Kenwood TM-255E - Manual (Page 29)

4 COMMUNICATION
PACKET OPERATION
One of the most exciting benefits of owning a VHF or
UHF transceiver nowadays is being able to use it for
data modes. Due to the ease of setting up a digital
station, even those inexperienced with this type of
operation can be on-the-air enjoying these new
modes quickly.
You can use almost any computer to control one of
the widely available Terminal Node Controllers (TNC)
since the computer primarily serves to input
commands and output received text data to its
display. Little real computing power is needed,
therefore a high-powered processor is not necessary,
and even a "dumb" terminal is satisfactory, at least to
start. As you become more experienced, you will
realize how well your interest in radio meshes with the
world of data communications.
Connecting to one of the many stations with gateways
to HF or satellite links can give you national and
worldwide messaging or conferencing capability with
other Amateurs with nothing more than your single
band VHF/UHF transceiver for the communications
link. Much reference material is available for getting
started in digital communications from any store that
handles Amateur Radio equipment. Or, if more
convenient, check the radio magazines for mail order
bookstores.
The most common mode used on VHF/UHF
frequencies for Packet operation is FM. However,
there is also some SSB PSK activity on the satellites.
Type Of
Keying
Modulation Mode /
Menu B, No. 77
Transmis­
sion Rate
Emission
Type
OMSK.
G3RUH,
etc.
FM/ON 9600 bps FID
FSK FM/ON or OFF 1200 bps F2D
PSK SSB/OFF 1200 bps G2D
Menu B,
No. 77
input Impedance Standard Modulator Input
OFF
10 kQ
40 mVp-p
ON
10 kQ
2 Vp-p
Consider the following before setting Menu B, No. 77:
• OFF: Transmit data input (PKD) sensitivity is
40 mVp-p. This is suitable for a typical
1200 bps TNC or other data
communications equipment.
• ON: Transmit data input (PKD) sensitivity is
2 Vp-p. This is suitable for most 9600 bps
TNCs. Set Menu B, No. 77 to ON if using a
TNC with dual speed capability that only has
a 2 Vp-p output. However, in this case, FM
mode must be used.
Using a modulator input level that is far different from
the optimum 40 mVp-p / 2 Vp-p specifications may
result in deterioration of S/N ratio or signal distortion.
This could result in increased errors or a complete
failure to connect with other stations.
If the modulator input level rises to approximately
4 Vp-p, the transceiver automatically switches to
Receive to prevent transmitting a distorted signal.
Transmission is not possible until the input level is
reduced by adjusting the TNC modulation level.
To receive after making the necessary connections as
explained in "PACKET EQUIPMENT" {page 5}, use
the following procedure and refer to your TNC manual
for further assistance.
1 Select the desired frequency.
2 Select the correct mode by pressing [AUTO/FM] or
[SSB/CW].
3 Select the correct Menu B, No. 77 (page 14}
selection based on the type of keying, transmission
speed, and TNC that you are using (see charts).
4 Turn the VOL control clockwise to set a
comfortable level if you plan to listen to the data
exchanges. Otherwise, leave the control fully
counterclockwise.
• The VOL control setting does not affect the
receive audio level from the DATA connector on
the Rear Panel.
5 Adjust the SQL control until the "BUSY" indicator
on the Display just disappears (threshold) while the
frequency is clear of any activity.
After beginning to receive stations, use the
Main Tuning control for minor frequency adjustments
to compensate for frequency drift while watching the
tuning indicator on your TNC. Do not use the RIT
control since it is important to keep your receive and
transmit frequencies equal.
To transmit, proceed to the subsequent steps.
6 Commands sent from your communications
terminal (either a computer keyboard or a "dumb"
terminal) to the TNC control your transceiver.
• Refer to your TNC instruction manual.
• Be courteous. Although packet protocol can
handle multiple stations on a single frequency,
overall throughput decreases due to packet
collisions during busy times of the day. Keep
transmissions short at these times.
7 Adjust the output level from the TNC while
watching the RF meter to avoid output power
saturation. Adjust for a maximum meter reading of
10 with a steady mark or space.
Note:
Inputting 9600 bps OMSK signals at too high a level or inputting
significantly distorted signals into the transceiver can cause
errors and a wide transmit bandwidth that may interfere with
other stations.
Always tune your transceiver to the exact frequency of the
station you are contacting. Failure to do so will result in
inaccurate demodulation of received packets and multiple
retries. Use the Main Tuning control (-5 kHz to +5 kHz) for
making fine adjustments as necessary.
22