Author Topic: Connecting a pair of Dialup Modems  (Read 1026 times)

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Online @rt

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Re: Connecting a pair of Dialup Modems
« Reply #25 on: July 14, 2018, 07:48:29 pm »
That links comes up “Forbidden” for me.
I have been told the 48V is present when the line is idle,
and drops to 8-12V for the voice circuit. Is this correct?
It does seem to explain why other diagrams use a single 9V battery.
 

Online 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #26 on: July 14, 2018, 07:56:54 pm »
That links comes up “Forbidden” for me.
I have been told the 48V is present when the line is idle,
and drops to 8-12V for the voice circuit. Is this correct?
It does seem to explain why other diagrams use a single 9V battery.

Sorry it is broken to me now too.. I uploaded image up there.
 

Online 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #27 on: July 14, 2018, 08:08:42 pm »
Originally it was like this:

1. 48V power
2. 48V high sensitivity relay. It serves as a current source, an AC block (high inductance), and also as an Off-hook detector
3. Voltage was 48-60V open 10-12V with phone off hook. Current draw was 10-20mA.

Good results can be had with 24V (many small inexpensive Panasonic in house switches had 24 V, better ones had 48V).
Also 12V/220Ohm combination should work quite well, it is lower impedance that in should be for the voice signal, but that is not a problem when locally connected.
You can create dial tone with arduino pin, lowpass filter it and attenuate to get 100mV ish level, couple it with a capacitor to the line.

It can be done, but it deceptively complicated to do it right..
 

Offline pamperchu

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Re: Connecting a pair of Dialup Modems
« Reply #28 on: July 15, 2018, 05:04:51 am »
Set Up A Dial-In Server ,

h++ps://www.howtoforge.com/linux_dialin_server
 

Online Berni

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Re: Connecting a pair of Dialup Modems
« Reply #29 on: July 15, 2018, 05:42:32 am »
Oh and there is also the easy way out of simply buying one of those household telephone exchanges that connects 2 to 10 phones to one POTS line. Connect all your modems to it as phones and simply dial the internal number of the other one. You get the dial tone, ring signal and perhaps even a caller id signal to both of the modems.

By the way 2N3055 I'm in your country on holiday. Good luck with the big soccer match.
 
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Online 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #30 on: July 15, 2018, 08:01:32 am »
Oh and there is also the easy way out of simply buying one of those household telephone exchanges that connects 2 to 10 phones to one POTS line. Connect all your modems to it as phones and simply dial the internal number of the other one. You get the dial tone, ring signal and perhaps even a caller id signal to both of the modems.

By the way 2N3055 I'm in your country on holiday. Good luck with the big soccer match.

Thanks a lot! I hope you are having a good time, enjoy the holidays... Lijep pozdrav!!
 

Offline kakabouras

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Re: Connecting a pair of Dialup Modems
« Reply #31 on: July 15, 2018, 08:22:36 am »
Back in 1989 I was assigned the task to make two dial-up modems to talk to each other over a leased line.

After a few experiments the following solution was implemented :

- I used one modem as a call initiator by using a command like ATX0DT5555555.

- X0 means do not detect dial or busy tone.

- DT5555555 means send DTMF tones corresponding to number five ie 1336Hz and 770Hz.

- At the other end I implemented a dual tone detector using two LM567 with their outputs tied in a logic AND configuration. I could have used a special DTMF detection IC like  CMD CM8870CSI  but those were not easily available in Greece then.

- When the DTMF signal was detected a hardware interrupt signal was used by the receiving end computer to send an ATA command to the receiving end modem.

It worked like charm for this telemetry project for many years until it was replaced by internet DSL lines in 2007.


When I described the idea to my boss he said  : "That's too simple to work".
Afterwards when we delivered the project he called the whole idea "the Columbus'  egg" .
« Last Edit: July 15, 2018, 08:30:39 am by kakabouras »
 

Online 2N3055

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Re: Connecting a pair of Dialup Modems
« Reply #32 on: July 15, 2018, 08:32:11 am »
You don't have to detect dialed number, just play dial-tone after off hook, insert few second pause, ring other side, and after it goes off hook connect them..
Or if you put ignore dial-tone in dial string for modem, no need for dial-tone at all.
 

Online @rt

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Re: Connecting a pair of Dialup Modems
« Reply #33 on: July 15, 2018, 02:40:07 pm »
Those telephone line simulators are neat,
but still surprisingly expensive on eBay for technology on the very edge of obsolescence.
 

Online CJay

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Re: Connecting a pair of Dialup Modems
« Reply #34 on: July 16, 2018, 07:45:26 pm »
Those telephone line simulators are neat,
but still surprisingly expensive on eBay for technology on the very edge of obsolescence.

Because that stuff is on the verge of being obsolete it's possible to buy office PABX boxes for cheap, they'd do just fine but they don't tend to be particularly small.
M0UAW
 

Offline Peabody

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Re: Connecting a pair of Dialup Modems
« Reply #35 on: July 17, 2018, 12:12:44 am »
This is going back a long way, but my memory is that the DAA, which is the part of the modem circuit that connects to the phone line, usually runs the lines through a FWBR because often Tip and Ring are wired backwards.  The FWBR makes the modem work with either polarity.  But with a direct connection with no voltage on the line, nothing gets through the two diode drops of the FWBR.  So you need to put some DC voltage on the line so the audio will get through.  But I'm confused that this was described earlier as putting the 9V battery "in series".  The battery would be *across* tip and ring.  Right?

And I also thought there were AT commands to get around the absence of dial tone and ringing.
 

Online @rt

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Re: Connecting a pair of Dialup Modems
« Reply #36 on: August 08, 2018, 08:08:33 pm »
The ring signal goes across the ring and tip, which puts the phone’s ringer solenoid in series I presume.
The circuit for speech/data is a series circuit (in series with either the ring or tip).

Just in case this thread is searched, the current arrangement worked well until I tried a PCMCIA card modem.
To get this working I use a pair of 12V SLA batteries in series with a 510 Ohm resistor.

Funny enough though, this is also a Fax modem, and when I tried it with Telstra’s fax test number,
it couldn’t interpret the fax to return it. The report was as if my modem was silent after connecting.
I had to go to a friend’s house for this, as he still has a POT line which is getting pretty rare in Australia, as they are being phased out.
 

Online NiHaoMike

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Re: Connecting a pair of Dialup Modems
« Reply #37 on: August 09, 2018, 12:55:46 pm »
I had a 28.8k building to building ppp link running for several years on some Hayes Optima modems over one spare pair.
Nowadays, it's pretty easy to hack some Homeplug adapters and send data at a few hundred Mbps through a few hundred feet of telephone wire, basically a cheap way to get a pair of really fast "short haul modems". Isn't technology amazing? :)
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 


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