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Connect at 300 baud?


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#1 logicerr

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Posted 20 August 2010 - 05:17 PM

I'm working on a microcontroller-based dialer. The modem I'd like to use is a single chip device. Datasheet is here if you're interested..

http://www.futurlec....MC145443P.shtml

It uses a Bell 103 modulation and connects at 300 baud. Will this thing be able to connect to any modems in the wild? I know some modems will still connect at 300 baud, but how many?

I'm not asking you to read the datasheet and tell me how to connect. My question is weather or not I'll be able to connect to much at 300 baud using Bell 103.

Thanks guys.

#2 PurpleJesus

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Posted 21 August 2010 - 09:37 AM

I'm working on a microcontroller-based dialer. The modem I'd like to use is a single chip device. Datasheet is here if you're interested..

http://www.futurlec....MC145443P.shtml

It uses a Bell 103 modulation and connects at 300 baud. Will this thing be able to connect to any modems in the wild? I know some modems will still connect at 300 baud, but how many?

I'm not asking you to read the datasheet and tell me how to connect. My question is weather or not I'll be able to connect to much at 300 baud using Bell 103.

Thanks guys.


Looks like it should be standard enough to connect with any 300 bps capable modem. It can do answer and originate modes. Should work.

#3 ThoughtPhreaker

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Posted 21 August 2010 - 04:24 PM

In practice, I've seen modems give me trouble when I need to connect to a number of 300 baud modems. Before you go ahead, I'd try connecting to a few modems with yours forced to use 103 mode, or something like it to make sure it's consistent. If you don't have any, check http://scanaday.gonullyourself.org .

If you don't mind a 28-pin DIP package, you could try finding one of these instead;

http://www.datasheet...310/105390.html

Nortel Millenniums have the PLCC version of these installed in them, they're quite nice.



#4 logicerr

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Posted 22 August 2010 - 02:59 AM

Thanks for the replies!

I expected some trouble using that speed and modulation. The long term goal for this thing is to have it scan for modems and report back somehow with the results. I'm hoping it works well enough to find some good stuff, but I don't expect it to work flawlessly.

My goals are to keep it reasonably low-power so it can operate on batteries and keep the cost low enough for it to be considered disposable, so that rules out a lot of other potentially good solutions. I'd like to keep the parts for each one under $25, and the micro is about $3.

I'd be fine with a DIP or a PLCC, but I can't find that chip anywhere.. There are a few eBay listings for a 73K212, but I can't find a datasheet for it to confirm that it's similar enough to use. It would be nice, too, because it will generate DTMF and do a few other things that would allow me to be lazy.

I really appreciate the advice. I lack the real-world knowledge needed to evaluate how well it might work, so I'm glad there are people willing to help me out.

Also, if one day I'm a bad enough dude to know the ICs in payphones well enough suggest them off the top of my head in various semi-related applications I'll be 100% satisfied in every aspect of my life.

#5 PurpleJesus

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Posted 22 August 2010 - 10:50 AM

Thanks for the replies!

I expected some trouble using that speed and modulation. The long term goal for this thing is to have it scan for modems and report back somehow with the results. I'm hoping it works well enough to find some good stuff, but I don't expect it to work flawlessly.

My goals are to keep it reasonably low-power so it can operate on batteries and keep the cost low enough for it to be considered disposable, so that rules out a lot of other potentially good solutions. I'd like to keep the parts for each one under $25, and the micro is about $3.

I'd be fine with a DIP or a PLCC, but I can't find that chip anywhere.. There are a few eBay listings for a 73K212, but I can't find a datasheet for it to confirm that it's similar enough to use. It would be nice, too, because it will generate DTMF and do a few other things that would allow me to be lazy.

I really appreciate the advice. I lack the real-world knowledge needed to evaluate how well it might work, so I'm glad there are people willing to help me out.

Also, if one day I'm a bad enough dude to know the ICs in payphones well enough suggest them off the top of my head in various semi-related applications I'll be 100% satisfied in every aspect of my life.


Sounds interesting.. Keep us posted on what you come up with. and happy canning ;)

edit:
To save power, you might want to have the micro dial in pulse. It's super simple, you already have to have the circuit anyways.. and you can have the micro sleep between them.. Also maybe you could leach a little power from the phone line to power it?

