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Interface 4

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Interface For The Yaesu FT-8900

This data interface circuit is based on Interface 3.  It is for use when the receive audio pin is the 9600 bps pin which is unsquelched.  The circuit provides an external squelch.

One difference in the "standard" amateur radio 6 way mini-DIN data port is as follows.  With the Kenwood TM-V7E, the 9600 bps receive data pin also has a standing DC voltage of about +5V.  The Yaesu rigs do not.  In the circuit below, IC1 and IC2 are used as switches.  IC3 is used as an amplifier and it requires a positive DC voltage to forward bias the input LED.  If pin 4 has a positive DC voltage, diode D2 won't be required.

The circuit was developed using a Kenwood TM-V7E.  When a RAYNET member with a Yaesu FT-8900 made the circuit, it didn't initially work.  Diode D2 made it work.  On the strip board, D2 was added as an afterthought using long insulated wires.

R3 should be 5.1 k Ohms if pin 4 has +5V DC and D2 won't be required.  If pin 4 does not have +5V DC, D2 will be required and R3 should be 1.8 k Ohms.


Component Side Of The Strip Board 


Solder Side Of The Strip Board



See interface 3 for component details as the circuits are very similar.

Circuit Notes

The external squelch for receive audio works as follows.  IC3 is an opto-coupler and is used as an audio amplifier.  The input LED requires a forward bias voltage which it obtains via D2.  Pin 4 PR9 is the audio input.  The audio output is developed across R4.  C1 connects the audio output to the receive audio volume control and transformer.  The DC power supply to this audio amplifier comes from the common serial port pin 5 and also from pin 4 DTR (Data Terminal Ready).  The common pin is 0V and the DTR pin is –12V for a desktop PC with its own serial port or –6V if a USB to serial port adaptor cable is used.

To give the negative voltage, EasyPal must be configured to lock DTR to the off (negative) voltage.

The DC power for the IC3 amplifier is only present when IC2 is turned on.  IC2 is used as a switch which is operated by the radio’s squelch (pin 6 SQC).  With the radio’s squelch closed, SQC is at 0V (or possibly slightly negative) with respect to the Data Earth (pin 2 DE).  IC2 is therefore turned off.  With the radio’s squelch open, SQC is at about +5V.  IC2 is on (output conducting) and so supplies the negative DTR line to IC3.

The double pole change-over slide switch allows the circuit to supply the recording trigger for the RAYNET Crash off-air recording program.  If there isn't enough room in the selected box to mount the slide switch, it should fit inside the box.  It will only be used occasionally, so the box lid can be removed to alter the switch position.
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Page updated on 09 January 2017

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