CEP AG
Revision 1.2
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Page 13 of 28
Data Set Ready (DSR)
An active DSR signal is sent from the modem to the application (DTE) to confirm that
a communications path has been established. DSR has two modes of operation,
settable using the AT command AT&S.
Data Carrier Detect (DCD)
DCD indicates that the DCE is receiving a valid carrier (data signal) when low. You
can define the exact behaviour of DCD with an AT command.
Ring Indicator (RI)
RI indicates that a ringing signal is being received by the DCE when low. You can
define the exact behaviour for RI with an AT command.
3 Operation
3.1 Switching ON the modem
There are two ways to switch on the modem, once power is applied.
• assert TO_IN to high level for > 1s
• activate the RS232 control line DTR
The modem is fully operational after 4 seconds. Logging onto a network may take
longer than this and is outside the control of the modem. The modem can be
configured to start up at the time power is applied by permanently tying power
connector signals TO_IN (pin 4) and VCC (pin 1) together. In this case DTR must be
used to switch the modem on again after it has been switched off or reset, while
power is still applied.
Note: DTR must be cycled from low to high.
Note: The TO_IN signal requires a positive “edge” (a sharp” signal transition from low
to high) to turn the modem on. This transition should be a rising signal from 0V
(GND) to VCC, or at least a large fraction of that voltage range, and must be applied
at the same time as VCC or after it. Very slow transitions (significantly slower than
many milliseconds) or very small transitions (e.g. only few volts instead of 0V to VCC)
will not turn on the module (since they are not considered to be a “positive edge”).
Although this will not be an issue in almost all typical applications of the modem,
please consider following points:
Large capacitors in your power supply which will lead to slow leading and
falling edges (issue does not apply with modern stabilized switching regulator
power-adaptors) AND TO_IN tied in parallel to VCC (instead of separate
dedicated digital signal)
Slow analogue signals used to assert TO_IN
TO_IN signal not before VCC
