Commodore Computers 1581 - 8520 A CIA Interface Details; Interface Signals and Their Roles

1581 SERVICE MANUAL
8520A COMPLEX INTERFACE ADAPTER
vss-
PAO-
PA1-
PA2-
PA3-
PA4-
PA5-
PA6-
PA7-
PBO-
PB1-
PB2-
PB3-
PB4-
PB5-
PB6-
PB7-
PC-
TOD-
VCC-
1
2
3
4
CJI
6
7
8
9
10 8520
11 CIA
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
-CNT
-SP
-RSO
-RS1
-RS2
-RS3
-RES
-DBO
-DB1
-DB2
-DB3
-DB4
-DB5
-DB6
-DB7
-02
-FLAG
-CS
-R/W
-IRQ
INTERFACE SIGNALS
02 Clock Input — The 02 clock is a TTL, compatible input used for internal device operation and as a timing
reference for communicating with the system data bus.
CS — Chip Select Input — The CS input controls the activity of the 8520. A low level on CS while 02 is high
causes the device to respond to signals on the R/W and address (RS) lines. A high on CS prevents these lines
from controlling the 8520. The CS line is normally activated (low) at 02 by the appropriate address combination.
R/W—Read/Write Input — The R/W signal is normally supplied by the microprocessor and controls the direction
of data transfers of the 8520. A high on R/W indicates a read (data transfer out of the 8520), while a low indicates
a write (data transfer into the 8520).
RS3-RS0 — Address Inputs — The address inputs select the internal registers as described by the Register Map.
DB7-DB0 — Data Bus Inputs/Outputs — The eight bit data bus transfers information between the 8520 and
the system data bus. These pins are high impedance inputs unless CS is low and R/W and 02 are high, to read
the device. During this read, the data bus output buffers are enabled, driving the data from the selected register
onto the system data bus.
IRQ — Interrupt Request Output — IRQ is an open drain output normally connected to the processor interrupt
input. An external pullup resistor holds the signal high, allowing multiple IRQ-outputs to be connected together
The IRQ output is normally off (high impedance) and is activated low as indicated in the functional description.
RES — Reset Input — A low on the RES pin resets all internal registers. The port pins are set as inputs and
port registers to zero (although a read of the ports will return all highs because of passive pullups) The timer
control registers are set to zero and the timer latches to all ones. All other registers are reset to zero