Amateur Radio - Free and Open Source Software

work, it req uires both stations 10 make adjustments
in the transmitter keyup delay value
(TX DELAY) in the TNC. (See the d iscussion
on setting TXDELAY later in this article) .
#3 - The Computer or Terminal
If you acquire a TNC you need a computer
or a computer terminal to display the data . If
you choose to use a computer terminal it
should be capable of asynchronous ASCII
operation. A terminal usually has everything
you need to communicate with the T NC builtin,
so all yoo need to connect the terminal to
the TNC is a cable (see below).
Computers are inherently dumb and must
be told what to do by software programs. If
you are operating with a multimode device or
a T NC you need a program for the computer
that allows it to operate as a terminal. Most
vendors of TNCs and multimode devices offer
programs for the more popular computers.
These programs are tailored to packet
operation and include features that make operating
packet radio more efficient, simple.
and enjoyable. On the other hand. almost any
program designed to allow the computer to
ope rate with a modem- such as C rosstalk or
Procomm- works fine .
If yoo get the add -in boa rd you'll need
some spec ial software. This software should
come with the boa rd , usually in the form o f
one o r more compute r d iskettes . Some
boa rds have publicly-available software .
Connecting the Tl"IiC to the Com puter
If you choose the add-in boa rd approach.
the connection is handled internally in the
computer. The manufacturer 's instructions
tell you how to properly confi gure the
jumpers or switches o n the board .
If, however , you have a TNC or multimode
co ntroller, obtai n a cable to co nnect the
device to your computer o r terminal. In order
for your personal co mputer to " talk" to a
T NC, make sure that it has true asynchronous
RS-232 serial port . (Some less expensive
computers do not adhere to RS-232 signal
level specs. These computers may not work
well with some types of T NC .) A se rial
port is one that accepts digital data seriallythat
is, one bit at a time. Most serial port
connectors on computers are either 08-25
males or 0 8 -9 females . Then buy o r configure
an RS-232 cable with the appropriate
connectors.
Some computers, such as the ever-popular
Commodore-Sa and the VIC-20, have TIL
signal levels at their serial ports. You have
several options here: find a TIU RS-232 Icvel
converter interface. or get a TNC/data
controller that use s TIL signal levels rather
than RS-232.
The simplest cable for connecting the T NC
to the terminal requires only three signals:
transmit data, recei ve data. and ground. On a
standard 08-25 RS-232 connector these are
pins 2. 3, and 7, respectively. Try the " three
wire" connection first to see if it works.
Connect the TNC to the terminal, tum on the
terminal, then tum on the T NC. Something
should appear o n the screen (although it may
be random " garbage" characters). If you get
16 73 Amateur Radio . October, 1989
absolutely nothing on the screen, your terminal
or computer probably requ ires some of
the RS~ 2 32 control signals in addition to the
transmit and receive data signals. Try connectin
g the signals "Data Set Ready" (pin 6),
" Data Carrier Detect" (pin 8). and " Data
Term inal Ready" (pin 20 ) together at the
terminal/computer end of the cable. You
should now get something on the scree n when
you tum on the T NC.
The next step is to ensure that the computer/terminal
and the T NC are sending data to
each other at the sa me speed (baud rate) and
format. Most packcteers use 4800 or 9600
bauds between the T NC and the terminal with
8 data bits. no parity. and one stop bit. Set the
terminal o r your terminal program to the desired
baud rate and these parameters. Some
TNCs have a switch on the bac k that lets you
set the data rate. In that case, set the baud rate
on the TNC to the des ired value.
Some TNCs have no external switch for
baud rate. These TNCs have an autobaud
routine that automatically determines the
baud rate used by the terminal . Usually you
wait until a legible message from the T NC
appears on the screen of you r terminal, then
you press a specific key on the keyboa rd. At
this point there should be communications
between the T NC and the terminal in both
directions .
