Part 2:  The Interface

ast month, we started this series with an overview of what it takes to inter-face a computer and a radio for purposes of handling signal content.  This is separate and distinct from using the computer to control the transmit/receive and frequency functions we've learned to use at Field Day.  This is more along the lines of using the computer to generate and read the various digital modes, send CW and play .wav files of most redundant exchanges, such as the Field Day exchanges.

To do this, there needs to be a way to link the computer's sound card with the transmit and receive audio ports of the radio in quest-ion.  This can be done by various means, but typically, it's wisest to do it in a manner that there is no common ground connection between the radio and the computer.  The device we're going to build here does just that.  I may have spoken too soon about the price of building the interface, but the basic circuit board should fit within the limits specified.

There are 2 sets of functions that need to be considered here.  First of all is the audio.  What we have here is a couple of audio interstage transformers, with simple attenuators attached; configured so that the pots that control the signal levels are on the far side of the transformers from the signal sources.  Additionally, there is a blocking capacitor on the lead for transmit audio, to keep DC from looping through the interface.  Receive audio comes from the Line Out pin on the radio's accessory plug (or the radio's remote speaker jack, if necessary), through T1 to R1, which sets the level going to the sound card's Line In jack.  Likewise, the transmitted audio goes to T2, then has its level set by R2, before going through C1 to the Line In pin on the accessory jack.

The other part of the circuit connects a serial port for controlling the Push To Talk functions, and also for keying the radio for CW; or if the radio supports it, the FSK functions for RTTY (otherwise, you're using AFSK, and the radio functions as it would on SSB).  Optoisolators perform the necessary switching functions for both of these operations.

There is a schematic of the inter-face presented, and the various paths described above should be fairly obvious.  Construction is pretty easy, too.  The biggest issues come with properly masking a PC board for etching, and with coming up with a board layout that fits the available components.  We are conducting an ongoing campaign to identify the best sources for some of the  components used, and to lay out boards appropriately.  The Radio Shack transformers I use are sometimes a bit difficult to work with, because of the fine wire leads involved (and it's problematic, stripping them with dentures).  Different transformers call for different board layouts.  Of course, there's nothing to say that you can't put one of these together on perf board, and that's how we prototype things.

The diagram just shows the actual interface itself, and not the intended network of jacks and switches that will allow it to be used with any computer, merely by fabricating the cables for the interface.  The cables are not difficult to make or buy.  On the com-puter side, you would use 2 -  3 mm stereo jacks, with the signal leads going to the tip connector in both cases.  The jack for the serial port is a 2.5 mm. stereo jack, with the common lead going to the sleeve pin on the jack, the tip pin going to R6, and the ring pin going to R5.  With the cable laid out as intended, that puts U2 on the RTS pin of the serial port, and U1 on the DTR pin.  The jacks on the computer side of the interface can be grounded to the enclosure, if you build this in a metal box.  On the radio side of the interface, it is intended to use an RJ-45 connector at the interface, and a short Category 5 cable (no more than 18 inches) to the accessory plug that goes in the back of the radio.

As soon as we can find our best source of transformers, a board layout will be prepared and published.  If someone can advise of better schematic drawing software that is either freeware or reason-ably priced shareware, I'd like to know; conversely, if someone were to re-draw this diagram to include all the jacks, that would also be nice.  If there's enough interest in this, we might also attempt to kit this as a building project for the club.

-AL7W

Cabling Pinouts:

Serial port to J1:  DB-9F connector, 6'stereo cable to 2.5 mm stereo plug.  Pin 4 (DTR) to Ring, Pin 5 to cable shield and sleeve, Pin 7 (RTS) to tip.

RJ-45 jack and cable to accessory plug: 
1 (wht/org)  PTT common
2 (orange)  PTT hot
3 (wht/grn)  FSK common
4 (blue) Line In or Mike jack (xmt audio)
5 (wht/blu) Xmt Audio common
6 (green)  FSK hot
7 (wht/brn)  Rcv Audio common
8  (brown)  Line Out (rcv audio hot)

Signal cables to sound card:  Standard 6' stereo cables with 3