Capturing an ISDN Signal

XGen

Greetings all,

I would like to capture the basic rate isdn signal and send it via usb onto a PC.

Could someone have a look at the following and tell me if there are any glaring errors.

There are 4 wires, 2 for tx and 2 rx, the line speed is 144kbit/sec for Tx and the same for Rx.
I would think the SX could handle 2 basic rate lines at the same time. Maybe more ?

So that would make it 576kbit/sec and 8 wires in total.
Im not sure how to get the line signal into 1's and 0's right now, as the line moves between +3v and -3v. I would have to have some analog circute to break it down into 4 wires with just 1's and 0's (0-5v) comming out of it. (any pointer here would be welcomed)

So here is my question:

How do i go about capturing the data without missing anything ?

a. Put the data through a Schmitt Trigger so that i dont get duplicate data.
I suppose i can use this to turn the signal into 1's and 0's (0-5v)
b. Interrupt routine running at 144khz that reads the values from the 4 line.

In an Ideal world there should be minimal amount of jitter between the signals comming in, but can this cause me to lose data ?

Thanks for reading this far
X

SteveW

>Im not sure how to get the line signal into 1's and 0's right now, as the line moves between +3v and -3v. I would have to have some analog circute to break it down into 4 wires with just 1's and 0's (0-5v) comming out of it. (any pointer here would be welcomed)

http://telecom.tbi.net/isdn-bu.htm shows some of the detail, It's not entirely trivial - as you've noticed.
You might get some useful receive hints from
http://www.national.com/pf/TP/TP3410.html - the silicon's a bit expensive, and contains a transmit sectio you don't want. If you're not building a product, just a toy for your own amusement, I'd imagine you can ignore an awful lot of the cleverer stuff, but you'll still need the basic 2B1Q receivers, you should be able to do everything else in an SX, though...

(And, if anyone here needs E1 line interfaces to play with, let me know. I have far, far too many...)

Steve

Post Edited (SteveW) : 6/12/2006 2:08:52 PM GMT

XGen

Thanks for the reply and links Steve,

I should have said that I want to monitor the S-Bus, or ST Interface.
That is ALOT simpler than the U interface. I know in some contries the U interface belongs to the Telco and you may not place any equipment on it.

I should go and read ITU Recommendation I.460 to see exactly what voltages I will be getting on the line.
Oh and i'm mistaken its 192kbits/sec not 144 as i said.

How would you go about connecting this setup to the SX ?
Would you implement the reading of the lines in a IRQ ?

Thanks once again
X

I've implemented an E1 logger, but that was simple. I used 2 LIU from Dallas to do all the hard work. They just spit out 1's and 0's

pjv

Hi XGen;

Depending on how precise you want your switching to be, each input line of -3V to +3V could be converted to 0/5 volt levels out of an SX quite easily using 2 port bits and a few passive components. Two resistors and one capacitor to be exact. And of course some software with an ISR is required. Each additional line would probably (I have not done the detailed thinking) only take 2 more resistors and one more port bit.

For fancier, more precise switching levels, the previous numbers would need to be doubled as then the plus 3 V would be handled independently of the minus 3V.

Again, the SX and some software could readily handle the simpler configuration at speed up to 1 Megabit per second, possibly 4 lines simultaneously. At 192 Kb/s it should be easy.

Cheers,

Peter (pjv)