Using Logic Analyzer on Old Caller ID...

Discussion in 'Embedded Systems and Microcontrollers' started by mwalden824, Dec 24, 2011.

  1. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    Hi. First let me explain what I am trying to accomplish. I want a device that will block calls or keep the phone from ringing. Maybe a button for when a caller calls and you press it then it will never let that number ring the phone again. Now I read a similar problem awhile back in a post here and someone, can't remember who at this time, built a device that would do this pretty much with a few added features I think. Now, I will go that route eventually but first I just wanted to experiment with an old Caller ID I found lying around my house. I thought maybe I could get this done relatively easy.

    My plan was to open it up and find the decoder chip and see if I could find the datasheet for it. I did and it's a CML fx602. I then located the RXD pin and was going to hook my logic analyzer (LogicPORT USB) up to it and read the data. The datasheet for the fx602 said it has two modes, where a MCU could clock out the data for situations where a UART was not wanted or available, or keeping the RXCK pin high, you could use a UART. So the reason I wanted to examine this before just pluging my MCU up to it and intercepting the data was to make sure how the data was being handled. Now everytime I hook the probes up to the fx602, one to the RXD and the ground to the Vss of the chip, it somehow shorts out or something because the phone connects and stays connected until I disconnect the ground probe. In other words, it won't let me call the phone so I can get the caller ID info to it.

    I do not have a regular phone line, I have VoIP with Vonage. I really thought this would be easy, just open the caller ID up find the RX pin, intercept the caller ID data sending it to my MCU where my program would examine it and compare to data stored in a memory chip, depending on successful compare it would trigger relay to disconnect line. Something like that. There was already a relay on this caller ID and everything for call waiting muting during the data transfer.

    So what the heck am I doing wrong? I am an amateur so I am probably doing something dumb.

    I also tried using my oscilloscope and it does the same thing. Now when I unplug the caller ID from the wall it doesn't do it anymore. Only problem then is the caller ID doesn't get the caller info unless it's plugged in, it only lets you see time and past calls on battery power. I guess I could get a 9 V battery and use that to power the Caller ID instead of the 9V wall wart. When I read the voltages across the pins of the chip with a DMM they are all 4.8 V or 0 V. So I dunno..?

    Any help would be appreciated.

    Thanks,
    Michael Walden
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Is one side of your scope or analyzer earth ground? If so, you are shorting the phone line to ground if you clip onto it.

    The caller ID is sent between the first and second rings as data, the "hardest" part is protecting the receive circuit from the high voltage "ring" current, and the standard ~48-50V that is on a live line (-50 if picky), but are referenced to earth ground. If you put a scope ground clip on either one, you are shorting that line (tip or ring/red or green) to earth ground.

    There are schematics around for DIY caller ID, talk to google about them, getting the search keywords is essential.
     
    mwalden824 likes this.
  3. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    The Analyzer I am using is a USB LogicPORT and the grounds on it are connected to the PC chassis through USB cable. And I am guessing the chassis to earth ground.

    So is there anyway I can read the data from this chip or should I just find a circuit and build another from scratch? I mean to use the logic analyzer you have to connect the ground leads which shorts the circuit, so I don't know.

    The recommended circuit through the datasheet is attached below...

    I guess I should have connected the ground to the ground of the MCU on the circuit instead of the Vss on this chip?

    I am just a little confused here. Let's say I start over and build this circuit below. I get it built and need to connect to my MCU, but I need to look at the RX data before I do so for some reason, how would I connect my LogicPort analyzer up to this circuit without shorting the Vonage phone line? I mean I would think RX and Vss. And the caller ID circuit has pretty much this exact circuit on their board, so If it doesn't work for that circuit then it won't for this one above. All I am trying to determine is how do I read the data with my analyzer if it is possible.?

    Thanks for your help,
    -Michael
     
    Last edited: Dec 24, 2011
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Battery powered scope, or hook the USB scope/analyzer to a notebook that is running on battery power.

    Another option is to use 2 scope channels, put channel A on pin1, Channel B on whatever you want the reference to, like the other phone wire, then display A-B, which can be done through menu on a newer scope, or pick INVERT and ADD on older 2-4 channel scopes to Add inverted B to A, which is the same as A-B, giving you the signal.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you can put your logic analyzer in differential mode, you can try that as well, if the voltage doesn't overwhelm/damage the inputs.
     
  6. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    Oh ok, I see. Cool. Thanks for your help!

    -Michael
     
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