P.C. Soundcard/Radio Interface notes for APRS.

[ Building | Connections | Debugging | P.C. Setup | Radio Setup | Software | Software Information | Final Notes | ]

This information is from Rob, KI0IO

Last updated: May 6, 2003

Read this entire document before starting. This document references a schematic you will find at http://www.w0ipl.com/ECom/APRS-schem.pdf


  • I only re-drew the schematics on the same page. Credit should be given to the folks that posted their schematics on qsl.net. I suggest you look at their original text. The current URL is: www.qsl.net/soundcardpacket/cabletx.htm

  • On the schematic, notice the ground on the PC and radio sides are NOT connected together. This is an important safety mechanism for both pieces of equipment.

  • I placed a four-pin header on the schematic as you will need to connect from here to your radio. I used the external jacks on my radio to connect to this header.

  • I took the schematics as posted, but did not follow them exactly. I used a zero ohm resistor (piece of wire) for R1 and R4. At the low volumes level I run, these parts were not necessary. Your installation will vary. Start with the parts as they can always be removed. Another method would be to replace R1 and R4 with a variable resistor to have adjustment here.

  • Speaking of removing parts. If you place all the discrete components (resistors & capacitors) on a 16-pin DIP header, you can easily remove/replace parts without un-soldering wires.

  • On the schematic, I used 2- 1/8" stereo jack to connect to the soundcard. In keeping with this method, I used a 1/8" mono jack between the parallel connector and interface card as well. Using the jacks makes for easy removal, but, you certainly don't need to use them.

  • A 2"x2" perfboard can accommodate all the parts shown on the schematic.

  • In total, I'd guess my interface card about $20. You can purchase a card/components for $25 by searching on the web. The main cost was the two 1:1 audio transformers. At Radio Shack, these were $4 each. If you have the parts in your junk box, so much the better. Either way you solder, unless you pay more.


  • To transmit, you need a connection to a serial or parallel port on your PC. On my schematic, I used the parallel port. Select which port you are going to use when you setup AGWPE. LPT1 is at address 0x370, LPT2 is at 0x278, and LPT3 is at 0x3BC. You will need to check your BIOS/SystemSetup to determine where your parallel/serial ports are before you setup AGWPE. Check the port address on parallel ports, one should match the addresses I gave here. I chose to use parallel due to availability, no other reason.

  • If you’re transmitting and your radio has a timeout timer, set this as short as possible. To five seconds minimum if you can. This way, if your transmitter gets stuck on, it will stop eventually. Additionally, it would be pretty simple to add a timeout monitor/control circuit just prior to R5 to control this with hardware. Do not remove R5 if you do this.

  • I used two stereo male-male shielded cables between my PC & interface card.

  • My cable out of the 4-pin header on the schematic has pins 2 & 4 connected together as my radio has audio-in and PTT on the same connection. This is a shielded cable also.

  • AGWPE can handle two independent radios by duplicating the schematic interfaces, but, using pin 2 on the stereo jacks instead of pin 1. Grounds of the two systems would still need to remain independent.


  • I debugged my interface by using the speakers from my PC and two clip leads. Starting with the receive side. I attached the shield of the speaker connector to ground and used a wire to the tip of the same connector as a probe. Follow the audio through the path starting at the input to R1. When you switch to the “other side” of the interface, don’t forget to move your ground lead. Once your SW packages are installed, you will see output in the UI-View monitor window when your receive side works. The monitor window is that white window in the bottom center of the UI-View display.

  • When debugging the PPT interface, a DVM (digital volt meter) works well. To check your PTT input, you should momentarily read 5volts on the input side of R5each time you send something. You should check this out prior to attaching your radio.

  • When you start to transmit and your station is operating correctly, you will see both the transmitted and received signal in the UI-View monitor window. The received signal is being re-broadcast from the digipeater. Note the letter ‘T’ or ‘R’ next to the timestamp at the beginning of each displayed line.

P.C. Setup

  • On your PC's volume control menu, turn the output and input controls for your sound card to about 1/3 scale to start. Keep in mind you could over deviate on your transmissions so if you have problems/inconsistencies, consider this as a possibility.

