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TN211-5.DOC
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1993-08-09
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TN211-5.DOC
GETTING STARTED
*******************
The following steps should be taken in the process of getting a TNPlus node
operational:
TNC Selection
"Net" type chip nodes work only with TNC-2's or clones. Except for the
PK-87/ PK-90, other AEA models and the entire Kantronics series will NOT work
as a TheNet node. Multi-mode type TNCs will NOT work for this application.
Use only a TNC-2 or clone (Pac-Comm, TASCO, MFJ) that has the full complement
of 32K memory installed. Using older 16K equipped TNCs will result in failure
of the node to work. The TNPlus node chip is a programmed type 27C256 EPROM
and replaces the TNC firmware chip U-23 in most TNCs. This chip should
have a label identifying the software version such as "TNC 2 1.2.6" or, "TNC 2
1.2.7."
CAUTION! CMOS handling precautions are necessary when removing/installing
these chips. Never replace components with power applied. The safest
procedure is to ground yourself, your work area and your TNC during the chip
replacement process. Keep the TNPlus chip in its protective container until
time to plug it into the TNC.
TNC MODIFICATIONS
Prior to modifying the TNC for TNPlus operation, make sure it is
functioning normally. Then install the TNC modifications. These consist of:
a. Connect a small wire from RS-232 pin 23 to the "common" side of JMP
9, (the three pins on the TNC facing the front panel).
b. Perform VHF or HF DCD modifications, as appropriate. These
modifications were developed and documented by Eric Gustafson, N7CL in the ARRL
7th Computer Networking Conference papers. They yield improved performance
and will improve node operation. For the TNC-2 or clone, the VHF modification
is extremely simple. It consists of adding a capacitor and two resistors to
the circuit board: Replace R-73 with a 180K ohm resistor. Place a 180K ohm
resistor paralleled with a .01 Mfd capacitor on the underside of U-20, between
pins 3 and 6.
Note: To perform the above modification, it will be necessary to de-install
the TNC circuit board. An alternative method of doing these mods would be to
purchase a TNC DCD modification kit from the Tucson Amateur Packet Radio
Association (TAPR), telephone (602) 749-9479. Specify the TNC model number
when you order.
c. On earlier version TNC-2s it will be necessary to increase the CPU
clock speed to 4.95 MHz. Check the TNC documentation on how to do this. TNCs
made after 1990 probably operate at the faster clock speed. Early version TNCs
had an LM324 opamp at U-3, which is not fast enough for 9600baud RS-232
operation. If the TNC has a different version, it probably will operate okay.
If it does have the LM324, it can be replaced by a TL074 or TL084.
Modification of the watchdog timer to increase its' time-out value does not
seem warranted as the 12-second timer value is sufficient. The Pac-Comm Tiny
2 TNC comes TheNet ready. It may require a more complex DCD modification kit,
also available from TAPR.
ALIAS SELECTION
Selecting the alias that is "just right" for node use has always been a
problem. From early on it was recognized there would be a high probability of
alias duplication if individual NodeOps were to make independent choices. One
method that's used widely, is to use airport designators, as these are
centrally assigned to prevent duplications from occurring. But this method has
not been universally adopted. Often the airport designators only had meaning
for local residents. They fail to adequately describe the node location for
users connecting from a distance.
There currently are node alias duplications in the network. This is not
too serious of a problem if the nodes affected are separated by some distance.
However in California we find two SFO nodes and two FAT nodes. This can
confuse node travelers. As the use of HF gateways increase, distance no longer
prevents duplicate alias confusion. For instance, there is FST in California
and FST in Texas; SAN in California and SAN in Maine, etc.
Some state packet groups have adopted the policy of prefacing their alias
with the state abbreviation. This certainly cuts down on the chances of alias
duplication. Another method might be to solicit the central data bank K4NGC
maintains. This data bank contains a listing of node aliases world wide.
Possibly this source would be willing to act as an information service to
find if a node alias already exists or not. In any event, it would be helpful
for maintaining his data base if NodeOps would advise by message of additions,
deletions or changes to node status by giving: type of node (G8BPQ, TheNet,
etc.), node alias, call sign, SSID, location and frequency of the affected
node. This information should be sent to K4NGC @ K4NGC.VA.USA.NA or,
via LL BBS at (703) 680-5970.
By convention, aliases for backbone nodes have the pound sign (#) prefacing
them. These are the so-called hidden nodes that typically are used to tie a
LAN frequency to a network trunk. "Pound nodes" are intended to be invisible
to the network traveler and will not appear in response to a standard user
initiated NODES command.
NODE RADIO CONSIDERATIONS
Radios selected for node use should be capable of heavy duty use. The T/R
switch circuitry should be able to handle virtually millions of operations
without failure. This means PIN diode T/R switching as a first choice followed
by high quality reed relay switching. Receiver front-end filtering should be
quite sharp if the node is to coexist with other radio services. One or two
tuned cavities may be required to cut down on front end overload and desense.
If the radio is operating on a simplex frequency, the cavities will aid in
reducing the effects of "white noise" generated by the transmitter. At
congested sites a circulator or diplexer may be required.
PLL synthesized radios should be avoided for backbone trunking service.
Two reasons: PLL settling time between transmit - receive is too slow for
optimum packet throughput. Assuming a TXD of 500 milliseconds (a not too
uncommon value), throughput would be cut in half. For instance, 9600 baud =
4800 baud. Also, the transmitter may be keyed before stabilizing on
frequency. This could cause interference to local receivers on different
frequencies.
Retired commercial service FM radios, such as the Motorola MICOR and GE
MASTR II, or later, make excellent node radio choices. These radios will
operate in moderate to high intensity RF environments. They are physically
rugged and reasonably priced on the used market. They are available in power
levels of up to 110 watts and are normally in different commercial frequencies
adjacent to the amateur bands. Thus be prepared to do some modification to
get one of these radios on frequency. The extra effort is usually worth it
since one will end up with a very stable and reliable node radio.
Most VHF/UHF FM radios are designed for voice grade service which isn't as
susceptible to phase distortion as is data service. As data rates exceed 1200
baud, the radio's relatively narrow IF filter adversely contributes to phase
distortion. Many commercial FM radios will perform satisfactorily to 4800
baud when using modems designed for amateur packet. Higher speed modems
(such as the K9NG and G3RUH designs) require radios with true FM modulators.
HAPN 4800 baud modems will work with phase modulators. At 9600 baud one's
choice is limited to radios with true FM modulators. None of the recently
introduced data radios for amateur packet come ready for full-duplex operation.
One reasonably priced surplus commercial radio that operates full-duplex
at 9600 baud is the Motorola Mitrek. However the modification process does
require access to high quality test equipment. A simpler (and perhaps better)
alternative is to use two radios, one for receive and one for transmit, in
full-duplex applications.
RADIO ALIGNMENT
Assuming the node radio is tuned and on frequency, setup for FM radios
consist mainly of adjusting the transmitter deviation for NO MORE than 3 KHz.
TNPlus versions 2.05 and later have a built-in tone alignment capability to aid
in setting deviation. A range of 2 KHz to 3 KHz deviation for alternate tone
frequencies is typical. Even if high quality crystals are purchased, normal
crystal aging effects may require frequency corrections over the next 30 to 90
days.