Stop Ring Voltage from reaching device (Phone, Modem, etc...)

@lon-C1oYAr • Oct 22, 2024

Oct 22, 2024

1.0K

I am sharing a POTS line with a MFD device and an outgoing-only fax modem. The fax modem is on an unsupported RISC box with unsupported proprietary fax & ERP software. We need the ability to occasionally send faxes from the unsupported (legacy) system to get old invoices, purchase orders, etc. The problem is that the fax modem answers on the first ring. The MFD does not have get a chance to pickup the line. We have no need for the fax modem to answer any incoming calls. I have no idea what the required modem string is to properly set this fax modem for the proprietary software. I have set the register zero to zero (ATS0=0) but then the fax modem stops working entirely. I know the proper "fix" is to send the entire correct AT command string including the ATS0=0 but, because we no longer have support on the software, the software company will not provide any help. I'm hoping there is a down and dirty analog circuitry work-around but don't have the skills to design it myself. Back in the day, I may have been able to but I have forgotten most of what I knew about analog circuits... So, finally, the question; Is there a simple way to stop the ring voltage from reaching the answering device (fax modem in this case). I'm thinking some sort of a diode across tip and ring? Maybe a resistor or load to drop some voltage? I don't have a test bench here to experiment with solutions so hoping one of you real engineers will provide a quick answer!
Many thanks - Sorry for the long-winded post! Lon

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Harshad Italiya

@harshad-ukH5ww • Aug 15, 2012

#-Link-Snipped-# can you tell me what the Voltage you are getting before clamping? And you can Use Series of Diodes or Voltage divider circuit to Clamp Voltage level.

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Lon McClure

@lon-C1oYAr • Aug 16, 2012

Hi Godfather! Thanks for replying to my post.

On-hook voltage is -51.4 VDC. Wikipedia says negative 48VDC is considered standard.

Also according to Wikipedia, "To ring the phone to alert to an incoming call, about 90 volts of 20 Hz AC current is superimposed over the DC voltage already present on the idle line".

The AC RINGING voltage I am seeing is about 45VAC.

Off-hook voltage is -5.7VDC.

I think if I can stop the high AC voltage from reaching the answering device, that would prevent it from seeing the Ring signal and thus not attempt to answer the call. Low-voltage AC would have to be allowed to pass so that the desired fax tones could be transmitted & received. An AC voltage clamp of some sort is probably the only way to do it. If I could prevent AC above 10-15VAC or so, I think that would kill the ring signal and still allow communications.

Thanks for any help!
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Harshad Italiya

@harshad-ukH5ww • Aug 16, 2012

Oh There is big mistake by me I suggest you to use Diode but it works only for positive Voltage drop. But its not proper way as Telephone signal is negative.
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Lon McClure

@lon-C1oYAr • Aug 17, 2012

I just wish I still had my old analog bread-boarding setup and cabinet full of discreet devices :-( I'd have this solved in no time! Thanks for the input, Godfather.
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Harshad Italiya

@harshad-ukH5ww • Aug 17, 2012

lonmcclure
I just wish I still had my old analog bread-boarding setup and cabinet full of discreet devices :-( I'd have this solved in no time! Thanks for the input, Godfather.
Please update this thread once you start working. Would like to learn.
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