Sometimes your terminal can see what the
TNC is sending but the T NC appears to ignore
everything you type . This is usually a
problem with the RS-232 connection. If this
occurs, try connec ting the " Request To
Send" signal (pin 4 ) to the " Clear To Send"
signal (pin 5) at the terminal or computer
RS-232 connection.
At this point the terminal and T NC should
be communicatin g. If every character you
type appears twice on the screen, set your
terminal or computer for full-duplex ope r­
ation.
Th is is a good time to set the terminal
control parameters of the TNC, Every TNC
or mulumodc con troller has some preset or
default values for its control characteristics.
You can change these parameters to make the
T NC more compatible with your terminal. If
you are using a C RT term inal or a computer.
enter the command BKO NDEL O N, After this
command the TNC erases characters on the
screen when you press the backspace key on
the keyboard . Some terminals generate the
ASCII rubout (delete) code instead of the
ASCII backspace code when the backspace
key is pressed (DEC VT-IOO and compatible
terminals). You can tell that this is happening
if you make a mistake then correct it with the
bac kspace key, and the TNC docs not recognize
what appears to be a perfectly valid command
. You can tell you r TNC to recognize
your backspace key with the commands
DELETE S7F or DELETE O~ . Read the manual
that comes with your TNC to find the appropriate
command.
Many terminals and term inal programs
provide some sort of line wrapping function .
Thi s means that if you type beyond the end
of a line you r typing continues on the next
line, The T NC also provides this feature.
If both are turned on, you see text on every
other line whenever the text extends beyond
the end of the line. To prevent this, either
tum off the line wrapping at the terminal or
send the following command to the TNC:
SCREENLO.
This is the time 10 experiment with send ing
commands to your TNC. You can get a feeling
for how it works before you hook it up to
the radio.
Connecti ng the T:\'C 10 the Radio
Perhaps the single most important thing
you cando to ensure reliable packet operation
is to properly connect and adjust the T NC to
the radio. First. decide where to connect the
two . Many transmitters offer a high-level
auxiliary input. Use this input instead of the
mic rophone input if possible. Many microphone
inputs do some equalization or signal
processing that can d istort the signal generated
by the TNC thereby making it difficult for
other stations to decode your packets . The
signal at the aux input often bypasses these
stages and provides bener performance.
The aud io from the receiver needs to get to
the TNC. Although the signal from the speaker
may work. it is almost always distorted by
the audio amplifier. The result is that the
TNC fails to decode otherwise good packets.
Many radios offer a low-level audio output
for a phone patch or selective calling unit.
This low-level output is a much better choice
than the speaker jack. If you r radio docs not
offer a low-level output, try laking the signal
from the " hot" side ofthe volume control.
Once you have the T NC connected 10 the
rad io, adjust the transmit sig nal level so that
the signal from the TNC proper ly modulates
the transmitter. Proper adjustment here is
critical for good results. T here arc two techniques
for setting the transmit level of an
NBFM transmitter.
The first technique requires a deviation
meter. Set up the deviation meter to read the
deviation of the transmuted signal. Turn on
the transmitter and the TNC and enter the
calibration command. If you r TNC offers the
option of selecting either the high or low
tone , select the high tone and adjust the signal
level at the TNC for 2.5 to 3 kHz ofdeviation .
If your calibration command alternates between
the mark and space tones at a rapid rate
(Kantronics' calibration command doc s this)
set the level at the TNC for 2.0 to 2,5 kHz of
dev iation.
The second technique does not require a
deviation meter but it docs require a receiver
and an AC voltmeter. Connect the voltmeter
to the output of the second receiver so that
you can get a relative indication of audio
output. Set the frequency of the receiver to
the output frequency ofyour TNC/radio pair.
Tum on the transmitter and the TNC and
enter calibration mode. Select the high tone
and increase the TNC's transmit signal level
until funhcr increase of the level at the TNC
no longe r results in an increase in the level
indicated by the AC voltmeter connected to
the second receiver. NOIe the reading on the
voltmeter. Now dec rease the signal level at
the TNC until the voltmeter connected to the
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