PKT Port Breakout Box

So here is the question: what is the best way to get audio out from the computer without using the speaker jack? The reason why we don't want to use the speaker jack is because when there is something plugged into the jack, the internal speaker is muted. Also, the maximum power output from the speaker jack is around 2 watts, which will easily blow out a sound card.

For a while we played with the HF DATA port on our FT-847, but its limitations became apparent quickly. It is not even supposed to work at UHF frequencies (even though it does), and we didn't know where the audio was taken from on the receiver. We also noticed that our IC-910 doesn�t have a HF DATA port, so we couldn't make our two earth stations identical, which was a main goal. After some more investigations, we settled on the PKT port. It is present on both radios, and even the pinout is the same! There is a discriminator tap on the PKT port, so in the future we could do 9600 baud easily without having to rebuild anything. However, be careful because the PKT port audio input levels for the different radios aren't the same.

Packet Port Basics

Here is the general PKT port pinout for amateur radios. The port on the radio uses a standard mini-din connector, the same connector used on PS-2 keyboards and mice. I would suggest that you to cut a connector off of an old keyboard instead of trying to solder a mini-din on to a cable. The colors listed in the table are the "standard" colors used in this type of cable. As always, please check your cable for consistency and make corrections accordingly. This table is provided here as a general guideline only; please read your owners manual for the specific pinout and voltages.

Pin	Function	"Standard" color	Description
1	DATA IN	black	audio input data to the radio
2	GND	brown	ground
3	PTT	red	push-to-talk line; active low (radio transmits when line is grounded)
4	9600	orange	receive line for 9600 baud operation; discriminator-tapped
5	1200	yellow	receive line for 1200 baud; same audio as the speaker jack; sometimes called AF OUT
6	SQL	green	squelch line; active high/low for different manufacturers; not necessary for this project
	GND	shield	solder the shield to ground to help with interference

As you can see with this table, the audio lines are easy to get to, no modification of the signals is necessary. However, the PTT line can't be used as easily, because standard serial ports cannot source current. All that is needed is someway to invert the signal coming out of the serial port.

Icom IC-910H: Please note that on the IC-910, the PKT port is called a "data socket," but it has the same pinout and connector. Also note that when the IC-910 is put into Satellite mode, the main and sub data sockets are reversed. Brilliant.

The Circuit

It is a really simple circuit and can be breadboarded within minutes for testing. Courtesy of the MixW Help Files.

The serial RTS line stays at -12v (a "low" or "0" condition) and rises to +12v when the radio needs to be keyed. The diode prevents the base of the transistor from going below about -0.7 volts, preventing the transistor from going into reverse active mode. The 2.2k resistor limits the current that can flow thru the diode to the serial port when the RTS pin is low. When the serial port goes high (+12v), the transistor goes into saturation and grounds the PTT line from the radio. This keys the transmitter. Any NPN bipolar transistor can be used, such as a 2N2222, 2N4401, or 2N3904. Since the PTT line on most radios is active-low, an external source just needs to pull that line to ground to key the transmitter. The RS-232 lines don't have enough drive to do this, so an external transistor is needed. Basically all this PTT circuit does is provide this needed oomph to pull the PTT line low, keying the transmitter.

Construction

Build your breakout box out of a small metal project box. Make sure to get a real ground on the box; as you can see in our pictures we drilled a hole in the box and used a bolt. Remember that the box is probably painted and that paint is non-conducting. For RFI, include small capacitors between the 1200, 9600, and DATA IN lines and ground. Also to combat RFI, wrap the 1200, 9600, and DATA IN wires around a toroidal core a couple of times. The SQL line is not used, and you can just leave it free floating within the box in case you want to use it later. Make sure it can't short anything.

Updated 21 May 2006