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Dial Up modem interfacing and PPP

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Newbie level 4
Jul 27, 2007
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I have to do a project where I need to connect my PIC micro to a dial up modem serial. What all lines I require to do the task?

TX and RX will do? or any other control lines?

I am new to modems even though have some experience with PIC serial ports.

Please help.

The number of control lines depends on the modem. If the modem can be configured for XON/XOFF or software flow control, then only TX and RX are required.

The bigger problem is PPP. This a protocol stack and is not a trivial piece of code. MicroChip had a sample protocol stack at one time, check their site.

If you are sending only small pieces of data, consider a simpler protocol like trivial FTP.

Thanks for the post and information.

Suppose the modem is not XOn /Xoff type, and if other hardware handshaking type, usually standard modems interface require how many other lines other than TX and RX?

Thanks for the warning about PPP. PPP is not my immediate task but have to start the set up with PPP in mind later.

Please suggesst

Thanks a lot

Most modems will default to no hardware flow control. Do not be confused by the connector. Many older external modems had 25 pin connectors on them. However, the default was just a sub-set of these.

Take a look at:

Here you will see a picture of the DB9 connector. First try just connecting RX and TX. Then see if you can talk to the modem. You can use only RX, TX and ground of course to connect the modem to a PC serial port. On the PC start a terminal program like Hyperterminal. Set the baud rate to the modem's baud rate and then manually type the modem commands into the terminal. Below are some links to modem commands, if you are not familar. Once you have the modem dialing from the PC, you can then convert these commands into code for your microcontroller. If the basic three wire connection does not work, try connecting pins 7and 8 together at the modem. If that does not work, try connecting pins 4 and 6 as well. What these connections do is loopback the handshake signal so that the modem does not wait for any flow control.
My experience is that you will not too much difficulty getting the modem hardware to connect. The real challenge will be running the protocol stack to transfer the data. Seiko made some chips several years ago that put the entire stack into an easy to interface piece of silicon. Unfortunately, sales were not that great and I think the part was discontinued.

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