Atari 400 Service Manual page 46

RS0 RS1 RwW
0 0 X
0 0 0
0 0 1
0 1 X
1 0 0
1 0 X
1 0 1
1 1 X
IRQA - IRQB
microprocessor IRQ line to provide interrupt requests. The IRQ lines
are open-collector so that several can be connected together to the
same input. When
microprocessor this line is set low. Control of the IRQ lines is
determined by the Control Registers A and B.
CA1 - CA2 Control lines. CAl and CA2 are the control lines for the
peripheral devices. CA1
device. The interrupt can be set by a negative or positive transition
of the input signal as determined by the Control Register A. Setting
the interrupt request line IRQA by CA1 is also controlled by CRA. CA2
can be used either as an input or output control line. In one output
mode CA2 generates a pulse each time data is read by the processor.
The pulse can be used to input sequential data. In a second output
mode CA2 is used in conjunction with CA1 to provide the "handshake"
between the microprocessor and the peripheral device. The CA1 input
is used to interrupt the microprocessor to indicate that data from a
peripheral device
reading the available data the CA2 output line is set low to signal
the peripheral device that new data can be made available.
4-10
CB1, CB2 Control Lines. CB1 and CB2 are peripheral device control
lines. CB1 is used as an input control line only. CB1 control line
can set IRQB on a negative or positive transistion as determined by
CRA2 CRB2
0 X
1 X
1 X
X X
X 1
X 0
X 1
X X
Interrupt Requests.
the PIA chip
is an
is available.
Function
Read or Write
Register A
Write into Peripheral
Register A
Read from A-side input pins PA0 -
PA7
Read or Write Control Register A
Write into Peripheral
Register B
Read or Write
Register B
Read from B-side input pins PB0-
PB7
Read or Write Control Register B
These lines
wants to
input interrupt
When the processor
Data Direction
interface
Interface
Data Direction
connect to
communicate with
only from an
has finished
System Service Manual
the
the
I/O