Campbell Scientific COM200 Telephone Modem - Manual (Page 21)

C-1
Appendix C. Theory of Operation
C.1 Theory of Operation
The COM200 modem is used to transmit data over bandwidth-limited channels
such as telephone lines by modulating audio tones, using Phase Shift Keying
(PSK) at 9600 or 1200 baud and Frequency Shift Keying (FSK) at 300 baud.
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 2 seconds and off for
4 seconds. The ring detection circuitry is continuously powered but draws less
than 2
µ
A. The ring signal is passed on to the datalogger through an opto-
coupler. The datalogger responds by raising the Modem Enable line which
internally switches the 12 VDC power to the modem. 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, it internally removes the 12 VDC power from
the modem circuitry, dropping power to the off-hook relay and thus placing the
telephone line on-hook.
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 coupling transformer.