WHAT DOES THE TNC DO
====================

   The TNC consists of a special purpose microcomputer, con-
taining all the necessary software and hardware to communicate
with your terminal, assemble a packet, operate your trans-
mitter and receiver to send and receive a packet, and decode
a packet. The special functions of the TNC which would be
difficult to implement with an ordinary personal computer are
the use of protocol to communicate with other TNC's and
real-time control.
   The encoding and decoding of packets involves a carefully
standardized set of procedures called "protocol". The proto-
col basically determines the exact form of the header and tail
parts of the packet. The header allows receiving TNCs to auto-
matically determine the purpose of the packet, e.g., net
check-in, part of a QSO, or ACK to a previous transmission. The
tail contains the FCS which allows the TNC to automatically
determine whether the packet was received correctly, and if so,
to automatically acknowledge it. Since the protocol is pro-
grammed into the TNC, the operator does not need to know exactly
what his packet looks like. In particular, he does not need to
know how the destination of his packet is indicated. The oper-
ator communicates with other amateurs by call sign, and the
TNC translates the call sign into the identification required
by the protocol.
   The TNC is required to perform a number of tasks simultan-
eously, including responding to events such as the receipt of
a packet or instructions from the operator in "real time", in
other words, as they happen. This makes programming in BASIC,
the common language of personal computers, undesirable. This
is because BASIC useĀ an "interpreter" which reads each line
of the program and translates it into machine-type instructions
every time the line is executed. The time required for the
translation would prevent a program from responding rapidly
enough in a packet radio environment. In order to meet the
speed requirement, an assembly-language program or equivalent
is required. While BASIC looks pretty much the same on any
computer, assembly language is different for every machine.
If the TNC were replaced by personal computers, program dev-
elopment would have to be redone for each variety of com-
puter. In addition to maintaining the right pace, the TNC also
must be constantly "listening" at both ports simultaneously
while putting packets together or taking them apart. The
hardware of personal computers may not even be capable of this
sort of multi-task application.
   Programming of individual TNC's must be as easy as possible,
since there will inevitably be unforseen problems in the
initial software. In addition, hardware changes may necessitate
software changes. For this reason, TNCs are designed around
erasable programmable read-only memories (EPROM's), which
normally function like the ROM of a personal computer, where
the vital software is storaed in an indestructible form.
However, if the need arises, they can be reprogrammed by
"burning in" the new program using special equipment.