A friend of mine has threatened for the past twelve months to find and then purchase for me a old TNC. He finally carried through on that threat and found one at the Reno, NV HAM swap this in May of 2013. He found a MFJ-1270B (manual here). For those that don't know what a TNC - think data connectivity over the airwaves. Prior to cell phone data plans and even before WiFi the TNC device was one the few ways to share data over the airwaves. I won't bore you with other details as there are other sources of the history and use of TNCs.
When I first started in on the TNC I discovered a few things.
First, this device is controlled by TWO Z-80 processors. These weren't the cheap knock offs either - these were actually made by Zilog back in the day. I have a special place in my heart for the Z-80 - I first learned assembly with the Z-80. I recommend learning assembly to any software developer who has interest in programming micro controllers. Also this knowledge has provided some utility even during my Windows 3.1 days programming in Borland C/C++.
Second, I don't own a serial DB25 cable. Not even a USB/DB25 cable. I do own a USB/DB9. Pretty sure this won't cause a problem - reading over the documentation of the TNC confirmed this. Luckily getting blank tips and soldering a DB25/DB9 cable turned out to be the easy part of this exercise.
Discovered that the connector in the back of the TNC is essentially a DIN 5 pin - the same used in a "midi" interface connector. The sad part here is that MIDI only requires three pins to be active so my old sound blaster joy stick/MIDI connector won't work as plug. Kind a of a bummer as I had already cut the old SoundBlaster cord when I discovered this problem. I checked a MIDI patch cable I had laying around, it too only has three of the 5 pins connected.
My radio - a FT-897D has a DIN6 interface. Finding blank DIN 6 and DIN 5 tips weren't difficult - discovered them at Fry's Electronics for under a quarter. As you can't simply go buy a cable like this you have to find, create, or re-purpose cables/tips from other sources. While I found the right connectors at Fry's electronics I quickly discovered that my soldering skills weren't up to the challenge of putting an 18 gauge wire on a flat tipped soldering post (seriously who thought that would even work) - well not without bridging the DIN 5 or 6 posts together. I ending up buying a pre-built cable from packetradio.com.
My first attempt and setting up packet radio sadly failed. I was successful in getting the device to work in "local" mode I discovered that I simply wasn't able to connect properly. Thinking I might have to make some adjustments I discovered this site: http://ohiopacket.org/index.php/Calibrating_Audio_Tones_for_MFJ_TNCs which provided some great tips in tuning the MFJ-1270B for optimal functionality. I discovered my audio tones were out of sync and was successful in getting those corrected.
Despite the confirmation that the tones were now properly tuned and that they were sufficiently loud enough (confirmed by a friend in nearby Lincoln, CA) I just couldn't hear myself on the air using packet radio. I even checked to see if the audio coming from the radio was strong enough for the TNC to pick up and process. No matter what adjustments I made I just couldn't get connected. While I haven't given up on making this work, I am pretty convinced that I need to look at another device or another method of doing packet radio.
My first attempt and setting up packet radio sadly failed. I was successful in getting the device to work in "local" mode I discovered that I simply wasn't able to connect properly. Thinking I might have to make some adjustments I discovered this site: http://ohiopacket.org/index.php/Calibrating_Audio_Tones_for_MFJ_TNCs which provided some great tips in tuning the MFJ-1270B for optimal functionality. I discovered my audio tones were out of sync and was successful in getting those corrected.
Despite the confirmation that the tones were now properly tuned and that they were sufficiently loud enough (confirmed by a friend in nearby Lincoln, CA) I just couldn't hear myself on the air using packet radio. I even checked to see if the audio coming from the radio was strong enough for the TNC to pick up and process. No matter what adjustments I made I just couldn't get connected. While I haven't given up on making this work, I am pretty convinced that I need to look at another device or another method of doing packet radio.
