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You can attach a USB modem to a SBC and go from there, that's probably the simplest way to approach it. The more complex way is to have an interface/isolation
by Kliment 11y ago
You can attach a USB modem to a SBC and go from there, that's probably the simplest way to approach it.
The more complex way is to have an interface/isolation circuit using two GPIO pins (one input for ring detection, one output that switches a relay into the rest of the circuit and also takes the line off the hook). You then have a single DAC or PWM output that plays the tone into the isolation transformer. If you are interested in more info on this sort of thing, contact me.
- chx 11y agoThat's where the "ridiculously expensive" part kicks in: the only modems that mention Linux support are 40+ USD. (Zoom 3095 etc)
- Kliment 11y agoThere are cheaper options, for example the Trendnet TFM-561U which claims linux support
- tyingq 11y agoThese $9/shipped generic USB modems worked on Linux for me: http://www.aliexpress.com/item/Adroit-New-USB-56K-V-90-V-92-External-Dial-Up-Voice-Fax-Data-Modem-for/32604327115.html http://www.aliexpress.com/item/Adroit-New-USB-56K-V-90-V-92-... Not great quality, but it sounds like you just want to go off hook and send a single DTMF tone. (ATH1, then ATD6). It should work fine.
- chx 11y agoThanks! I will need to figure out how to configure mgetty to do an ATDT6 on dial.
- tyingq 11y agoSure. Looking at mgetty's man page, it looks like you would want to a) Disable Auto Answer on the modem, manually: ATS0=0 b) Set the "answer-chat" setting in mgetty to send the ATH1 then ATDT6 I imagine you'll also have tons of errors, since no actual modem-to-modem data connection will ever get established in this scenario. So maybe some options to suppress all that.