Understanding
Soundcard Interfacing
by Ernie Mills, WM2U. http://www.qsl.net/wm2u
Please print this article and use it as a hand out at Club talks and lectures

This page is an attempt to correlate the interfacing schemes for various Radio models, and Sound Card configurations. Since 26th December 1998, when the sound card version of PSK31 was first introduced to the Ham community, and after much dialog, experimentation and confusion, an interfacing pattern slowly emerged. I feel that an understanding of this pattern is valuable and can save you a lot of time and grief. Please note that each circuit references various letters which will correlate with the Radio pin-outs on the various popup tables available. Just click the manufactures button below, pick your radio and read off the hook-up points.

I Urgently need other Radio hook-up info.and corrections.  Please e-mail me, WM2U , or submit this Form for it's inclusion onto this page.

You have a Kenwood Radio and your hookup information is not here! Don't panic! Download this PDF file direct from Kenwood. If it is not on this list, it was never made.

If you can't find it here try this great TNC to Radio Hook-Up page by Buck Rogers,K4ABT

Ok! Ok! so you prefer to buy (that's a dirty three letter word) a ready made Computer to Radio Interface. Check out this nice package by N1ZZ and K1UHF. They call it the RIGblaster

Wait!!! are you hooking up a fully computer controlled Radio? One with a virtual consol? TheKachina, Pegasus or whatever? Hey! first read Georges Notes
STOP!!! If you are a FT1000MP owner, look no further. Doug McCann, VA3CR has a web page dedicated solely to that fine machine. This great site is a one stop information authority and missing it will be your loss. You are warned :) Check out http://www.qsl.net/va3cr.
First off, keep it as simple as you can. This circuit shows a single connection between the Radio Audio Output and The Sound card LINE IN, and a simple 100:1 attenuated connection between the Radio Aux. Mic IN and the Sound Card LINE OUT. The most simple Sound Card hook-up. Use this first. If system works OK with no ground loops then stop right here. Your finished!
Oh! before we start a little word on the component abbreviation used on these diagrams. Since the decimal point was not showing up too well in cases like 4.7k ohm I adopted the European system which uses the 'k' as the decimal point. So the 4.7k ohm now will be marked as 4k7 ohm.
The 100:1 divider is very important because the output from the Sound Card can be in the order of 1volt into the microphone input which is nominally 10mvolt. It follows therefore that without the attenuator, microphone input overload can result, causing the normal narrow bandwidth to increase dramatically producing unwanted splatter.

Try this hook-up first. If you are operating clean and you are having no ground loop problems, that's it. Read no further. Your done! There are a lot of Hams who operate very well using just this circuit.
So! you have ground loop problems eh? Well the next step is to isolate your sound card from the radio. To achieve this, check out this well proven schematic shown at right.
With this configuration the VOX is used to trigger the PTT line for Transmission.

 OK, with a bit of luck all is ok now. If not you must add isolation on your PTT and/or CW line. Carry on!
Now, a very simple transistor circuit can be added to automatically switch the PTT circuit on the Rig. This uses the RTS line in the active Comm. Port. The voltage swing on this is -12v/-5v through +12v/+5v so this line is always 'hot'.  To isolate the RTS line from other devices being used on the same Comm. Port, you might need to incorporate a switch.
Important: Please check that the PTT line, and/or the Keying line on your rig has a positive voltage on it and it requires a pull-down to ground to activate. These circuits will not work otherwise.
What does that mean? Well, you don't need to use a separate Comm. Port just for this application. I have a Kantronics KAM (all mode TNC) connected to Comm. Port 1 and use the same Port for my sound card modes. The KAM is switched off during sound card mode operation.
Instead of using the VOX to operate the PTT line, this circuit will automatically switch over by using the Comm Port RTS line.

I take off the RTS, DTR and GND lines from the Comm. Port at the RS232 connector going into the back of the KAM. Since the KAM will drag the DTR line down, leave it disconnected at the KAM connector. The KAM does not use it anyway! 
Since you might already have used the RTS line you can use the DTR line for other switching chores. Very handy for sending Morse Code.

Ok! Included on this web site is a really nice keyboard CW program. I like the idea of being able to simply click an Icon and start operating one of a growing number of Sound Card modes now available. So CwType uses the Comm. Port DTR line to send Morse Code.
So! you have realized that with the inclusion of the TTL circuits shown above that you now don't have 100% Radio to Computer isolation.

