MARS Automated HF Modem Switch

Driving Requirement for the MARS Automatic HF Modem Switch

As a result of the above I decided to design and build a unit that will handle the interface between most HF transceivers and HF modems.  Here are the requirements I settled on.

  • The switch shall be able to interface to up to 3 HF modems or peripheral devices simultaneously
  • The switch can allow all to be able to receive audio at the same time
  • The switch shall only allow a single device to furnish transmit audio in priority order
  • The audio connections for the three peripheral devices may can be either balanced or unbalanced
  • The HF transceiver connection may be either balanced or unbalanced
  • One device channel shall be capable of using VOX keying or RS-232 RTS/DTR keying
  • All PTT inputs shall have a soft diode blocked pull-up to 5 VDC
  • One device may be a sound card with no additional sound card interface required
  • The switch shall be powered by the stations 12VDC power supply (13.8 VDC)
  • The DC power input shall be overcurrent protected
  • The DC power input shall be reverse polarity protected
  • The design shall neither generate EMI nor be affected by it
  • No surface mount components
  • All components shall be available from a single source (Digikey in this case)
  • The design shall accommodate either perf-board or printed circuit board construction

The design shall not interfere with the data connections between HF modems on accompanying computer or terminal

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