For the purposes of building a TARPN, the $40 TNC-PI kit is a base level device. Don't spend much more than that for a 1200 baud TNC and don't suffer with a worse working device.
Addresses Each packet includes, at the start of the packet burst, the callsign of your station and the callsign of the destination station. That means that if you chose, your station can reject any packets that are not to you. Secondly each packet station only transmits for long enough to get across its short message. Thus several hams can use a single frequency for conversations without having to listen to each of the other conversations.
In normal packet operation you would type a carriage return after each line of text that you are sending to another station. After you type the carriage return your packet station will wait for a quiet moment on the frequency and then send its message. If you have specified an intermediate station in the path to your friend then the intermediate station will hear its call in your message and will retransmit your message, only if the message is received perfectly and after the intermediate station sees the frequency quiet. Then your friends station will hear the message and send back an acknowledgment which is picked up and echoed by the intermediate station. When your station gets the acknowledgment it will go on and send your next line of text when you hit the next carriage return. If you have already hit the next return then your station will immediately start looking for the frequency to get quiet and will then transmit the next line. If your station waits for a preset amount of time and doesnt get an acknowledgment for its packet it will send another one. This will repeat until the message gets through or your station sends RETRY amount of times, (usually 10). The form of communications where your station waits for a quiet moment and then transmits its message is calledCarrier Sense - Multiple Access or CSMA
On the other side of the circuit, a TNC has a computer connector. TNCs made before year 2005 exclusively used RS-232 to talk to the computer. More modern TNCs may be USB or some specialty bus made for an embedded computer. The TNC-PI, which is made for the Raspberry PI, uses I2C which is a multi-station TTL-level serial bus, permitting several TNC-PI to be connected to a single Raspberry PI, via the expansion connector on the Raspberry PI.
Click to zoom on these images.
On the left photo from the top:
All TNCs will run on 12v unregulated DC. D4-10 radio needs regulated, I think.
There are many more TNCs made before 2005 or so which can be supported using the same cabling. Kantronics, for instance, made a line of dual-radio TNCs supporting 300 and 1200 baud, made to support an HF radio and a VHF radio at the same time, or 1200 on one radio and 9600 on the other radio, at the same time. I'm pretty sure we can run both radios at a time with a single TNC on the G8BPQ node code. We'll have to try it.
There are several companies making modern TNCs which are sold to support APRS packet radio. Some of these do not fully support the protocols required by our node software. These include models by Argent Data. I own a pair of Argent 1‑watt UHF 9600 baud integrated radio/TNCs based on the Tracker-3 family. These do not allow transmission of full‑length packets and completely fail in node service. That's too bad because they were under $100 and had USB connectors.