     $DD01/56577/CIA2+1:   Data Port B (User Port, RS-232)
       +-------+------------------------------------------------------+
       | Bit 7 |   User Port PB7 / RS-232 Data Set Ready              |
       | Bit 6 |   User Port PB6 / RS-232 Clear to Send               |
       | Bit 5 |   User Port PB5                                      |
       | Bit 4 |   User Port PB4 / RS-232 Carrier Detect              |
       | Bit 3 |   User Port PB3 / RS-232 Ring Indicator              |
       | Bit 2 |   User Port PB2 / RS-232 Data Terminal Ready         |
       | Bit 1 |   User Port PB1 / RS-232 Request to Send             |
       | Bit 0 |   User Port PB0 / RS-232 Received Data               |
       +-------+------------------------------------------------------+

    Kernal-Reference:

     BIT $DD01   : $EF0C $EFEB $EFF9 $F006 $F05B
     LDA $DD01   : $EFFE $F068 $F070 $F453 $FED6
     STA $DD01   : $F003 $F06D $F48D