Campbell Scientific DC112 Telephone Modem - Manual (Page 6)

DC112 MODEM
4
5. IF PROBLEMS ARISE
If any of your telephone equipment is not
operating properly, you should immediately
remove it from your telephone line, as it may
cause harm to the telephone network. If the
telephone company notes a problem, they may
temporarily discontinue service. When
practical, they will notify you in advance of this
disconnection. If advance notice is not feasible,
you will be notified as soon as possible. When
you are notified, you will be given the
opportunity to correct the problem and informed
of your right to file a complaint with the FCC.
For assistance in installation or for repair,
telephone (801) 753-2342 or write to Campbell
Scientific, Inc., P.O. Box 551, Logan, Utah
84321. To comply with FCC Rules and
Regulations, all repairs on the DC112 Modem
will be performed by Campbell Scientific, Inc. or
an authorized agent of Campbell Scientific, Inc.
6. THEORY OF OPERATION
The DC112 modem is used to transmit data over
bandwidth-limited channels such as telephone
lines by modulating audio tones, using Frequency
Shift Keying (FSK) at 300 baud, or Phase Shift
Keying (PSK) at 1200 baud.
The telephone company gives a 40 to 150
VRMS, 20 Hz signal on the phone 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 uA. The ring signal is passed on to the
datalogger through an opto-coupler. The
datalogger responds by raising the Modem
Enable line which enables the 5Vdc 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, the 5Vdc power is
removed from the modem circuitry, dropping
power to the off-hook relay and thus placing the
phone line on-hook.
To reject noise common to both phone lines
and to satisfy registration requirements, the
modem circuitry is electrically isolated from the
phone lines by using an opto-isolator and
coupling transformer.
6.1 JUMPER CONFIGURATION
The DC112 has 5 user-changeable jumpers;
one at location C8 and four at location H15 as
shown in Figure 4. As shipped, only one jumper
is in place, H15 1 to 8. The following table lists
the jumper meanings. After removing a jumper,
store it on a single pin of the terminal.
FIGURE 4. Jumper Locations