add connections

src/ham/hw/usdx.md

@@ -2,6 +2,8 @@
 
 ## CAT mode
 
+Only available with OLED IO Board.
+
 Firmware needs the following:
 ```c
 #define CAT              1   // CAT-interface
@@ -13,6 +15,13 @@ NOT TESTED yet:
 //#define CAT_STREAMING  1   // Extended CAT support: audio streaming over CAT, once enabled and triggered with CAT cmd, samplerate 7812Hz, 8-bit unsigned audio is sent over UART. The ";" is omited in the data-stream, and only sent to indicate the beginning and end of a CAT cmd.
 ```
 
+### Connection
+
+- GND/RX/TX to an USB FTDI
+- Audio OUT (mono) to USB sound card
+- Mic/Key to USB sound card: T(ip)=Key, R(ing)=Mic, S(leeve)=GND, so connect Ring and Sleeve to the soundcard
+- PTT is handled by CAT, nothing specific here
+
 ### Generic settings
 - TS-480
 - 38400 bps