Edited by PurpleJesus, 22 August 2010 - 11:05 AM.


#6 ThoughtPhreaker

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Posted 22 August 2010 - 07:38 PM

http://shop.ebay.com...ngle chip modem

The datasheets for these seem to be a tad more common, but a bit prohibitive if you want to spend just $25 for the whole project. Tell us if this works out, it looks promising :) .



#7 logicerr

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Posted 23 August 2010 - 04:43 PM

Thanks for the advice, guys.

I guess dialing in pulse makes sense. I'd also like to leech some power from the line, but I can't find a circuit that I'm confident will do this well in less than 15+ parts..

That chip does look good. I have some of the ICs I mentioned in my first post on the way already, but if I get some extra cash I'll pick up some of those too.

Progress is a bit slow for various reasons but I'll keep you updated.

#8 ThoughtPhreaker

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Posted 23 August 2010 - 09:45 PM

If you're going to be dialing in pulse, just be careful. I'm not sure exactly how the phenomenon works, but on some lines, a phone will interperate dialpulses as ringing. If you're going to be using said device from a line that you'd rather avoid causing a disturbance on, make sure you can avoid this problem.

#9 PurpleJesus

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Posted 24 August 2010 - 11:52 AM

Oh.. doubtful this is what you need, but with some ingenuity I think you could use it to scan for tones other then just carriers and maybe keep the total cost down.

From the jist of it, you wire the audio to an ADC port of the micro. Then watch for when the voltage crosses zero. Take the average, do some math, you have a frequency ... Assuming the audio stays still long enough that is.

Not sure if you are wanting this, but I thought it was interesting.

FSK modem simulator

#10 systems_glitch

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Posted 24 August 2010 - 01:28 PM

You /should/ be able to do this without a modem chip. In fact, I'm pretty sure you can do it with a comparator and a filter, or an op-amp with a filtering feedback network. Modems usually answer the line by blasting 2100-2200 Hz down the wire (that's what you hear when you first dial one), so you can create a tuned feedback network around an op-amp with its peak at 2150 Hz. When you get an AC output of a certain level (peak detector/comparator/uC ADC would work here), you know something is sending 2100 Hz. As long as the roll-off of the tuned circuit is sharp enough, you should be able to get a reliable detector out of it.

Now I really wish I had a POTS line...

#11 logicerr

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Posted 24 August 2010 - 02:58 PM

Interesting about the pulse dialing.. Maybe because it ends up looking like an AC signal somehow?

That FSK appnote is cool as hell.

I am using the AVR, so I could probably use the ADC to look at zero crossings, but the math to get that done is way beyond me right now.. I've seen the code but I don't nearly understand it.

I'd also considered looking for specific tones with some simple external hardware, but I'd have to work with a finite set of tones, and I'm not sure what exactly I would look for. What would you guys say would take priority? And faxes would also get marked positive for the connect tone.. Not that it would be so difficult to weed them out. Around here it's probably about 10 fax machines to 1 data modem.

It turns out I actually have a 73K212AL kicking around the house. It's in an old Z80-based credit card terminal that I was messing with, but I might remove it to play with it. The modems I ordered should get here in the next few days, too.

I've been working on the code to get things started. I think it should be easy enough to generate DTMF, so I'll probably just do that. That also gives me an easy way to report numbers that connected. Could leave a list as a message in a voice mail or something similar.

I'll keep working on my code for the MC145443s in the mail. It's reasonably modular, so adding support for new modems shouldn't require a total re-write.

Once I get the basics working, is there a toll free range you'd recommend testing it on? That's still probably pretty far off, but it doesn't really stop me from thinking about it.

#12 dinscurge

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Posted 08 October 2010 - 05:34 PM

Interesting about the pulse dialing.. Maybe because it ends up looking like an AC signal somehow?


maybe dials to fast?

#13 ThoughtPhreaker

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Posted 08 October 2010 - 07:11 PM

I doubt it, 10 IPS has been standard since before forever ago. If you're within spec, usually it means something else is at fault.

By the way, if you're still looking for a range to try it out on, take a look at 440-233-34xx. It's not toll-free, but it's a Macy's data center. You'll find a ton of modems in it.






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