Or how I (almost) interfaced an Yaesu FT-891 radio to an Icom tuner.

I recently decided to replace my Icom IC708G in the car, as it had a weird ALC fault on transmit. It is afterall, a radio that came out in 1997, making it approximately 24 years old. I reviewed what was available in the field, and settled on the Yaesu FT-891.

I selected the FC-50 tuner as a suitable tuner, and was dismayed that my research didn’t reveal that it is useless. It can only tune within a 3:1 range, so is simply not suitable for the 3m helical whip I use on the back of the Xtrail.

Not to worry, I already have an Icom AH-4 tuner in the car, and it eats the tuning job for breakfast. So – How to plug a round plug into a square hole (Kind of literally – The Yaesu is round Mini DIN8, while the Icom is a rectangular Molex 4 pin)?

FC-50 Physical Connection

The first step was to break out the cable between the FT-891 and the FC-50. A little research showed me that the cable had the following pinout;

• Pin 1 – +13V
• Pin 2 – TX Gnd (Pulled low by the radio on TX)
• Pin 3 – Ground
• Pin 4 – TX Data (TTL Level)
• Pin 5 – RX Data (TTL Level)
• Pin 6 – Tuner presence detection (Grounded by the tuner)
• Pin 7 – Reset (Pulled high by the radio)
• Pin 8 – TX INH – (Grounded by the tuner)

Data is sent at 4800 baud, so the signals were broken out with a piece of veroboard and examined with a scope and an Ardiuno monitoring the data lines.

I had a massive step up from research that was done by John Price – WA2FZW, in a document that he published in the files section of the FT891 Groups page (FT-891 to MFJ-939Y Tuner Interface Test Box).

Yaesu Logical Protocol

Pressing the TUNE button on the radio starts the following process when communicating with the FC-50;

Radio	Tuner	Function
0xFF		Wake up tuner (delay of 50mS required)
0xF1		Set Frequency to zero
0x00
0x00
	0xA1	Tuner Tuning
0xFF		Wake tuner
0xF2		Set Frequency
0x14
0x03		14.03Mhz
	0xA0	Frequency Ack – Starts tuning
	0xA1	Tuner finished tuning

AH-4 Physical Connection

The AH-4 uses a 4 pin Molex

• Pin 1 – Key – Output from tuner
• Pin 2 – Start – Output from radio
• Pin 3 – +12V
• Pin 4 – Gnd

Yaesu Logical Protocol

To tune, the radio starts by asserting the <Start> line. Approximately 300mS later, the tuner asserts <key> – This causes the radio output to keyup at a reduced power level. (This bit is important)

The AH-4 then tunes, and uses <key> to let the radio know if the tuning was successful. This is an example of a successful tune.

If tuning fails, the AH-4 returns an extra 20ms wide negative pulse on the <key> line

Tune Power

And this is where we hit a roadblock….. The AH-4 is designed to tune with a maximum input power of 10 Watts. The Yaesu will merrily output a 50W CW level while tuning.

This may be an issue.

More later…..