KD7LMO - Payload Computer


Payloads Payload Comp Overview Block Diagram Processor GPS TNC Local I/O Repeater Diplexer Commands Source Code Mechanical Telemetry Data Logger Micro Beacon Cut Down HF-APRS Video Link Pico Beacon NSRV Beacon APRS Beacon

TNC

The TNC (Terminal Node Controller) is a MODEM connected between the payload computer and a radio. It modulates and demodulates a 1200 baud AFSK (Audio Frequency Shift Keying) signal. The serial bit stream is provided to the payload computer that processes the packet radio data. In addition, it sends APRS (Automatic Position Reporting System) packets that provide location and system status information. The best source of specifications and information on packet radio and APRS is available from TAPR (Tucson Amateur Packet Radio). The following are the key documents used to implement a TNC.

	AX.25 Link Access Protocol, Version 2.2
	APRS Protocol Reference, Version 1.01

These documents are saved as a Adobe Acrobat PDF file. If you don't have the Adobe Acrobat Reader, you can download it for free from Adobe. Adobe has a PDF viewer for Windows, Linux, Solaris, SGI-IRIX, and even the mac.

Lessons Learned

Rather than reiterate the information contained in the AX.25 and APRS specification documents, the following are lessons learned during the implementation of the TNC MODEM.

NRZI Encoding Very little information is provided about the physical connection of the TNC system. One note of interest is the data stream is encoded using the NRZI (Non-Return to Zero Inverted) format.

AFSK vs. FSK The TNC utilized an AFSK (Audio Frequency Shift Keying) modulation scheme rather than a FSK (Frequency Shift Keying) system. AFSK allows the use of any transceiver with an external audio connection. In a traditional FSK transceiver, the carrier is at one of two frequency to indicate a zero or one data bit. The AFSK modulator utilizes two audio tones at 1200 and 2200Hz.

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