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Title:  DRSI DPK-2 19.2 k baud Modification
Author: Steve Sampson, N5OWK
Date:   June 1994

   The Kantronics D4-10 radio comes equipped with both an Analog and a TTL
interface jacks on the back.  The Analog port is for speeds up to 9.6 k baud
and the TTL port is for 19.2 k baud.  This radio has a large bandwidth and is
designed to swing +/- 9.6 kHz deviation.  A TTL logic 1 gives +9.6 kHz and
a logic 0 gives -9.6 kHz deviation.  The task then is to provide a serial
data stream up to 19.2 k baud maximum to the TTL port.

  All TNC-2 clones can be converted to this speed.  In fact most are equipped
with a switch on the front of the PC board which is used to specify the data
rate.  These rates vary from 300 to 19200 baud and normally you never touch
this because the modem chip onboard only works at 1200 baud.  If you purchase
a 10 MHz CPU/SIO upgrade then normally these rates are doubled and you can
run up to 38400 baud, in which case you will need more radio to operate at
that baud rate.

  The DRSI DPK-2 is an all-CMOS version of the TNC-2 and is in the $120 price
class.  J4 is where an optional header is installed for external modems.  This
modification doesn't use a header, but does use two points on the header for
soldering a wire jumper.

        1.      Remove cover (4 Philips head screws).
        2.      Remove battery jumper (disable battery backup).
        3.      Remove PC board (4 Philips head screws, two hex screws).
                To remove the PC board you must remove the hex screws on
                the back holding in the DB-9 serial port connector.  Also
                you should use a screwdriver and slowly push each front
                panel LED out of the panel.  Once this is done, you can
                lift the front and pull out the power and DB-9 jacks from
                the rear.  Remove the metal DB-9 cover and set it aside.
        4.      Remove the PC board trace (using a sharp knife) between pins
                17 and 18, and 19 and 20.
        5.      Locate R-18 and C-8 on the board and remove the side going to
                J2 (the radio jack on the back).
        6.      Solder a wire jumper (wire wrap wire is good) from the removed
                R-18 side to J4 pin 17, and the C-8 removed side to J4 pin 19.
                Do not cross these wires in layout (to minimize cross-talk).
        7.      Put the metal shield on the DB-9 connector and put the PC
                board in the case and slowly push the LED back into position.
        8.      Replace the battery backup jumper (enable it).
        9.      Replace cover.

Theory of Operation

  Before modification the TNC would only operate 1200 baud AFSK (Audio
Frequency Shift Keying) to an FM radio.  This modification disables the SIO
data from reaching the internal modem and redirects the data out the rear radio
jack (J2).  It provides a switched baud rate to the rear radio jack using TTL
(Transistor-Transistor Logic).  The TNC will now only operate with radios that
are equipped with TTL data input.  The Kantronics D4-10 is a radio of this
type.  You can also modify any radio to convert TTL to the proper AFSK tones
and deviation.  For example to be compatible with most packet radio users, your
converter should translate TTL to either 1200 Hz or 2200 Hz at 3.0 kHz
deviation.  Probably a good start would be a BayCom modem installed right into
a radio wired to a DB-9 connector for the TNC.  But I digress, as the intent of
this modification is to use a TNC-2 for 19.2 k baud and the D4-10.  By the way,
the RF Carrier Detect pin must be used on the radio jack to the D4-10.

Functional Checkout

  When I first set up my 9600 modems I spent quite a bit of time working with
the deviation setting.  It seemed like it took a long time to even get the data
to flow across the house.  When I converted two TNC-2 modems and plugged them
into the D4-10's the result was instant gratification (my lifes goal) and I
only had to adjust the Transmit Delay on both TNC's to a point where they both
operated without retries.  This is truly a plug and go operation.  My next job
will be to seperate the radios over range and see how reliable they remain.  So
far I am quite impressed with the data throughput across the house.  I plan to
run these in KISS mode with KA9Q TCP/IP for large packet sizes and speed, which
can't be duplicated using Net/Rom or Rose.

Good luck,
Steve  (BBS) n5owk@k2gkk.ok or (Internet)


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