PAL modems and PC-DC connection

Hi,
I've recently been trying to connect my DC to the Internet through a Raspberry Pi. I've written some software that listens for the DTMF dialing tones from the Dreamcast and then answers automatically. I developed this using a phone plugged into the USB modem through a line voltage inducing cable. So far so good...

When I connected my Dreamcast however, the USB modem doesn't recognise anything. Blind answering with mgetty or minicom doesn't work. The Dreamcast will occasionally say the line is busy if I answer at the right time, but I'm yet to have it connect.

So I plugged a phone into the Dreamcast (using the LVI) and I can hear the Dreamcast dialling. To clarify:

- The Dreamcast dials when connected to a phone
- The USB modem hears tones generated by a phone
- The USB modem doesn't hear/understand the Dreamcast

I've experimented with different voltages (6, 9, 12) and various power levels on the line (19mA to 80mA). Still the USB modem doesn't react to the DC.

I'm starting to wonder if PAL modems even work with a simple line voltage inducer? I'm fast running out of ideas as to why the USB modem doesn't pick up the DC.

I tried measuring the line with an oscilloscope with the idea of comparing the DTMF tones of the DC with those from the phone, but I couldn't figure out how to get a reading.

Does anyone have any idea what the problem could be? People have successfully used PAL modems without a full line sim right?

 

It sounds like the DC is waiting for dial tone, and not seeing it...

 

No I have blind dialling on, and it *is* dialling because I can hear it when I connect a phone to it.

 

How do you connect the phone to the dc? You plug it straight to the telephone jack?
How do you connect the phone to your modem? You plug it to the line jack or the phone jack?

Give more details how do you hear the dc dialing (cabling).
Give more details how you have tested you app with a phone.

 

When I connect the phone to the DC, I have this:

DC -> Line with voltage -> Telephone (I can hear dial tones)

When I connect the phone to the modem:

Phone -> Line with voltage -> USB modem (I can generate dial tones that are received by minicom)

I literally just switch out each end of the setup. I just can't see why DC -> Line with voltage -> USB modem somehow doesn't work

 

You generate dial tones by pressing keys on the phone or with your software?

 

By pressing keys on the phone, the USB modem receives them. I've also done it in software and listened to them on the phone.

 

I think maybe you need more line voltage - you should be able to see the DTMF on the scope if it's dialing. Some of the DC modems required significant line voltage (> 20V).

 

Are you sure? That seems weird because the off-hook voltage on a BT line is 9v... on hook voltage is much higher but obviously when the DC dials it's off-hook. I can maybe try a high voltage but I thought 12v was pushing it..

 

Yeah, but obviously I'm talking about on-hook voltage - when you go off-hook the voltage should drop to ~10V - but in some cases if the open loop voltage is too low it won't even go off hook.

A simple test is to just monitor the line voltage and see if it drops when you issue an ATD or ATA command.

 

If I double the voltage (by linking to 9v batteries in series) I'm not going to blow up anything am I? Should I change the resistor in the line voltage inducer?

 

What resistor are you using at the moment? The normal design impedance of something connected to a phone line is about 600R - so if you use 600R of source impedance for every 10V you should be fine.

But first of all, just see if you detect any voltage change across the modem when you go off hook.

 

Would the Dreamcast even dial if it couldn't go off-hook?

 

This is a fairly interesting project, I'd love to hear more about it when you've got it working. I have a spare first-gen Pi laying around here somewhere...

 

When you say line with voltage, what do you mean?

Can you explain exaclty how have you connected everything together?

I am asking because it is weird, normally if you plug a phone to a modem you don't hear anything. Modems, i think, don't send any voltage down the line to power the telephone.
Hearing anything would be not possible because the phone itself doesn't have any power.

Do you mean that you have a telephone line and you connect the dreamcast and the phone in parallel so you can hear when dreamcast is calling out?

 

Here, this explains the power: http://dreamcast.onlineconsoles.com/phpBB2/guides_pcdcwin98.php#10

 

As you can see in the link that you posted, it says that if you use a usb modem you don't have to use Line Voltage Inducer.

Also if you read further down you will see that you have to use a pstn line to fool dreamcast into dialing and then plugging it to the modem that you have.

Reading again that link, i think that i am writing crap but check it out!

 

Doubling the voltage to 18v worked!!!

I've wired two 9v batteries together and used a 330 Ohm resistor. This gives a current of around 50mA.

I'm going to look into designing a PCB for this, one that doubles the 9v voltage with a booster circuit rather than two batteries, and a 3D printable case.

Next step though is getting my program to answer correctly, and to play a dial tone down the line! Then I'll have the perfect DC <> PC server set up!

 

So where exactly does the Raspberry Pi come into this? I'm a confused idiot.

 

DC -> Magic Voltage Generator -> USB Modem -> Raspberry Pi -> Ethernet