A Radio designer Dave Benson, NN1G, provides the answer: “The diode (D2) between the base of Q1 and ground acts as a ‘shunt diode’ (see Figure 1). The RTS or DTR pins on COM ports can drop to about -10 V in the ‘off’ state, which may be sufficient to get Q1 to go into reverse breakdown. The results could be a rig that is locked in transmit, or otherwise be erratic in its keying characteristics. Adding the shunt diode will prevent this from happening.”
Figure 1—A COM port switching circuit modified with the addition of a shunt diode (D2) between the base of transistor Q1 and ground.
From QST August 2000
