FlexNet driver documentation
connecting PTT

PTT signalling using Soundcard/DSP drivers

There seems to be some confusion on how drive PTT with my Soundcard and DSP driver. So here are some instructions on how to do it.

Standard PC Soundcards

  • Serial Port 
  • Parallel Port 
  • Game Port

    • Note: The MIDI port must be MPU401 compatible. Old Soundblaster models have a proprietary MIDI port which cannot be used to generate a PTT signal.
  • Vox circuitry 

    • Personal Sound Architecture (PSA) Soundcards

  • Game Port 
  • Vox circuitry 
  • Using the FLAG_OUT pin of the DSP chip (see ADACOM Magazine #6) 

    • Texas Instruments TMS320C26 and TMS320C50 DSP Starters Kits (DSK)

  • Serial Port 
  • Vox circuitry 

    • DG1SCR DSP Board

      Note: newer versions of the DG1SCR monitor program use the RTS line for hardware handshaking, therefore RTS is no longer available as PTT signal.
    • Vox circuitry 
    • The Board's PTT outputs 

    Analog Devices EZKIT Lite

  • Vox circuitry 
  • Using the PFx output pins of the board 

    • The Parallel Port Solution

      The following signals are available at the parallel port:
      Signal Name
      Pin
      Description
      DATA0
      2
      PTT
      DATA1
      3
      DCD from the modem
      DATA2
      4
      CON from the FlexNet kernel
      DATA3
      5
      STA from the FlexNet Kernel
      GND
      20
      Ground
      [Schematic PTT using Parallel Port]

    The Serial Port Solution

      The following signals are available at the serial port:
      Signal Name
      Pin (9pin)
      Pin (25pin)
    		 Description
      TxD
        3
      2
    		 PTT (only Soundmodem)
      RTS
      7
      4
    		 PTT
      DTR
      4
      20
    		 DCD from the modem (only Soundmodem)
      GND
      5
      7
    		 Ground
      [Schematic PTT using Serial Port]

    The Game Port Solution

    MIDI out is the output of the serial midi interface, which runs at 31500 baud. The DSP does not output anything if PTT is off (level is inactive high) and it keeps sending zero-bytes during PTT on (format is 8 data bits, 1 start bit and 1 stop bit). The MIDI out can be found usually at the game port connector, pin 12 (TTL levels), ground is found at pins 4 and 5 and +5V is found at pins 1 and 8. Since MIDI out does not deliver a constant level at PTT, one must use a retriggerable monoflop with about 5ms impulse duration. It has to be triggered on the falling edge. 
      [Schematic PTT using Game Port]

    The Vox Circuitry

      [Schematic PTT using Vox circuitry]

      Please note that the Vox circuitry should only be used for 1200 baud AFSK. It is not well suited for 9600 baud FSK

      You must adjust the output level of the right channel to the highest possible volume in order to have enough signal voltage to turn on the MOSFET transistor.