Campbell Scientific VS1 Telephone Modem with Voice Synthesizer - Manual (Page 32)

E-1
APPENDIX E. THEORY OF OPERATION
The VS1 Voice Synthesizer 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
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 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 5 VDC 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.
This manual does not attempt to be a primer on
the "AT" command set. The commands are
therefore only summarized below. For most
applications, the modem commands are issued
automatically by the PC208 software. The
commands apply only when the modem is used
as an originate modem, which is not the primary
use of this modem. Except as noted, all
commands begin with "AT" and end with
carriage return. As an example, the command
ATDT5551212 followed by carriage return will
tone dial the number that follows the ATDT,
then go on-line and wait up to 30 seconds for
the remote modem carrier. If the carrier is
detected, the result code 1 will be returned,
otherwise the result code 3 will be returned, and
the modem will return to the command mode.
COMMANDS:
AT Prefix to all commands (except /A)
A/ Repeat last command (not followed by
carriage return)
D Dial
P Pulse
T Tone
0..9 Telephone number digits
W Wait for another dial tone
; Return to command state after dialing
, Pause
Z Reset to default settings
A Answer without ring
H Hang up
O Go to on-line state
COMMANDS TO CHANGE DEFAULT MODES:
R Call an originate modem
B CCITT mode
B1 BELL 212A mode
C Carrier off
C1 Carrier on
E Commands not echoed
E1 Commands echoed
F Half duplex
F1 Full duplex
Q Return result codes
Q1 Disable result codes
Sr? What is the value in register r
Sr=n Set register r to value n
V Digit result codes
V1 Word result codes
RESULT CODES:
0 OK
1 Connect
2 Ring
3 No Carrier
4 Error
REGISTERS:
S0 Rings before auto-answer (0..255)
S6 Wait for dial tone (2..255 seconds)
S7 Wait for remote carrier (1..255 seconds)
S8 Comma pause time (0..255 seconds)
S10 Delay time between loss of carrier and
hang up (1..255 in 10th of seconds)