Another design with transistors by Bill, G3MCS. Thanks for sending it in !


                  ICOM CIRCUT CAT PLUS CW KEYER
                                  by
                                 G3MCS
 ICOM CAT SYSTEM and CW keyer for RS-232  computer COM Port. 
 
 DB25S (Computer)                            Jack plug (Radio,cat)
                                 C
                           B | /-------------------------- CENTER PIN  
  2 TXD     -----/////-------|<                        ICOM JACK PLUG
                  10K        | \ E     NPN 2N2484
                                |      or BC172 
                                |
                                |
                              __|__GND
 
  7 GND     ----------------------|---------------    BRAID TO ICOM GND
                                __|__ GND
                          C       
  3 RXD--|  ----------------\ :B
         |                   >|------//////----------------- CENTER PIN 
  4 RTS--|                  / |       47K                ICOM JACK PLUG
                           |
                           |         NPN 2N2484
                         __|__ GND   or BC172
                  
                                C
                           B: /--------------------------Screened cable
 20 DTR---------////////----|<                              to Rig KEY.
                  4.7K      |  \E
                               |
                               |
                             __|__ GND
                                               /-------\  Plastic transistor.
                                               |C  B  E:  BC 172
                                               ----|----
Connect both jack plug connections together ..Icom only use one screened wire
feed for both signals.
This circuit allows use of pin 20 for cw keying.
You can easily make this circuit in side a RS-232 DB25 plug top.
This interface is compatible with the software CAT for the ICOM radios.
This is a tested circuit and works fine on IC-775 DSP.
It's possible that the circuit works on different radios.....with slight 
modification ??.
NO RESPONSIBILITY ACCEPTED for above circuit !!  it works FB for me.
73 de Bill.