Electronic tinkering comes in many flavors. There is building from "scratch", there is kit building, and there is modifying store bought stuff. There is (at least) one other type of tinkering, the one that I want to address in this article, and that is hooking up two or more pieces of gear to work as a system. Specifically, this article is about hooking up a computer, a terminal node controller (TNC) and a transceiver in order to "go digital".

By going digital, I mean using such modes as AMTOR, PACTOR, RTTY, etc. I realize this is a giant leap from the little 40 meter transceiver that was presented in the first twenty, or so, issues of "Wanna Tinker?", but we (you and I) can handle it.

I have used a home brew TNC as well as a couple of commercial models, and they all present the same kind of challenges. Fundamentally, what you need to "go digital" is shown in Figure 1.

The transceiver can be either HF or VHF, the problems you will encounter are much the same whether you are on 80 meters or 2 meters. What is NOT shown in Figure 1 is the computer program (software), the cables and the connectors required to make the whole thing work as a system. Software is beyond the scope of this series of "Wanna Tinker?" articles, but the switching and cable hook-ups are not, and thereby hangs a tale ...

The first problem you are going to encounter is with the manual(s). The tech writers try to do a good job, I am sure. In spite of their efforts, however, the folks who write the manuals seem to leave out (or hide very well) those key items that you r-e-a-l-l-y need in order to get things working. Some manuals are better than others, but all seem to need improvement. Lest I be accused of promoting one brand over another, I will not name a particular product here. Instead, I will lump then all together and refer to a generic TNC.

Here’s an example of what I’m talking about: One commercial TNC uses an 8-pin DIN for the connection to your HF rig. There is absolutely nothing wrong with using an 8-pin DIN connector, but the plot thickens. 

I am using audio frequency shift keying (AFSK) as opposed to "true" frequency shift keying (FSK), so I need a cable from the TNC to my transceiver microphone connector. In the "getting started" section of the manual, I found the 8-pin DIN connections as shown below.

Pin 1 - Transmit Audio

Pin 2 - Ground

Pin 3 - Push to Talk

Pin 6 - Receive Audio

The manual also shows microphone pin connections for ICOM, Kenwood and Yaesu rigs. This is great because I’m using the TNC with my ICOM, so I wired the cable from the TNC to my ICOM as shown in Figure 2.

Nothing worked!

Why? Because (silly me) I assumed the TNC used the same 8-pin numbering scheme as did ICOM, Kenwood, and Yaesu.

Big mistake!

The pins used by TNC are shown in Figure 3.

How did I find the pin numbering scheme? The hard way, buy circuit tracing. 

Later (much later) I found, way back on the next-to-last page in the manual (no page number, and no listing in the table of contents) "Connectors", which revealed the following …

This is just what I needed to know BEFORE I started wiring the cables! How did they come up with this numbering scheme? I have no idea. At least, they got pins 5, 7 & 8 right, and three out of eight ain’t bad these days.

I know, I KNOW! I should have read the WHOLE manual before starting, but who does that?!

Sigh.

Want more? OK, how about this …

When I (finally) got my TNC up and running, another problem reared it’s ugly head. I could send short (2 minute, or so) digital messages, but on longer messages the TNC would drop the PTT line for no apparent reason. The "diddle diddles" kept coming on the TX audio line, but they weren’t going anywhere because the transceiver was in receive mode with the PTT line dropped. By watching the clock, I discovered that this happened about two and a half minutes into every message. "Probably a software timer of some sort.", I thought to myself.

Nope. 

After carefully reviewing all the commands (twice) I found nothing about a PTT timer. If there is no software timer, then it must be a hardware timer. Well, whadda ya know, a "Watchdog Timer" is listed in the index. Sure enough, by reading a couple of paragraphs in the hardware manual, I learn that the TNC comes from the factory with jumpers installed that limit transmission time to two and a half minutes. Easy fix, but there is no mention of the watchdog timer or jumpers in the start-up procedures.

Sigh (again).

The moral to all this? Read the (whole) manual, then read it again, BEFORE you plug in your soldering iron. (I know, nobody does that, but I have to say it, anyway.)

Connecting boxes with (correctly) wired cables is, unfortunately, not enough to get things going smoothly. I suppose it would be enough if all you wanted to do was digital, but you may, occasionally, want to use voice or CW. You certainly don’t want to disconnect, then reconnect all the cables every time you change modes, and that brings us to the next topic, which is SWITCHING.

I suspect that if you looked at all the thousands of digital stations you would find thousands of different switching arrangements. Several good articles about switching from digital to voice, from manual to computer control, etc. have appeared over the years, and I don’t plan to repeat that effort here. 

Next time, I will have a few more words about switching, then move on to other things. ‘Til then …

73, Dick, W6BKY