Campbell Scientific COM310 Voice Communications Modem - Manual (Page 35)

Appendix A. Theory of Operation
The COM310 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 COM310 has four connectors:
1. RJ11 – telephone line
2. Orange Tip, Ring, GND – alternate telephone line and EARTH GROUND
3. CS I/O – datalogger serial comms and 12 VDC power if more recent
wiring panel (see Section 6.2)
4. Green Power – 12 VDC (alternate in case wiring panel doesn’t furnish
12 VDC power)
The telephone company places a 40 to 150 V
RMS
20 Hz signal on the telephone
line to signify a ring, which is typically on for 2 seconds and off for 4 seconds.
The COM310’s ring detection circuit is continuously powered but draws less
than 2
μA. The COM310 passes a detected ring signal on to the datalogger
through an opto-coupler. The datalogger responds by addressing the COM310
synchronously (pins 6 and 7) which switches on 5 VDC power to the
COM310. The COM310 then negotiates a connection with the calling modem
and remains ‘off-hook’ until it loses the carrier or until the datalogger sends it
a shutdown command. The datalogger sends the COM310 a shutdown
command upon receipt of an E (end call) command, after 40 seconds without
receiving any command or if the VoiceSetup SecsOnLine setting times out.
The shutdown command switches off internal 5 VDC power to the COM310,
dropping power to the off-hook relay and ‘hanging up.’
To reject noise common to both telephone lines and to satisfy registration
requirements, the modem circuits are electrically isolated from the telephone
lines by using an opto-isolator and coupling transformer.
A-1