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Interface For General Use

This data interface circuit connects a PC to a standard amateur radio 6 way mini-DIN data port.  It can be used for e.g. EasyPal or DIGTRX data transfer, APRS or SSTV software when the PC's sound card is used to generate and decode the required audio frequencies.  It uses the 1200 bps received audio pin on the mini-DIN.

Both audio paths use transformer coupling to prevent PC battery charger noise introducing data errors.  The PTT signal and Crash recording trigger voltage use opto-isolator ICs.

 

Component Side Of The Strip Board

 

Solder Side Of The Strip Board

 

Maplin's UK Component Part Numbers

            I won’t bother listing part numbers for the fixed resistors and diodes.  Any small signal diode will work e.g. 1N914.

6 wire alarm/signal cable       XS54J         @ 69p per metre     £ 0.69

9 way D socket                    RK61R         @ £1.19                 £ 1.19

9 way plastic D cover            KE94C         @ £1.59                 £ 1.59

6 way mini-DIN plug             JX04E          @ £1.19                 £ 1.19

Plastic box                          KC92A         @ £1.89                 £ 1.89

Preset pot 50 k Ohm             UH26D        @ £1.41                 £ 2.82

2 x Transformer LT44            HX82D         @ £1.89                 £ 3.78

2 x Opto-couplers SFH618-2   CY94C         @ £0.89                 £ 1.78

2 x 3.5mm stereo plugs         FJ99H          @ £1.99                 £ 3.98

Screened cable                     FS24B          @ 1.29 per metre    £ 1.29

(The plugs and cable cost £5.27 but to save work soldering, perhaps buy a 3.5mm to 3.5mm adaptor cable for £5.99 and cut it into two.  Code L29BA)

Plus strip board.  Various sizes.  The interface circuit requires 24 holes x 17 holes.  I bought JP50E for £3.34

Suggested Construction Method

 

  1. Obtain all the parts.
  2. Cut the strip board to size.  With the copper strips vertical, there needs to be 24 holes vertical and 17 holes horizontal.
  3. File the cut strip board edges so that it fits in the bottom of the plastic box.
  4. Cut the 17 gaps in the copper strips.  A special tool is available, but most people use a twist drill.  If you use the drill bit in your hand, it doesn’t take much effort to cut each gap.  If you use an electric drill, you’ll probably drill right through the board.
  5. Before soldering, check and double check that you insert the components in the correct holes.
  6. Fit and solder the 12 wire straps.  Most will need to have insulation.
  7. Fold the transformer lugs underneath the transformers.
  8. Fit the transformers.  Only one winding of each has a centre tap.  Double check they are in the correct holes, then solder the transformers.
  9. Fit and solder the other components.
  10. Optionally, solder a few millimetres of wire as test points.
  11. Solder half of the 6 wire cable to the 9 way D socket and fit its plastic cover.
  12. Solder the other half of the 6 wire cable to the 6 way mini-DIN plug.
  13. Using Tippex or a white pen, draw a line on the mini-DIN to indicate the top.
  14. If you bought separate 3.5 mm stereo plugs, solder each to half of the screened cable.
  15. Cut or drill 4 cable entry holes at one end of the plastic box.  All holes at one end means the strip board can easily twist, hinge and fit under the cables.
  16. Remove the cable sheath from the end of the 4 cables.
  17. Solder the 13 connections to the outside world.
  18. Decide if you require two holes drilling in the plastic box to allow a screwdriver to adjust the two pots.  As the adjustment probably requires doing only once, it may not be worth it.  You can always remove the box lid to do it in the future if necessary.
  19. Don’t put the lid on, as you have to adjust the pots. and remove the 37 faults you put on while soldering.
  20. When you do put the lid on, the springiness of the four cables holds the strip board quite firmly.  There is no need to use mounting pillars and screws.
  21. Mark the two 3.5 mm plugs as mike and headphone or use a colour code.  Green tape for headphone (like the socket) and red tape for mike (pink socket).

Adjusting The Attenuation

The method used depends on what test equipment is available and if other RAYNET members within direct or repeater range are already set up to use EasyPal.

