Monday, 10 March 2014

Datamode Interface built

This weekend I finally got around to sorting out my digital/data mode interface for the Yaesu FT-857D.

To transmit and receive digital/data modes you need to connect the radio audio in/out to the computers sound card in/out, the computer then runs the necessary software to encode/decode the signals. I want to try out WSPR, PSK, JT65 and some SSTV for starters I have spent too long just receiving and decoding...

There are a number of inexpensive commercial interfaces available, but many of them use the same basic design originally intended for eQSO/Echolink operation. I nearly succumbed but I had built an eQSO interface many years ago when using PMR446 and had most of the parts to build another.

I nearly took the easy route and got a commercial one since connecting up some home built circuitry to a £20 hand held is slightly less daunting than plugging it into an expensive rig!  My original interface has been modified and reused over the years and was a bit of a mess, but being brave I decided I could tidy it up and I couldn't really damage anything if I took my time... actually the truth was I discovered I didn't have the necessary optocoupler IC so couldn't build a new one just yet...

A simple internet search for digital/data mode interfaces will bring up a great deal of information, schematics and ideas for home brew solutions. The basics can be found here for example.

The simplest form of interface is just a simple direct lead with the transmitter operating in VOX mode. However levels can be a problem as the line/speaker output from a computer can be too high for a transmitters microphone input. Also connecting a radio to a computer directly can lead to problems with ground loops and interference.

The computer can be made to control the Push-To-Talk (PTT) on the transmitter using a serial port with the software controlling one of the handshake lines (RTS/DTR) Some data mode software support CAT to allow control of the PTT as well as tuning the transmitter, but the serial port method is more universal.

The preferred interface, and the one I had built isolates the computer from the radio by using two audio transformers and an optocoupler. There is no direct connection between the two devices so keeps interference to a minimum.

I could have used the microphone and speaker output on the radio, but the FT-857D has a convenient data connector on the back. This is a 6-pin DIN socket as used by older (PS/2) computer keyboards/mice. Note the diagram shown in the FT-857D manual (as below) is the view as you look at the socket.

FT-857D Data connector as in the manual
The connector has two "data" outputs but they are simply fixed level audio outputs from the receiver. The one of interest for most modes is the 1200 baud output (the 9600 baud output is more akin to a discriminator tap and is only of use for FM packet) There is a data-in (TX-audio) and a PTT control line.

Like many people I initially thought I could cut a lead off a mouse/keyboard and repurpose it, however I discovered most only use four wires and they don't use the necessary pins! You might be lucky especially with older keyboards or alternatively if you have an old keyboard extender cable they usually have all six wires present. Alternatively the plugs are readily available from the likes of CPC/Farnell.

I had a hunt around in a junk box and located a suitable keyboard extender cable. I chopped off the useless end and metered out the pins to identify the appropriate wires. Remember when looking at the plug the pins are swapped left-to-right compared to the diagram which is the socket view.

Well here is the insides of the interface.. and as you can see I completely failed to tidy it up! Not my best work, but I did put it in a new box and I did tape up all those unused wires!

The messy internals of the interface
One annoying issue I had was the audio from the computer wasn't getting to the radio, it worked and scoped out okay when out the box, but in the box it stopped working. I suspected a bad solder joint and redid them all, but still showing the same intermittent issue. I did notice flexing the board slightly out the box cause the fault and soon located a track on the vero-board which appeared to have a hairline crack, I couldn't see anything obvious but a wire soldered along it cured the issue.

It looks better with the lid on..
I soon had it connected up and fired up WSPR

Computer, radio and interface

It was straight forward setting up WSPR to use a combination of CAT for tuning and the RTS PTT control and soon had some encouraging results, in fact these are some of the spots of my 5W signal on 10m/20m and 30m, I was grinning from ear to ear!

10M Spots

20M Spots

30M Spots
I had simply set up the FT-857D for basic USB transmission, however it does have a dedicated DIG mode, the manual refers to setting it up for RTTY/PSK, I very briefly had a go with PSK31 and Digipan and was successfully decoding signals and put out a few CQ calls (again on 5w) but got no immediate response. However checking PSK Reporter later it seems I had been heard by VC3S, OH1FOG and ES1JA maximum distance was around 3260 miles

Looking forward to spending some more time experimenting with the data modes.

1 comment:

Anonymous said...

Which schematic did you use or was it your own version?