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I want to send a signal through the serial port to restart a modem, enfora gsm1218 (see datasheet http://www.testech-elect.com/enfora/pdf/GSM1218HR.pdf) is it possible?

I know that an AT command like AT$RESET exists to reset the software configuration, but I would like to hardware reset the modem itself as opposed to a software reset.

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I have implemented this kind of thing using the modem control signals. You can wire DTR or RTS to a circuit that drives the RESET line of the system. Naturally you have to make sure your serial communications software isn't going to use those signals, or rather is prevented from controlling those signals until you want the desired reset to occur.

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  • \$\begingroup\$ That requires a reconnection of some cables,right? I'm trying to do it via a "software" signal without modify the device. is it possible? \$\endgroup\$ – Rafael Machado Aug 26 '13 at 20:59
  • \$\begingroup\$ It depends; do your cables have all the signals present? Are the signals doing anything on the modem side? you would have to add a diode (at a minimum, depending on the modem) to wire the signal to a reset line, but without specifics in the question this is about as good an answer as I can give. If you don't want to wire anything and have no control over the software on the modem there is little you can do. \$\endgroup\$ – akohlsmith Aug 26 '13 at 23:31
  • \$\begingroup\$ I haven´t enough information about the modem's internal parts and connections. Also, i can´t modify any of the hardware. I reviewed a lot of manuals, guides and datasheets and did not found a specific signal or use of some pin that be useful. Thanks anyway. \$\endgroup\$ – Rafael Machado Aug 27 '13 at 16:11
  • \$\begingroup\$ Why not use a second serial port to do the 'out of band' signaling. If you connect the TX line from the second serial port to some kind of debounce circuit, a simple echo 'switch' > com2 should produce enough transitions to trigger the denounce circuit and control the reset line on the modem. \$\endgroup\$ – Andyz Smith Aug 28 '13 at 17:24
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If the command set does not specify such a command: No; AT$RESET is a normal command like any other.

In most cases you will tie the hardware reset or some similar pin to a GPIO of your controller and use this to fully reset the device.

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You need to get the full modem documentation, and look for whether it's possible to configure the modem to reset whenever you deassert the DTR (data terminal ready) signal. Your host's serial driver should include an API for controlling that signal.

If supported, the modem will essentially go offline unless you're asserting DTR. When you want to reset, just deassert DTR longer than minimum time required by the modem, and then reassert it.

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  • \$\begingroup\$ I haven´t enough information about the modem's internal parts and connections. I reviewed a lot of manuals, guides and datasheets and did not found a specific signal or use of some pin that be useful. Thanks anyway \$\endgroup\$ – Rafael Machado Aug 27 '13 at 16:13

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