$DD01/56577/CIA2+1:   Data Port B (User Port, RS232)
   +-------+------------------------------------------------------+
   | Bit 7 |   User Port PB7 / RS232 Data Set Ready               |
   | Bit 6 |   User Port PB6 / RS232 Clear to Send                |
   | Bit 5 |   User Port PB5                                      |
   | Bit 4 |   User Port PB4 / RS232 Carrier Detect               |
   | Bit 3 |   User Port PB3 / RS232 Ring Indicator               |
   | Bit 2 |   User Port PB2 / RS232 Data Terminal Ready          |
   | Bit 1 |   User Port PB1 / RS232 Request to Send              |
   | Bit 0 |   User Port PB0 / RS232 Received Data                |
   +-------+------------------------------------------------------+

Kernal-Reference:

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