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Diagram 2

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On the receive rig's 6 way mini-DIN socket, pin 6 is the squelch open / closed indication.  When an RF signal is received which opens the squelch, pin 6 switches from 0V to about +5V.  That lights the LED in the opto-isolator IC and draws about 2mA of current from pin 6.  Its output transistor conducts to pull down the transmit rig's pin 3 to earth potential and that rig transmits.

The transformer provides DC isolation in the audio path.  The volume control to set the transmit deviation level can be either be a preset type which requires a screwdriver or the type with a knob.  The latter is likely to be rotated accidentally.

It should be noted that mobile FM rigs with a data port socket have a vastly different transmit audio sensitivity, depending on the rig menu setting the data rate, i.e. 1200 bps or 9600 bps.  The data port is not being used for data, just for its audio and PTT connections.  The data rate menu on the transmit rig should be set to 1200 bps.  It then expects about 40mV p-p of transmit audio.  If it was set to 9600 bps, it would expect about 2,000mV p-p.

The 6 way min-DIN plugs at each end of the cable should be marked "TX" and "RX" as the circuit only operates in one direction.  It may not matter, but is it usually best to know which rig will be the receiver and which will be the transmitter.  When the rig is switched into transmit mode from its data port, the microphone is usually disabled.  It may be best to check that this is the case.  If not, local noise in the car will be received by other stations as well as the talk-through audio.

This same circuit, with different connectors, can be used with the CRASH program.  It records off-air RAYNET FM traffic.  The simple method is to configure CRASH to record when receive audio is present.  That works, but it may miss the first syllable at the start of each transmission and also after a pause in speech.  Using the squelch=open condition to start recording audio will give better results in some situations.  The CRASH program is described here.

It is suggested that the 6 way mini-DIN plug for the receive rig is wired for both pin 4 (unsquelched receive audio) and pin 5 (receive audio under squelch control).  That gives a choice of which to use.  Pin 5 seems to be the obvious one to use, but the talk-through audio sounds bassy with little treble.  Pin 4 sounds better, but there is a disadvantage.  As that receive audio is always open squelch, if the talk-through operator uses the mike on the transmit rig, other stations will hear both the voice and also unsquelched audio from the receive rig.  While the transmit rig's mike is used, the receive rig must be turned off.  An alternative is to add a switch between the top of VR1 and the receive audio.

The Standard Data Port Connections

This diagram shows the solder side of the plug, which is the same as looking at the socket on the rig.


Diagram 2


Similar Circuit for the CRASH Program


Diagram 2 Plus Connections for the CRASH Program

It is difficult to use a single optocoupler to work with both talk-through and the CRASH program at the same time.  Two optocouplers work OK.  Apart from wasting hours trying to get a single optocoupler to operate both talk-through and acting as a CRASH recording trigger voltage, things got very confusing later.  The problem is one of standards and how standard is a standard?

The selected optocoupler is a low input current device.  It will work with an input current of 0.5mA to 50 mA.  The problem was that the sink current available from the data port's SQC (Squelch open/closed state) pin is very different between Kenwood and Yaesu rigs.  Putting an ammeter between the SQC pin and earth (frame / chassis) to measure the maximum sink current with an open squelch gave the following results.  Both rigs had an open circuit PD of about +5V.  A Kenwood TM-V7E could sink 4.7mA, but a Yaesu FT-8900R could sink only 1.05mA.  The optocoupler requires a minimum of 0.5mA to switch on.  In theory, the 1.05mA could perhaps give 0.5mA to two optocouplers, but in practice it wouldn't, no matter which values of R1 and R2 were used.  The TM-V7E had no problem in working with two optocouplers.  If the FT-8900R tried to switch two optocouplers, none switched on.

Not wanting to fit a battery in the interface cable box, another solution was used.  A switch was inserted to disconnect the second optocoupler.  The transmit rig is unimportant.  If the receive rig is a Kenwood TM-V7E (and possible any Kenwood rig), with the switch closed, the circuit can be used at the same time for both talk-through and making a CRASH recording of RAYNET traffic from the receive rig.  If the receive rig is a Yaesu FT-8900R (and possibly any Yaesu rig), only talk-through can be used and the switch must be open.

The above refers to using CRASH with a DC voltage to supply the trigger to start each period of recording.  That gives slightly better results than configuring CRASH to start recording when any audio voltage is present.  A configured CRASH delay of 0.1 seconds is best.

SW1 is used to disable CRASH recording if a Yaesu FT-8900R is used as the receive rig.  SW2 allows the choice of using the two available receive audio pins, which sound slightly different.  The PR9 pin (9600 bps receive audio) is always open squelch and has no audio filtering.  The PR1 pin (1200 bps receive audio) is under manual squelch control and has audio filtering.

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Page updated on 09 January 2017

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