RL-20 DB9 Pinout:

1 - Right (Aux or Tone) Output

2 - Left (Main or Voice) Output

3 - COS Logic Input - (pull near ground required for valid input)

4 - CTCSS Logic Input - (pull near ground required for valid input)

5 - PTT Output

6 - Discriminator /Detector or Receiver Audio Input - (level controlled by potentiometer R9)

7 - Not Used - brought to a solder pad to be used optionally.

8 - Ground - (Digital, Analog, and Power)

9 - Not Used - brought to a solder pad to be used optionally.

MIC +5VDC is a larger pad/hole near the DB9 connector to allow connection for power 
to a DTMF microphone. This pad connects to the USB power (5 volts) and is bypassed by 
the 1uF Tantalum capacitor C20. It can optionally be jumped with a wire to the hole 
marked "9" below it to supply 5V to the DB9 pin 9.

AUDIO AMP +5V is a larger pad/hole near C17 to allow connection for power 
to an audio amplifier. This pad connects to the USB power (5 volts) and is bypassed by 
the 1uF Tantalum capacitor C20. It can optionally be jumped with a wire to the hole 
marked "7" below it to supply 5V to the DB9 pin 7.

Jumper JU1 = Remove for PTT LED false illumination protection.

A partially illuminated Red PTT LED can happen when a radio adapter is connected to 
a radio that has a PTT switching voltage (bias) of less than 5 volts. This situation 
causes the PTT LED to partially or fully illuminate. Full illumination can also happen 
with just about any radio when the radio's power source is removed or turned off - falsely 
making it look like the radio is transmitting. A BAT43 Schottky diode has been added 
to the RL-20 that eliminates the false illumination from happening. A jumper has been 
added to disable the function and bypass the PTT protection circuitry if the diode 
creates a problem for a particular installation. Jumper "JU1" simply shorts 
out the protection diode.