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Adding a Data Port to the Kenwood TM-281a

by Lloyd Gregory - W4LCG

The purpose of this article is to share how I added a data port to an inexpensive Kenwood VHF radio. These radio's run about $130.00 brand new and it's the cheapest way I can find to use APRS mobile with 65 watts and still use the radio for VFO and repeater use. The Kenwood TM-V71A includes a data port built in but costs $200 more than the TM-281A. Off the shelf parts from companies like Argent Data Systems would make it very easy to connect a TNC to the TM281A using the speaker and mic jack, however the radio would not be able to hear voice alert calls on the APRS frequency or easily switch to VFO/repeater opertion. This would be fine if we just dedicated a radio to APRS only operation like my digipeater but I wanted keep the radio/antenna count low in my truck so I opted to expand the capability of  the budget priced Mobile Radio to function more like it’s higher priced sister.

Figure 1

The first order of business after opening up the radio was to drill a hole for my cable to pass through.  I used blue masking tape (sticky side out) to catch any aluminum shavings that were created while drilling the hole for the cable to pass through. This worked really well and I would highly recommend it to keep conductive items from getting into the radio and shorting it out.

Figure 2

Figure 3

After drilling the hole in the case I installed the cable (figure 4). The cable I used is a PS2 extension cable for a computer keyboard or mouse. It will have a male and female connector making it perfect as you have both sides to make a cable to connect to your TNC also. These can be found on Ebay for a few bucks and they are the same as the TNC connector on some of the Japanese radios. This makes widely available TNC to mini Din 6 cable plug and play if you wire the plug correctly. You could use any cable connector combination for this like a DB-9 serial connector and still be somewhat industry standard.

Figure 4

This would be a good time to use a DVOM to confirm what pin each wire color corresponds to. Do not go by my wire colors as your cable may be different. I found a small paperclip made it easier to access the female pins of the mini din 6 connector.

I made the wires plenty long so they could reach all the way to the faceplate when it tipped out to avoid any wire splices inside the radio.

As seen in (figure 6) I soldered a red wire to the board where it is marked “DTMF IN’. This is the discriminator audio tap. Direct any question about why this is maked “DTMF in” to Kenwood. My soldering was awful on this but I’m sure you can do better. If you don't have any experience soldering on a board like this consider practicing on an old computer motherboard or some other kind of scrap piece first to hone your skills. Even my soldering works fine even though it is not always pretty.

Figure 5

Figure 6

I carefully soldered the ground wire to that pin in the back as seen in (Figure 7). I used liquid electric tape to protect the bare ground wire since I could not find my heat shrink. A zip tie on the inside around the cable keeps the wire from pulling through.

Figure 7

Figure 8

Figure 9

Figure 10

My first connection on the back of the faceplate was for PTT as seen in (figure 9). I did not bother to remove the ribbon cable to the faceplate as there was plenty of room to complete this without damaging anything.  Then I soldered the brown wire to the TX audio tap as seen in (figure 10). 

That's it 3 connections plus ground and I have a mini din 6 connector like the more expensive radios. 

I use the Mobilinkd TNC external and made my own cable to connect from the TRRS connector on the tnc to the mini din 6 I added to the radio. You could use a different connector to the radio like a DB9 serial port if that works better for your application. 

The Mobilinkd TRRS TNC plug pin out is:
Sleeve is ground.
Ring2 is audio out (TX)
Ring1 is PTT
Tip is audio in (RX)

(Figure 11) was final assembly and testing. The PS2 cable was left way too long and needs to be shortened still to this day. I typically use this mobile with an android phone/tablet running APRSdroid with really good results. Keep in mind the tablet will need GPS or else APRSdroid will have no idea where you are. This connection could be used for other data modes than APRS like checking Winlink email through a VHF node or PSK is even possible. 

Figure 11

If you would like to make your own TNC to work with your modified ™-281 you could follow Gary Griffin’s excellent instructions at

Thank you to Arcom who’s instructions made this project possible can be found here.

Information about the mini din 6 port can be found here

Mobilinkd makes great TNC’s that work for a variety of applications and it is one of the few companies around that encourage you to hack their products.

APRSdroid is the best phone app bar none

You are reading this on the club website, please take some time and check out the other projects and resources. Also stop by the club facebook and let us know what you think

Constructive Criticism or fresh ideas are always welcome, I can be contacted at W4LCG.APRS at gmail or through facebook.

APRS is a registered trademark Bob Bruninga, WB4APR.