*** TNC2C & TCM3105 on 2400bd ***

			  TNC2c on 2400 Baud





  - If you have a TNC with TCM-3105 chip, you can be qrv on 2400 bd and
    enjoy this speed after minor modifications to your TNC, you just have
    to replace some components,and realigne your TNC.

  - Change the original Xtal (4.43 Mhz) to 6.5536 Mhz.
    It is easy to find this Xtal.

  - On the original Xtal you find 2 capacitors of 33 Pf,put a
    capacitor of 47 Pf parallel with ONE of the 2 33 Pf C's.

  - Change the HDLC baudrate (original 1200 baud) to 2400 baud,
    (the second strap of the first 6 inline).

  - Realigne your TNC as followed:

      - Use TAPR 1.1.7 or equal software.
      - install a jumper between pin 1 and pin 3 of the 5 pin DIN plug
      - Type "Cal" for Calibrate.
      - Type "D" for Diddle (2 tones are active).
      - Connect an osciloscoop on pin 8 of the TCM-3105.
      - Trim the 50Kohm potmeter for 50% duty cycle.
      - Type "Q" to quit the Calibrate function.

  - You can now operate 2400 baud AFSK,the tones are 1775 Hz & 3250 Hz.


  - Here are some "VERRY IMPORTANT" points:


    1)  Our PTT regulations DO NOT allow us to use the 2400 Bd speed,
	but you can get a special authorization for 1 year.
	Please introduce a request to experiment the 2400 Bd
	(or even 4800 Bd and 9600 Bd) on UHF and SHF.
	Please introduce your request to RTT right NOW !

    2)  We beg you to reduce your TXDELAY at the lowest possible value.
	For example if you loose the connection with a TXDELAY of 11
	and got a good copy for 12, then use a TXDELAY of 15
	(you have then a 30 % safety margin).

	 In order to get full advantage of the speed increase, please
	 reduce your TXDELAY. Lets compute the time for a  AX.25 frame
	 and lets compare it with the usual 300 ms TXDELAY :

					bytes   bits  time at     300 msec
						       2400Bd   represents
   short frame (just "RR" for example)     19    152    63 ms        476 %
   long frame  (256 bytes info frame)     276   2208   920 ms         32 %

   You see that the TXDELAY is really a WASTE of TIME, but it is necessary
   to be sure that the PLL of the transmitter is correctly locked and it
   is necessary to be sure that the squelch circuit of the receiver is open.
   (note that both time constants have been designed for
   "phone transmissions",but not for data !).


   - If you have a TNC with AM7910 chip, you can be qrv and enjoy this
     speed after minor modifications to your TNC, refter to the 2400 bd
     bulletin of ON7PC @ ON7RC.




 73 Walter ON1AOT @ ON6AR



 PS: If you can not find the Xtal of 6.5536 just send me a message
     @ ON6AR.