Even worse maybe you  still have a ground loop problem. Well simply substitute one or both of these schematics for there equivalent TTL version. They are the same as the two TTL circuits above except that they use an Opto Isolator to switch the circuit.
The Opto Isolator is a 4N25, 4N29, ECG 3084, PS2601 or equivalent from Digi-Key, Newark or Radio Shack, I used a TIL111.

 If you still have some ground loop affects after using the 'isolated' circuit above, then this Opto Coupled PTT line switch offers complete system ground isolation

To maintain 'complete ground isolation' you should use this circuit if you are using the DTR line for auxilliary switching.
An e-mail off Ken, W7LAR gives us a great tip by the way. If you want to get the Optoisolator from Radio Shack it will probably not be a store stocked item. You can get it direct mailed to your home by asking the shop clerk to order it for you from there warehouse. It is number 11305190 and costs about $3.00
WM2U Tip!!
I periodically get e-mail telling me that certain parts are no longer available from Radio Shack. I suggest therefore that you go to the Bux CommCo  Catalog where you get one stop shopping for all your component needs including the hard to get connectors. Oh! by the way Buck sells the sound card, ISO-KIT and Little Rascal. Hey! for about $25 you get all the components you need. You can't buy them yourself for that price! Check out the ISO-KIT info here. After reading Bucks' page, go to his Kwik-Select guide
The N9ART Interface
designed by Jim Mitrenga
Well! you are now really biting at the bit and want to jump right in there, build a new sound card interface, download that fancy new windows software and join the Digital Revolution. Hold it! hang loose a minute. Are you one of those Hams that uses a non-soundcard, serial port type, interface, and you are really comfortable with that DOS software? You are! well  join the crowd.... but you have realized that there is a problem. Yep! you will get tired of switching cables from one system to another.
Uhmm! well, I think we have the answer to that for ya!. Lets take a look at Jim's interface. This interface will give you the best of both worlds. You can certainly join in the soundcard Digital Revolution and guarantee clean signals but at the click of a switch you cut the the sound card out and fetch in the serial port interface that feels and acts just the way you remember. 
Check out the circuit diagram below. err! look familiar? It should. This interface is from an article that appeared in the November issue of QST, entitled "A Flexible Digital-Mode Interface" and is reproduced by kind permission of the author, Jim Mitrenga, N9ART.
A Flexible Digital-Mode Interface as presented in November QST 2000, by Jim Mitrenga, N9ART

 

The PCB photo of the Digital-Mode Interface

For a complete list of components, click here. If you would like a printout of this list, simply right click on the popup margin and click print.
Ok! the construction of this interface although somewhat more complex than the standard isolated interface, is nonetheless well within the capabilities of most Hams. All the components can be found at Radio Shack and a ready made PC Board is offered by FAR Circuits at http://www.cl.ais.net/farcir/ for only $5.00. Since the original QST article can be downloaded from this site in .pdf format, I will not attempt to cover the circuit theory or the test and alignment procedure in this write-up. 
Please download this file and print it off. Jim has made an excellent job of describing his circuit theory, schematic, construction details, alignment, and the summary is full of useful links relative to both DOS and Windows software.
Hey! as Jim said "Now you have no excuse for not trying Amtor, Fax, Hellscheiber, MT63, Pactor, Psk31, Rtty, SSTV, et al." For more information you can e-mail Jim at: n9art@arrl.net.

I have operated with most of the modules listed above and have ended up with the Isolated Circuit and both the Opto Isolated versions for RTS and DTR control. That configuration works great in this shack.
After my initial hook-up, and all ground loops removed, I still had problems with RF getting into the computer. I found that winding the cable to the computer speaker system through a Ferrite Core about 4 turns, removed this problem.
However, other Hams were not that lucky. I remember two other cases I was helping with where a Ferrite Core was needed in the Sound Card LINE IN lead and the other was wound in the Keyboard cable before a cure was effected. This process seems hit-and-miss at best so the best approach appears to be one of trial and error.
To help me collect the different Radio hook-ups please send me, WM2U or submit this Form with your Radio's hookup information for publication on this page. Any comments or help would be really appreciated.
I hope this page grows with the your help and that it is found useful when interfacing you system,
 73 de Ernie (WM2U)