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Author Topic: Modem 56k to Arduino R3, serial data  (Read 3550 times)
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Hi, i have a question about modem 56k.Please dont tell me..google it.. i made it about 32159127579 times and didnt find anwser to my question. :p.

so, if i use an old 56k modem with rs232, do i only have to tell the modem to call a number (with AT command), and send my data on the tx pin of the rs232 and get them on the other modem on the Rx pin? Because everyone says oh its easy. but i just want to know how. yeah i understand how the modem work but will it work if i made it. :p

is this easy as i think? or i cannot do that?

arduino -> serial data -> tx pin of the modem -> modem modulate data in frequency -> phone line -> (other modem get the data) modem demodulate the signal - > rx pin of the modem -> second arduino.

thanks a lot.
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I think, you would need max232 chip to convert arduino TTL rs232 to regular two polar +- 15V rs232.
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ok thanks i read about it, but do the output signal on the second modem is the same as the input of the modem who transmit the data?
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Quote
arduino -> serial data -> tx pin of the modem -> modem modulate data in frequency -> phone line -> (other modem get the data) modem demodulate the signal - > rx pin of the modem -> second arduino.
I'd correct a little bit
arduino Tx -> serial data -> Rx pin of the modem -> modem modulate data in frequency -> phone line -> PSTN swirch -> phone line -> (other modem get the data) modem demodulate the signal - > Tx pin of the modem -> Rx second arduino.

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output signal on the second modem is the same as the input of the modem who transmit the data
Not quite. First, AT command "dial the number" of course would stay at the transmitter side, its not goes to line after PSTN to second modem. Second, to transmit a data, there should be some kind of protocol, X, Y, Z modem , which ensure the data integrity.
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do the output signal on the second modem is the same as the input of the modem who transmit the data?

Not exactly the same.  If there is no error then the characters sent to the transmitting modem will come out the other side.  Errors can cause characters to get lost. Character timing may be changed because of buffering so the time between characters at the receiving end might not be the same as the time between characters at the sending end.  The character format on the receiving side might be set different than the sending side so the parity and stop bits may be different.
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One thing you should know about a 56k modem is that they cannot talk 56k to another modem on an analog phone line.  They can only get 56k to an ISDN line (digital phone line, not the same as DSL).   I think the fastest you can do on an analog phone line is 33.6kBPS using a V.34 modem.

Hopefully your 56k modem also does V.34 or at least V.32.
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ok, but dont the speed doesnt matter

but whats the easiest way to send serial data on a phone line.

Please dont tell me with a modem (i know lol)

because very want to do it and i dont have the modems now. and before going to far in my project i want to be sure if its possible. some tell me that its not exactly the same.. if i want the second arduino to recognize the signal, how im gonna do it?

thks for your anwser ( and i will read about the pstn switch)
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a ok, i understand the pstn stwitch, but i just wrote down the signal path without the system between the 2 modem.
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Just make sure that one of your modems can be set to Answer Mode.  It has to be able to 'pick up the phone' when the line rings and negotiate a data connection with the sending modem.
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do somebody have tried this? yet
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up!
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Not quite so easy.  As other said you will need to convert ttl-rs232 levels via a max232 or similar.  Your also probably going to need at least a few control signals to implement flow control and get status etc.  Depending on the modems that you get they can automate much of the work, most models can be configured to auto answer some can auto dial and redial.  Start with something like http://www.usr.com/support/3453b/3453b-crg/Preface%20-%20Product%20features.htm USR was a high end modem it has a lot of features you might find useful.   Modem init strings were the eletro incantations of the dial up BBS days some of them could just about do your taxes via at commands.

Overall it's pretty easy to get working, making it work reliably can be a problematic.  Better modems were always more reliable but the PSTN is a harsh place. You can setup the modems to pretty much just work by themselves and not spit out anything to the serial ports besides the data sent over the wire and use the CTS to signal it's ready to accept data (meaning it's dialed the other modem connected and it's buffer is not full). The setup is here http://www.usr.com/support/3453b/3453b-crg/chap%2016-dedicated_leased%20line.htm for the USR modems though many others can be setup in a similar fashion.  Depending on what sort of data your sending you will need to deal with re-transmission, error detection, etc.
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ok thanks! but if i use a modem like that:
it's a ttl seria dial up modem.
http://www.radi.com/modular29.htm

Why do i need to implement a protocol or something..
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