$180C/6156/VIA1+12:   Peripheral Control Register

   +----------+-----------------------------------------------------+
   | Bits 7-5 |   CB2 Control:                                      |
   |          |     000 = Input negative active edge                |
   |          |     001 = Independent interrupt input negative edge |
   |          |     010 = Input positive active edge                |
   |          |     011 = Independent interrupt input positive edge |
   |          |     100 = Handshake output                          |
   |          |     101 = Pulse output                              |
   |          |     110 = Low output                                |
   |          |     111 = High output                               |
   | Bit  4   |   CB1 Interrupt Control: 0 = Negative active edge   |
   |          |                          1 = Positive active edge   |
   | Bit  3-1 |   CA2 Control: see Bits 7-5                         |
   | Bit  0   |   CA1 Interrupt Control: see Bit 4                  |
   +----------+-----------------------------------------------------+

   CA1 (Input): ATN IN (make Interrupt if ATN occurs)

ROM-Reference:

 STA $180C  : $EB2F