TxD ---*-----------------------------
     (3)    I                            I
            ---                          ---
            I I                          I I
                I I 2k2                      I I 10k
            ---          1N4148          ---
            I       I\I                  I
            *-------I-I-----*---------   I
        \   I       I/I     I        I   I
         \----\             I +      I   I
             /\  \           ---    14 I   I 4
               /  \    5V6      ---   --------------
               ----         10u  I    I  Vdd Vpp   I
                 I  ZENER        I    I            I
                 I               I  5 I            I
          GND ---*---------------*----I Vss        I
          (5)           22k           I            I
                       -----       12 I            I
          RTS ---------I   I----------IRB6 (clock) I
          (7)          -----          I            I
                       -----       13 I            I
          DTR ---------I   I----*-----IRB7 (data)  I
          (4)          -----    I     I            I
                        2k2     I     I  PIC 16C84 I
          CTS (8)---------------I     I------------I