Setting the audio levels is quick.  You are not initially bothered about decoding or correctly transmitting a data file.  Once the levels are set, things will work.

Connect the interface circuit to your radio’s data port, your PC’s serial port and your PC’s mike and headphone socket.  Run EasyPal or DIGTRX.  With Vista, you will probably crash EasyPal if you insert or remove audio plugs while the program is running.  It is assumed that you have already configured EasyPal or DGTRX.

On the transmit side, set the Windows Wave Volume to maximum and Master Volume to half way.  On the receive side, disable any mike amplifier and set the Windows Recording Volume to half way.  Disable any sound effects, non-flat equalisation, tone boost or base boost.

When doing volume adjustments, only do so when a data file is being transferred.  Not on voice and not on waterfall signals.  You need to adjust VR2 to set your transmit deviation to +/- 2.5 kHz.  Another digital station may know the receive audio level he sees when receiving a +/- 2.5 kHz signal, so can talk you in, i.e. up a bit, down a bit.  If desperate, set your digital deviation to sound a similar level to your voice deviation level.  That has now set your transmit audio.  For ever?

Adjust VR1 to set your receive audio level.  With another digital station transmitting a data file using a deviation of +/- 2.5 kHz, set VR1 to receive –15 dB with DIGTRX or at the vertical bar near the right hand side of the EasyPal RX bar.  That has now set your receive audio.  For ever?

Put the strip board in the box.  Put a cable tie around the four cables inside the box for strain relief.  Possibly put another cable tie outside the box.

Performance Tolerance

With the Windows volume sliders set to the half way points and the interface attenuation pots. set correctly, an amazing thing happens.  You shouldn’t need to adjust any volume settings again, unless a digital station you are transferring files with is totally out of adjustment.  If that is the case, the other station should change its end, not you.

Assume that you and another station have your volumes set correctly.  If you transmit a data file, the other end should obtain and maintain sync. even if you change your Windows Master Volume to maximum or almost zero.  When decoding a data file, you should be able to obtain and maintain sync. even if you change your Windows Recording Volume to maximum or almost zero.

On my Vista PC, I can set the Recording Volume to 1% (Vista volume controls are the only good thing about Vista) and still decode a file transmitted to me.  There is no indicated receive level.  There is nothing in the waterfall. All black.  The audio spectrum analyser shows no peaks.  I can decode stations which aren’t even there!

As such an EasyPal display looks forged, the screen dump below was taken when 26 out of 27 segments of a file had been received and the Vista volume was on 2%.

 

Faulting Notes

I forgot.  None of the above applies because you haven’t fixed the 37 faults you put on whilst constructing the interface. Also, you didn’t solder in any Test Points, so you will require five hands and a plastic vice for the tests.

The following should help.  The measurements were taken using a DVM (Digital Volt Meter).  I use a USB to serial port adaptor and the output voltages on the DB9 socket are about +/- 6V.  A PC with its own serial port will have output voltages of about +/- 12V.  Don’t expect exactly the same measurements shown below, but they should be somewhere near.

Transmit Audio:  Unplug the mini-DIN plug and DB9 socket.  The DC resistance between TP1 and TP6 should be about 61 Ohms.  Half of the transformer’s centre tapped winding is 61 Ohms.  All of that winding is double that resistance, so k to m should be 122 Ohms.  Turn VR2 fully clockwise until you feel and hear the clutch slipping.  All of the transformer’s i to j winding is now in parallel with 50 k Ohms.  The 50 k Ohms doesn’t count as it is so high compared with the low transformer winding resistance.  Measure the DC resistance between TP8 and TP9.  That transformer winding has a resistance of 1,300 Ohms.  Keep measuring.  Turn VR2 fully anti-clockwise.  The resistance measurement should have steadily increased to 50 k Ohms.

