When it comes to portable packet, SIZE is probaly one of the most important factors. Going with that, I find that the PacComm PicoPacket is the best TNC for the job. It uses standard TNC-2 commands and is very much like a Kantronics or MFJ TNC. PacComm also offers the HandiPacket which is much like the Pico except it has a built in mailbox, much like a KPC-3 and also contains a rechargeable battery pack.
For a computer or terminal, I have found many different kinds and shapes. The most common that come to mind are small laptops and Palm OS systems. While I am a user of these, I have also found there are many other palmtops out there that will work. One, they need to have a built in terminal program or the ability to have one downloaded to it. I have several examples of both situations. My Sharp OZ-3000 came with a terminal program built in. My Sharp OZ-770 came with the ability to have programs downloaded into it. I was able to find a simple terminal program online for it and viola! Any of my Palm units had to be loaded with a Palm OS app terminal.
Another featured required would be the ability for the unit to send and receive data on a RS-232 level. You can find a good page about RS-232 here. Most units will be able to do this out of the box with either a homemade cable with the correct configuration or a cable that does level conversion of the signals. Palm units made by 3com all have true RS-232 levels from the com port. Sharp Wizards and like models have TTL out and in but only require the factory PC-cable to conver the signals. My true test of this was with a Handspring Visor. Although it runs Palm OS, the PC interface is true USB-TTL configuration and required a serial to TTL/TTL to serial cable witch I had to make on my own. The schematic can be found here.
The PacComm Pico Packet is the primary TNC that I like to use when I'm operating packet in a portable setup. The TNC will run on a nominal voltage of 6-15 vdc according to the manual. I found that at 6 vcd the TNC likes to "act up", a lot, usually looses all parameters. For portable operation the lightest power supply is most appealing. A 9 vdc battery will run the Pico for about an hour. Oddly enough, for it's size, it seems to drain the current fast. When I'm running it in the car a 9 vdc power plug runs the Pico just fine. But it does not like power spikes that much.
For the radio and computer connections, the Pico uses standard 6-pin connectors. I usually run the 6 conductor flat wire from the Pico to 9-pin DB connectors housed in a typical case. The only difference is the housing has a 8 pin socket on the back side to accept the 6 conductor connector.
Below is a diagram of the 6 conductor connector and the coresponding plugs for a standard dual-pin connection to a Icom/Yaesu/RadioShack like HT. The two plugs are 1/8" for the incomming audio and 3/32" for the audio out and PTT control. When wiring for a Yaesu VX-1R I was able to leave out the 0.1uF cap and 330 ohm resistor. These would normally be used to seperate the audio and PTT signals from crossing each other. Some radios MAY require these. For the VX-1R I wired for a normal dual-pin configuration and used the dual-pin to single pin adapter available from Yaesu. If you wanted to use the VX-1R as your primary radio, you could easily replace the dual-pins for a single 4-conductor 1/8" plug. These can be found at www.pl-259.com.
Normally an HT would be ideal for a portable setup. I like to use my Yaesu VX-1R, it's about the size or a pager and puts out 250mW on the internal li-ion battery. The interface is easy to wire. It uses a special 1/8" plug that has 4 metal bands instead or the typical 3 band stereo plug. I use a adapter that takes this single plug to the standard 1/8" speaker jack and 3/32" mic jack. This way I can wire my TNC to radio cables to work with ALL or my HTs.
With almost all palmtops, you must use a null-modem configuration for your TNC to serial connection cables. All this means is that the TX and RX RS-232 lines are reversed. This can be achieved by wiring the serial connector for the required configuration or a null-modem adapter can be put in-line to allow the cables to stay in a standard configuration. This is somtimes ideal when a palmtop and a laptop are both used regularly. Since the the laptop does NOT require the null-modem, a standard wired cable can be used and when a palmtop is used, a null-modem adapter can be easily implemented. Keeps everyone happy :o)
For the Sharp OZ-3000 I had to solder a few jumpers inside of the PC-Sync cable. This cable was capable of converting the TTL to RS-232 levels but needed to be modified some in order for it to talk to the TNC. This allowed for RS-232 handshaking. It came with a DB-25 female connector. I chopped it off and replaced it with a DB-25 male. I also reversed the TX and RX pins to allow for the null-modem configuration, so I would'nt need an external adapter.
The OZ-700 was the most simple to set up. I downloaded the terminal program to it, put a null-modem adapter in the line and hooked it straight to the TNC and it kicked right in. No mods, right off the shelf equipment. It makes a great station to monitor packet activity and connect to local BBS's. I have also used this to connect to friends and talk keyboard to keyboard while in my truck.
On the left I have a DB-9 male that is setup with a null-modem configuration to connect to any of my palmtops, no adapter needed.
On the right I have a straight serial DB-9 female setup to be connected directly to the com port of any computer. Since the computer is setup for the serial input, no null-modem adapter is needed.
Here are all 3 Palm PDAs I have: Handspring Visor Deluxe, 3com IIIe and Handspring Prism (color).
Here are the connectors on my serial cable I made to convert the TTL signals to and from the HS units. The gray cable is the factory cable for the 3com units.
Here are the cradles for HS and 3com units