$1800/6144/VIA1+0:   Data Port B (ATN, CLOCK, DATA, Device #)

   +----------+---------------------------------------------------+
   | Bit  7   |   ATN IN                                          |
   | Bits 6-5 |   Device address preset switches:                 |
   |          |     00 = #8, 01 = #9, 10 = #10, 11 = #11          |
   | Bit  4   |   ATN acknowledge OUT                             |
   | Bit  3   |   CLOCK OUT                                       |
   | Bit  2   |   CLOCK IN                                        |
   | Bit  1   |   DATA OUT                                        |
   | Bit  0   |   DATA IN                                         |
   +----------+---------------------------------------------------+

ROM-Reference:

 BIT $1800  : $E8D2 $E902
 CMP $1800  : $E9C3
 LDA $1800  : $E780 $E78E $E873 $E87B $E8DB $E99C $E9A5 $E9AE $E9B7 $E9C0
              $EA0B $EA5D $EA63 $EB3A $EBE8 $FF20
 STA $1800  : $E878 $E8E0 $E8FF $E9A1 $E9AA $E9B3 $E9BC $EA50 $EBDC $EBED
              $FF15