You have now DC tested T2 and VR2.  To AC test the transmit audio, first connect the 6 way mini-DIN plug to the radio’s data port, then plug the correct 3.5 mm plug into the PC’s headphone socket.  Leave the radio turned off.  Have the PC turned on.  Set the DVM to AC Volts between TP8 and TP9.  Use EasyPal or DIGTRX to “transmit” a file.  You should measure about 80 mV AC.  If not, adjust the Windows Master Volume for 80 mV AC.  Now measure between TP7 and TP9 and “transmit” a file.  You should measure about 25 mV AC.  If not, adjust VR2 for 25 mV.  Measure between TP1 and TP6 and “transmit” a file.  You should measure about 5 mV AC.  If your DVM is measuring tiny induced signals, it may indicate slightly higher than 5 mV all of the time, even though you aren’t sending audio from the PC.  It may perhaps show 7 mV.  If so, turn VR2 clockwise slightly while “transmitting” a file and you should see the measurement increase above the induced 7 mV.  When the program has ceased transmitting the file, the measurement will drop down to 7 mV again.

You have completely DC and AC tested the transmit audio circuit.  All two components of it.

Receive Audio:  Unplug the mini-DIN plug and DB9 socket.  The DC resistance between TP4 and TP5 should be about 61 Ohms.  Half of the transformer’s centre tapped winding is 61 Ohms.  All of that winding is double that resistance, so f to h should be 122 Ohms.  Turn VR1 fully clockwise until you feel and hear the clutch slipping.  All of the transformer’s d to e winding is now in parallel with 50 k Ohms.  The 50 k Ohms doesn’t count as it is so high compared with the low transformer winding resistance.  Measure the DC resistance between TP1 and TP2.  That transformer winding has a resistance of 1,300 Ohms.  Keep measuring.  Turn VR1 fully anti-clockwise.  The resistance measurement should have steadily increased to 50 k Ohms.

You have now DC tested T1 and VR1.  To AC test the receive audio, first connect the 6 way mini-DIN plug to the radio’s data port, then plug the correct 3.5 mm plug into the PC’s mike socket.  Turn the radio on.  Have the PC turned off.  Set the DVM to AC Volts between TP1 and TP2.  Have another station use EasyPal or DIGTRX to transmit a file. You should measure about 80 mV AC.  Now measure between TP1 and TP3 and ask the other station to transmit a file.  Initially, you should measure zero.  Adjust VR1 clockwise for 25 mV.  Measure between TP4 and TP5 while the other station transmits a file.  You should measure about 5 mV AC.  If your DVM is measuring tiny induced signals, it may indicate slightly higher than 5 mV all of the time, even when you aren’t receiving audio from the other station.  It may perhaps show 7 mV.  If so, turn VR1 clockwise slightly while the other station transmits a file and you should see the measurement increase above the induced 7 mV.  When the other station has ceased transmitting, the measurement will drop down to 7 mV again.

You have completely DC and AC tested the receive audio circuit.  All two components of it.

PTT:  Unplug the 6 way mini-DIN from the radio.  Switch the DVM to a DC resistance range and connect it with the red lead to TP12 and the black lead to TP1.  Connect only the DB9 socket to the PC’s serial port.  Don’t connect either of the two 3.5 mm plugs.  Run EasyPal or DIGTRX on the PC.  The measurement should show infinity Ohms.  Transmit a file.  The measurement should show about 136 Ohms.  If that fails, switch the DVM to DC Volts and connect it between TP11 (red lead) and TP10 (black lead).  With EasyPal in receive mode, the measurement should be about –5.8V (or –12V if the PC has its own serial port).  With EasyPal in transmit mode, the measurement should be about +7.0V (or +12V if the PC has its own serial port).

Crash Recording Trigger:  Plug the 6 way mini-DIN into the radio’s data port.  Don’t connect the DB9 socket or any 3.5 mm plugs.  Switch the DVM to a DC resistance range and connect it with the red lead to TP14 and the black lead to TP15.  With the radio squelch closed, the measurement should show infinity Ohms.  Open the squelch.  The measurement should show about 136 Ohms.  If that fails, switch the DVM to DC Volts and connect it between TP13 (red lead) and TP1 (black lead).  With the squelch closed, the measurement should be 0V or slightly negative.  With the squelch open, the measurement should be + 5V to +7.8V.

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Page updated on 09 January 2017

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