Registered jack interface to FPGA

[Solidus]

Hey folks,

I've recently picked up a new project on the side, and that is creation of a software modem in an FPGA.

The software side of things isn't the main issue at this point (until I have hardware, any software is useless). What I need help figuring out is how to wire the RJ11/RJ14/RJ25 jack into a DAC/ADC pair for digitizing of the signal.

It's fairly easy to find a pinout of those telephony jacks online, but the problem is that I can't find an explanation of the analog protocol. To summarize the questions, I'll try and go through them below:

1) How do I transmit power over the lines? I can't see any power lines demarcated on the pinouts so I'm assuming this is a sort of passive, in-line power transfer similar to phantom power over XLR jacks. Is this the use of the transformers often seen on modems?

2) If I transmit power, will the transformer insulate the transmit/receive side from the high voltages? (correct me if I'm wrong, I've read somewhere that the lines can swing up to 48V)

3) Other than power configuration, is the 'wiring' as simple as wiring the transmit/receive lines into an ADC/DAC for digitization? Or are their other things I should be thinking about here as well?

My apologies if the main topic or questions seem a bit disordered. This is the first time I've ever ventured into this area of electronics (that is, dealing with PSTN signals), so I'm not 100% sure what to ask or how to ask it.

 

[Gryd3]

Hey folks,

I've recently picked up a new project on the side, and that is creation of a software modem in an FPGA.

The software side of things isn't the main issue at this point (until I have hardware, any software is useless). What I need help figuring out is how to wire the RJ11/RJ14/RJ25 jack into a DAC/ADC pair for digitizing of the signal.

It's fairly easy to find a pinout of those telephony jacks online, but the problem is that I can't find an explanation of the analog protocol. To summarize the questions, I'll try and go through them below:

1) How do I transmit power over the lines? I can't see any power lines demarcated on the pinouts so I'm assuming this is a sort of passive, in-line power transfer similar to phantom power over XLR jacks. Is this the use of the transformers often seen on modems?

2) If I transmit power, will the transformer insulate the transmit/receive side from the high voltages? (correct me if I'm wrong, I've read somewhere that the lines can swing up to 48V)

3) Other than power configuration, is the 'wiring' as simple as wiring the transmit/receive lines into an ADC/DAC for digitization? Or are their other things I should be thinking about here as well?

My apologies if the main topic or questions seem a bit disordered. This is the first time I've ever ventured into this area of electronics (that is, dealing with PSTN signals), so I'm not 100% sure what to ask or how to ask it.

You should be avoiding this project...
Connecting unapproved equipment to a carrier's lines can cause some problems.
Telephone lines have a DC offset, and an AC element that work together to power the equipment, generate the ringing tone, and handle signalling.
Note: A modem does not inject power onto the line, don't attempt this!

(If you are interested in telephony, perhaps you first project should be making your own personal exchange in your house / between offices. Again, don't mess with the carrier's network.)

 

[Solidus]

You should be avoiding this project...
Connecting unapproved equipment to a carrier's lines can cause some problems.
Telephone lines have a DC offset, and an AC element that work together to power the equipment, generate the ringing tone, and handle signalling.
Note: A modem does not inject power onto the line, don't attempt this!

(If you are interested in telephony, perhaps you first project should be making your own personal exchange in your house / between offices. Again, don't mess with the carrier's network.)

Ah, failed to give you enough background. (It's for a work project, so I'm trying to keep the details fairly 'generic' until I have a better idea of what I'm getting into). I could do this project completely on my own up until the analog/registered jack interface - that's what's stumping me.

The RJ connection won't be going into a mainline. The idea is to make an FPGA interface to allow the system to talk to a normal telephone. FPGA takes the signal off the registered jack and modifies it and puts it out over another communication interface such as RS232, etc. It also does the same in the reverse direction.

The reason why I was going into power issues is because there are a good number of telephones that aren't powered via their own DC source (they get it off the line), so in the interest of being complete, I'd like to figure a way to at least mimic that to some extent.

 

[Gryd3]

Ah, failed to give you enough background. (It's for a work project, so I'm trying to keep the details fairly 'generic' until I have a better idea of what I'm getting into). I could do this project completely on my own up until the analog/registered jack interface - that's what's stumping me.

The RJ connection won't be going into a mainline. The idea is to make an FPGA interface to allow the system to talk to a normal telephone. FPGA takes the signal off the registered jack and modifies it and puts it out over another communication interface such as RS232, etc. It also does the same in the reverse direction.

The reason why I was going into power issues is because there are a good number of telephones that aren't powered via their own DC source (they get it off the line), so in the interest of being complete, I'd like to figure a way to at least mimic that to some extent.

That's better
'modem' threw me off, as I had thought you were wanting to connect to the phone line.

The complexity will depend on the services you want to offer the phone you wish to connect to the phone.
Your device will need to be able to provide a DC voltage up to 40-50 V, as well as be able to impose a 90V AC voltage on the same power line to make the phone 'ring'.
The transformer you see in use in modems is typically for coupling a signal to the phone line... as the twisted pair will carry power and signal.
Unfortunately, I know just enough to cover the basics from my SIP course... so I can't help to actually build the device, let alone point you in the direction for support of something like Call Display (Which you could implement to show custom data to your phone)

You could starts looking into keywords like PSTN, POTS, and TDM ... Or get lucky and have a reply from someone who's worked on the hardware on the other side of the phone


Edit: Can also look into making your own 'exchange'
http://www.aholme.co.uk/Exchange/Design.htm
Should give you some better ideas of what you will need to make an outbound connection to a phone.