Commodore VIC-20 Programmer's Reference Manual page 239

INDEPENDENT INTERRUPT INPUT MODE:
As above, the CB2 interrupt flag will be set on a negative
transition on the CB2 input fine. However, reading or writing to
PORT B does not clear the flag.
INPUT MODE:
The CB2 interrupt Mag {IFR bit 3) wifl be set on a positive
(low-to-high) transition of the CB2 line. The CB2 flag will be cleared
on a read or write of PORT B.
INDEPENDENT INPUT MODE:
As above, the CB2 interrupt flag will be seton a positive transition
on the CB2 line. However, reading or writing PORT B does not
affect the flag.
HANDSHAKE OUTPUT MODE:
The CB2 line will be sel low on a write to PORT S. It will be reset
high again when there is an active transition on the CB1 line.
PULSE OUTPUT MODE:
The CB2 line is set low for one cycle after a write to PORT B.
MANUAL OUTPUT MODE:
The CB2 line is held low.
MANUAL OUTPUT MODE:
The CB2 line is held high.
R: CB1 CONTROL
This bit selects the active transition of the input signal applied to
the CB1 pin. If this bit is 0, the CB1 interrupt flag will be set on a
negative transition (high-to-low). If this bit is a 1, the CB1 interrupt
flag wil[ be set on a positive (low-to-high) transition,
S: CA2 CONTROL
s
BIT # 3
0
0
0
0
s
2
0
0
1
1
s
1
0
1
0
1
DESCRIPTION
Interrupt Input Mode
Independent Interrupt Input Mode
Input Mode
Independent Input Mode
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