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World of Ham Radio 1997
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packet14.doc
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1997-02-01
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INTRODUCTION TO PACKET - Part 14 - by Larry Kenney, WB9LOZ
TNC COMMANDS - continued from Part 13
MAXFRAME Sets the upper limit on the number of unacknowledged packets the
TNC can have outstanding at any time. (The outstanding packets are those
that have been sent but have not been acknowledged.) It also determines
the maximum number of contiguous packets that can be sent during one
transmission. Value can be set from 1 to 7. The best value of MAXFRAME
depends on the frequency conditions. The better the conditions are, the
higher the value you can use. If conditions are poor due to the amount of
traffic on the frequency, noise, or other variables, (shown by lots of
retries) MAXFRAME should be reduced to improve throughput. The best value
of MAXFRAME can be determined through experimentation. MAXFRAME of 1
should be used for best results on HF packet.
MFILTER This command allows you to enter up to four ASCII character
codes, 0 - $7F, for the control characters that you want eliminated from
your monitored packets. Codes may be entered in either Hex or Decimal.
Here are the ASCII codes for some of the more troublesome control
characters found in monitored packets:
HEX DEC FUNCTION POSSIBLE RESULT
$07 07 Control G Rings your bell or "beeps" your speaker
$0C 12 Control L Form feed - could clear your screen
$13 19 Control S Can cause your screen to stop scrolling
$1A 26 Control Z Can clear your screen
$1B 27 Escape Can cause your cursor to move to a random
point on your screen and can raise havoc
with printer control.
AEA has added a new code, $80, that will not allow ANY control characters
to be displayed on the user's screen from monitored packets.
MHEARD An immediate command that causes the TNC to display a list of
stations that have been heard since the command MHCLEAR was given or the
TNC was powered on. This command is useful for determining what stations
can be worked from your QTH. Stations that are heard through digipeaters
are marked with an * on most TNCs. On the AEA PK-232, the stations heard
direct are marked with the *. (Check your TNC manual.) The maximum number
of stations in the list is 18. If more stations are heard, earlier entries
are discarded. Logging of stations heard is disabled when the PASSALL
command is ON. If the DAYTIME command has been used to set the date and
time, entries in the MHEARD list will show the date and time the stations
were heard.
PASSALL Causes the TNC to display packets that have invalid checksums.
The error-checking is disabled. If PASSALL is ON, packets are accepted for
display, despite checksum errors, if they consist of an even multiple of
eight bits and are up to 330 bytes. The TNC attempts to decode the address
field and display the callsigns in standard format, followed by the text of
the packet. PASSALL can be useful for testing marginal paths or for
operation under unusual conditions. PASSALL is normally turned OFF.
SCREENLN n This parameter determines the length of a line of text on the
terminal screen or platen. Value may be 0 to 255. A (CR-LF) carriage
return and line feed are sent to the terminal in Command and Converse modes
when n characters have been printed. A value of zero inhibits this action.
If your computer automatically formats output lines, this feature should be
disabled.
TXDELAY n This parameter tells the TNC how long to wait before sending
data after it has keyed the transmitter. All transmitters need some start
up time to put a signal on the air. Some need more, some need less.
Synthesized radios and radios with mechanical relays need more time, while
crystal controlled radios and radios with diode switching require less
time. External amplifiers usually require additional delay. Experiment to
determine the best value for your particular radio. TXDELAY can also be
useful to compensate for slow AGC recovery or squelch release times at the
distant station.
There are many additional commands available to you. I've only covered the
ones that I thought would be the most useful to you. Spend some time
reading the owner's operating manual that came with your TNC to discover
some of the surprises the other commands offer. New versions of the TNC
software have added several commands that you might find useful in your
packet operating.
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