# Pi to modem adapter

**URL:** <https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397>\
**Category:** Raspberry Pi\
**Created:** [22 January 2019 14:29 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397 "2019-01-22T14:29:28Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Dale78162](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/dale78162/32/2217_2.png) [@Dale78162](https://forum.core-electronics.com.au/u/Dale78162)\
**Post date:** [22 January 2019 14:29 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397/1 "2019-01-22T14:29:28Z")

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I am looking to utilize my Pi to ‘screen’ incoming calls. My hope is to use the ‘Ring Indicator’ to trigger an interrupt. So far I’ve been unable to find a TTL to RS232 that provides access to the DB9 pin 9 that I need.  
Could anyone recommend a device?

Many Thanks,  
Dale

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**Author:** ![Stephen](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/stephen/32/1599_2.png) [@Stephen](https://forum.core-electronics.com.au/u/Stephen)\
**Post date:** [25 January 2019 00:13 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397/2 "2019-01-25T00:13:08Z")

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Hi Dale,

I’m not aware of any device like that. It seems that most if not all TTL to RS232 converters just convert the RX and TX lines used for serial communication. You could get crafty and make your own circuit, or rewire a converter so your pin 9 goes through the RX or TX connection line. It really sounds like you might be moving into custom PCB territory!

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**Author:** ![Dale78162](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/dale78162/32/2217_2.png) [@Dale78162](https://forum.core-electronics.com.au/u/Dale78162)\
**Post date:** [25 January 2019 15:39 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397/3 "2019-01-25T15:39:43Z")

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Hey Stephen,  
Thanks for responding. I considered soldering a jumper from pin 9 but my limited electronics knowledge warns me against the voltages involved. Could you tell me how to basically do what the ic on the adapter does in limiting the input voltage to my Pi?

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**Author:** ![Clinton](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/clinton/32/1515_2.png) [@Clinton](https://forum.core-electronics.com.au/u/Clinton)\
**Post date:** [26 January 2019 23:05 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397/4 "2019-01-26T23:05:57Z")

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Hi Dale,

If you are just wanting to sense when the ring signal is active you could use a voltage divider to step it down. You can have a look at our [tutorial on resistors](https://core-electronics.com.au/tutorials/analogue-electronics/how-to-choose-the-right-resistor.html) to see how. I’d probably pick values in the k ohm range because you need almost no current for sensing.

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**Author:** ![Dale78162](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/dale78162/32/2217_2.png) [@Dale78162](https://forum.core-electronics.com.au/u/Dale78162)\
**Post date:** [27 January 2019 13:32 UTC](https://forum.core-electronics.com.au/t/pi-to-modem-adapter/4397/5 "2019-01-27T13:32:28Z")

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What’s up Clinton,  
Thanks for the suggestion. I will definitely look into that. I seem to vaguely recall something about voltage dividers way back in school.