Scandata faq q4

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Q4: How do I build or acquire a simple decoder interface circuit?

If you want to decode trunk control, MDT, some pager, fax and/or
other digital signals, you will need to build or buy a simple
2-Level FSK Interface to go between the NFM Discriminator/
Baseband Audio of the scanner and a standard PC serial port.

See Question #5 for a more sophisticated 4-Level FSK Interface.

   If you are not capable of building your own, or if you
   don't have the time to mess with it, I can supply either
   a Kit of all essential parts and detailed instructions,
   and you build it yourself, OR I can supply an  assembled/
   tested Decoder Interface, built inside a DB-25 shell and
   connector with a 3 ft shielded cable terminated with an
   RCA plug.  See Question #9 for details.

The DB-25 shell connects directly to a DB-25 comport on the
PC, or, if your comports are DB-9's, you can get a
DB25-to-DB9 adapter for it. You should, however, be able to
make your own following the below instructions.

This is a tried and true Data Interface circuit that's good
for everything we've tried, including a wide variety of
scanners. External power is not required.  This circuit
draws its power from the comport. It is similar to the
"Hamcom" and "data slicer" interfaces, but is optimized for
more scanners and more applications.

Copyright (c) 1998 Bill Cheek  <Revised 12/04/98>

               Note 4                               COM2
                                                 Serial Port
Scanner  |<------circuit----------------->|      DB25    DB9
                                                 ====    ===
Note                      o----o--|<--------o----->20 DTR  4
 5     Note 1             |    | 1N4148     |
NFM    10-uF         |\   |    o--|<--------|--o--> 4 RTS  7
 >-----+|[------o----|2 \ |    |            |  |
DSCR            |    |    \    o--+|[-->>>  |  |   Note 9
        100k    |    |LM  7 \     10-uF     |  |
 >--o--/\/\/----o    |741  6 >----------o---|--|--> 5 CTS  8
GND |                |      /           |   |  |
    o--/\/\/--o------|3  4/ Note 2      |   |  |
    |   3.3k  |      |  / |             |   |  |
    | Note 8  |      |/   |    Note 3   |   |  |
    |         o-----------|----/\/\/----o   |  |
    |                     |     100k        |  |
    |                     |                 |  |
    o---------------------|-----------------|--|--> 7 Gnd  5
    |                     |                 |  |
    |                     o-------->|-------o  | o-<2 TxD  3
    |            10-uF    |      1N4148        | |
    o-------------+|[-----o-------->|----------o |
    |                                            |
    |      Circuit                               |
    o--->>>Ground     o--------------------------o
    |                 |
 - -|- - - - - - - - -|- - - - - - - - - - - - - - - - - -
    |   o-------------o
    |   |
    |   |      Optional for TrunkFollower
    |   |      Control of a second scanner
    ^   ^       (See Note 10 and Q&A #8)
  Gnd  RxD
  or other
  port for Trunk
  Follower control


   --o-- = soldered connection/junction

    --o  = turn of the trace in the drawing

   --|-- = crossover (no connection)
     +|[ = capacitor, polarized (note the +)
      >| = diode (anode->|-cathode)
      |< = diode (cathode-|<-anode)
 -/\/\/- = resistor
   -->>> = ground

 1.  Use tantalum capacitors, +|[ polarity as shown

 2.  Radio Shack's LM-741 op-amp  works fine. Others might not!

 3.  Use 1/4 or 1/8 watt resistors to conserve space

 4.  Build circuit on a trimmed and fitted piece of perf
     board inside a DB-25 shell for max efficiency and space
     savings. DB-25 connects to PC's comport and a shielded
     cable from the scanner goes to the input; shield is
     ground and center is audio.

     Hi-res laser printed Instruction Sets available with precise
     schematics, drawings, perfboard layout, graphics, wiring,
     and solder points. Inquire:  or see:

 5.  Many handheld scanners have no space to install a phone
     jack for the NFM Discriminator Output.  For such tight
     spaces a strip of 3 pinline sockets can be melted into
     the plastic case and wires soldered to the inside
     protrusions. The middle pinline socket is not connected
     to anything; it's just there as strength and support
     for the pinline plug you make, with the shield ground
     soldered to one end and the audio-center soldered to
     the other end:

     GND---o o o---NFM DSCR (audio)

 6.  For base scanners, install a female RCA jack on rear

 7.  For handheld scanners where there is space, use a 3/32"
     mini or a 1/8" standard monaural phone plug.

 8.  3.3k is an optimized value for most scanners.  If the
     baseband audio output measures less than 0.350-V AC, with a
     voltmeter set to AC-volts at a 1v range, you should decrease
     the 3.3k resistor to 2.2k, or even 1k if if the measurement
     is under 0.250v AC.

     I measured 0.150v AC out of the baseband audio tap in a
     Sony ICF-SC1PC handheld scanner. The required resistor
     from U1 Pin 3 to ground worked out to 1k.

     The NFM baseband audio signal of most scanners should
     show 0.400v AC or higher, in which case the 3.3k resistor
     is just right.

 9.  Connect the interface to a comport on the PC.  Plug the
     input cable in to the scanner's NFM Discriminator
     Output Jack.  Tune to a desired signal and run the
     decoder software of your choice.

10.  If you are interested in using the "data decode"
     scanner to control or "TrunkFollow" a second scanner,
     then run a pair of wires off the Data Decoder Interface
     from unused TxD (Pin 2) and Ground (Pin 7) out the back
     of the circuit. This pair of wires will feed the RxD
     and Ground pins of another COMport on the PC.

Schematics, and sources for some pre built units, can be found on our Data Slicers article

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