), and DPSK (Differential Phase Shift Keying).
The telephone company gives a 40 to 150 VRMS, 20 Hz signal on the telephone lines to signify a ring, which is typically on for two seconds and off for four seconds. The COM220 is supplied with 12 V from the datalogger’s CS I/O connector or from the COM220’s external power connector. The 12 V is then regulated to +5 V to give power to the ring detect circuitry. The ring detection circuitry is continuously powered but draws less than 2 ∝A. The ring signal is passed on to the datalogger through an opto-coupler. The datalogger responds to a “ring” by raising the Modem Enable line, which internally switches the regulated +5 V supply to the rest of the modem’s circuitry.
The modem then answers and remains off-hook until it loses the carrier or the datalogger lowers the Modem Enable line. The datalogger lowers the Modem Enable line by remote command or after 40 seconds in the absence of a command. When the Modem Enable line goes low, the COM220 internally removes the +5 V from the modem circuitry, dropping power to the off-hook relay and thus placing the telephone line on-hook. The COM220’s ring detect circuitry is still powered at this time.
To reject noise common to both telephone lines and to satisfy registration requirements, the modem circuitry is electrically isolated from the telephone lines by using an opto-isolator and a coupling transformer.
