TNC-X: An Expandable Microcontroller-Based Terminal Node Controller

摘要

TNC-X is a new Terminal Node Controller design based on the Chepponis/Karn KISS protocol. It is implemented using a Microchip PIC 16628 microcontroller, a CML MX614 Bell 202 modem chip, an 8K Ramtron FRAM, a Maxim MAX232A level converter chip, and a dual op-amp which provides active audio filtering for the modem. From the beginning, this TNC was designed to be small, have low power consumption, and, most importantly, be expandable. Expandability stems from two sources. First there is an on board socket for a DLP-USB232M USB module which provides the TNC with a USB port. Drivers that are shipped with the module make it appear to the host PC as a standard serial port. Thus PC software that expects to see a serial port on the computer will interpret the TNC-X as being connected to such a port, even when the PC has no serial ports, or they are all used by other applications. In addition, when the USB option is used, the TNC can be powered from the USB port of the computer; no other power supply is needed. TNC-X is also expandable via an expansion header that allows the addition of "daughter boards". Power is provided to the daughter board through the expansion header. In addition, signals that would otherwise go to or from a host PC can be intercepted by the daughter board at the TTL level and processed. The I/O on the expansion header speaks "KISS" so that any daughter board only has to send and received data packaged in KISS format (see section 4, below) to access the core module. This makes the development of daughter boards fairly simple and inexpensive. For example, one could use a PIC (or other microcontroller) to create a daughter board that would convert TNC-X into a stand alone digipeater, simply by intercepting the data stream that would normally be sent to the PC, reformatting it (perhaps with call sign substitution) and send it back out the expansion header. The developer of such a board wouldn't have to worry about the usual nasty details of CRC calculations, bit stuffing, buffering data until the channel was clear, and timing the bits to be sent to the radio, because that would all be handled by the "core" TNC-X module. The parts count of such a daughter board would be extremely low. The most expensive item would likely be the printed circuit board itself. In this configuration, a portable digipeater could be constructed for emergency operation that is extremely cheap and has very low power consumption.