Some of you have asked about my "digi in the computer" projects. Here are some photos of the latest one, in progress. Actually, this one will be a dedicated Linux-based APRS machine, running on 486 computer (even though it says "16/286" on the front).

Power ConnectorsThe power for the TNC is derived from the computer's power supply. Shown here are two of the methods I use for making the connection. The cable on the left was made from an old CPU fan "pass through" power cable, while the one on the right started life as an adapter for powering a 3.5" floppy drive.

In both cases, the 12v wires (yellow/black) have a 5.0/2.1mm coax plug installed. This is the "standard" connector used by Kantronics and some other TNC manufacturers for power, but you should use whatever matches your particular TNC.

You can directly modify a drive cable coming off of the computer's power supply, if you do not have one of the above in your junk box. I chose this route because it isn't a permanent change to the computer case itself, and I had a LOT of the old floppy adapters laying around...

The Kantronics KPC-1, KPC-2, KPC-4, KPC-2400 and KAM (non-Plus) all use the same profile case, in different overall lengths. It is just slightly smaller in width and height than a half-height 5.25" floppy drive. So, what better way to handle it than mounting it in a drive bay?

Mounting holes  in TNC casesThis picture shows the holes drilled in the Kantronics TNC cases for holding them into the drive bay. The KAM case (top) only has one hole, while I got ambitious with the KPC-4 case (bottom), and drilled a "full set" of holes. Either way, make sure you remove the electronics before drilling and tapping, and clean up any burrs or shrapnel from the inside before reassembly!

Also note that the rubber feet have been removed from both cases, so as to not interfere with other devices mounted below the TNC.

My ultimate goal is to have everything, including the radio, mounted in the same case. However, that isn't always practical. So, the TNC's radio connection(s) need to be extended out to the back of the computer. Below is the cable set I use for the KPC-4, which has two DB-9F radio ports, and a DB-9M serial port.

TNC radio and serial cables The radio ports are wired straight through, with pins 1, 3, 5 and 6 connected between the DB-9F and DB-9M connectors. I used Cat. 5 network cable, because it provided sufficient wires (4 pairs) for this dual-port TNC.

The DB-9Fs mount to cut-outs in the back of the computer case, while the DB-9Ms are simply pushed onto the back of the TNC. Any cable designed to plug into a Kantronics TNC will then be usable directly.

TNC connection to serial port boardThe serial cable started life as a standard "motherboard to backplane" cable, use in many computers to bring out extra serial ports when the back of the I/O card doesn't have enough room... or the serial ports are on the motherboard itself. The picture on the right shows how this looks, with the parts outside the computer case.

In this case, I had to change the normal DB-9M to a DB-9F. Many computers have "extra" DB-25M versions of this cable, which is perfect for most Kantronics, MFJ, or whatever TNCs. This is my way of eliminating external cable clutter.

Below, we have the "finished" computer/TNC combination. In order to get the TNC inside the case, I had to remove the rear cap on the extruded case. For RFI purposes, though, it should be put back on, after the TNC is in place; this can be a problem, unless you have a very open case, or special tools (I have an angle-drive screwdriver that works well for this).

Computer with TNC mounted

I have built several APRS digipeaters using these methods, using Wes Johnston's APRSDIGI program for the software. Any old 80286 or better computer with a floppy drive can be used for this. This particular computer is getting a motherboard upgrade, and will be running RedHat 6.1 Linux and Xastir, although it might run XAPRS when it is ready.

page updated Tuesday, 1 August 12, 2000. Copyright 1999, 2000 by Jeff Brenton KA9VNV