In my previous article [1] was published scheme packet modem to work in УSW range from 1200 Baud. Later the scheme was successfully tested and the LW range at a speed of 300 Baud. that has allowed to develop a universal modem 1200/300 Baud. characterized by simplicity and reliability.
Schematic diagram of the modem is shown. Compared to [1] it does not undergo any significant changes except for two moments: added switching nodes of the reference oscillator frequency 4.43 MHz /2.215 MHz and indication of operating modes of the modem, PTT and DCD, providing additional convenience in work. Switch LW and УSW (speed 300/1200 Baud) is made by the switch SA1.
Connector X1 is used to connect to the COM port of the computer connector x2 - connect LW transceiver, connector X3 - connect УSW radio station.
In addition I would like to clarify data on the concept [1]: the value of resistors R2 and R14-2.Km.
Literature
1. Teeruk V. Modem for the package. Hams.- 1997. No. 10. S. 37.
Author: V. Teeruk (YL2GL), LV-5402, Latvia, Daugavpils; Publication: N. Bolshakov, rf.atnn.ru