  • If you have both microphone and line inputs on your soundcard, use line-in as you can get away with a higher input level from your radio.

Radio Setup

  • Set your radio to 144.390MHz, turn off the squelch, and turn the volume way low. So low that if you hold the radio to your ear, you can barely hear noise and the packets. ARES D10 is talking about using a couple of frequencies for various purposes – stay tuned for more information as it develops.

  • On the HandieTalkie I use at my installation, I used the external speaker, microphone, and PTT jacks for connection.

  • Antennas. As with everything else in HAM radio, antennas are important. I use an Arrow J146/440. I’d agree with the reviews on Arrow’s website. Best antenna I ever bought for $45 including shipping. Received it the next day too.


  • Configure the AGWPE SW first. APRS uses 1200 baud and a serial/parallel port for PTT. TNC mode is “Soundcard” and I selected the “KISS Simple” mode. Under the TNC menu, select 1200baud and allow the program to adjust parameters.

  • Configure UI-View. The SW makes you read the help pages before you can use the software. Follow what they say to do. It seems to me when I setup the ”Comm Setup” menu. First set 1200baud, 8bits, 1stopbit ,no parity, and no Comm Port. Then set the Handshake mode to NONE and HostMode to AGWPE. I believe this cured some problems I had.

  • Configure the Station Setup item next. Seems to me enabling the main beacon is like the main transmit On/Off control. Unproto address is: APRS, WIDE2-2. 2-2 means a maximum of 2 digipeaters will be used in any transmission. Most of us beacon station text at the default 30 minutes.

  • Since we are using the sound card, you must DISABLE the Audio Announce feature of UI-View under the Options menu I believe. The interface won't work too well if you don't.

Software Information

  • AGWPE is SW for your PC's soundcard that essentially turns it into a TNC. Technically, it takes the audio and converts it to a software "socket". Sockets allow various programs to interface to your PC. AGWPE is freeware. I don't remember the website, but, if you search the web using AGWPE, you'll find it.

  • UI-View displays the information to you and controls your station. Internal to the PC, UI-View connects to a socket and this is how it connects to AGWPE. UI-View16 is shareware. Cost is around $16.50 depending on the exchange rate to become registered. Registered users can additionally download/run UI-View32. I found several enhancements over the 16bit version that make it even better and well worth the few dollars. I thought the 32bit version ran faster too. That said, either version would operate equally as well. The website is: www.ui-view.com.

  • Since we are talking about sockets, one cool thing you can do if you have a LAN at your installation is to have a radio connected to one PC and view the data on a different PC. This is possible because of sockets.

  • If you setup a weather station, I recommend checking out www.weather-display.com. Brian has some very universal SW for many weather stations. His SW currently costs $55 and generates the file you need for APRS. APRS interface is via a shared file called WXNOW.TXT. Weather-Display software includes a socket server so you can view weather on other PCs in your house as well if you have a LAN.

  • You can add more maps to UI-View. I suggest you contact Dave (K0DLB) or Richard (K0RAW). I got mine from Dave and use the Northern Colorado map most of the time. Another benefit of registering UI-View is the map interface so you can get very detailed.

Final Notes

Finally, if you're like me and want to try something quickly. Make a cable that makes the proper connections between your soundcard line-in connection (1/8" stereo) and your radio audio output/ground. Configure AGWPE as described. Set your radio and PC as described. Using your new cable, connect both. AGWPE creates a little MODEM looking icon in the lower right hand corner. You should see a "little green lite" flash intermittently if you are receiving. Don’t forget to turn OFF your radios squelch – very important. AGWPE uses the constant noise to determine what is a packet and what isn't. Now install and configure UI-View. When you have everything right, icons will appear on the map and text will start scrolling through the monitor window of UI-View. To transmit, you’re going to need to build the interface.

These are all the notes I have right now. Your additions are always welcome to keep this list up-to-date. Please e-mail ki0io@arrl.net with any suggestions/additions/questions. Around the Northern Colorado area, you can usually find me on 145.115 weekdays at commute times.

Rob, KI0IO