Receive Interface Power
                -----------------------


The receive interface uses these pins of the serial port.

        Signal    25 pin     9 pin   
        Name    connector  connector    Function
        ------  ---------  ---------    --------
        TxD        2           3        * see below
        RTS        4           7        *
        DTR       20           4        *
        GND        7           5        ground
        DSR        6           6        amplified audio to computer


Notice that the serial port has three output lines (TxD, RTS,
and DTR) which are used to power the op amp, and in some cases,
provide audio output and push-to-talk (PTT).

Different interfaces use different combinations and polarities
for the power.

         Power taken from
         ----------------
       TxD     RTS     DTR     Interface
       ---     ---     ---     ----------
                +       -       SSC PC SSTV       
                +       -       AEA FAX
                +       -       EZ SSTV (version 2)
        -       +               SSTV Explorer
        -               +       VESTER
                -       +       HAMCOMM
                -       +       JVFAX
                -       +       MSCAN
                -       +       GSH-PC

Note that the earlier systems used the RTS pin for positive (+) 
power and either DTR or TxD for negative (-).

Most later systems have settled on DTR for positive (+).  This  
is the default for EZ SSTV version 3 but the polarity can be
reversed for use with the older interfaces.

To change the polarity, pick "SSTV Interface" from the "Setup"
menu, then click on the "Power" button.


What happens if it is set wrong?

1. Circuits with the transmitter's push-to-talk (PTT) activated
   by the RTS or DTR will turn on the transmitter when the  
   program is in receive mode.

2. An attempt to receive will result in a message that there
   is no receive interface or audio present.  This is because
   the interface is not receiving the necessary power to 
   amplify the audio and apply it to the computer's serial 
   port.