Pinout

Reference signals

Below are simple pictures of the different signals on the CIA#1 MOS 6526 chip which I use for reference/comparison during fault finding. For further details about the signals on the different C64 custom chips I will highly recommend Sven´s techsite. Information about the pinout is taken from: https://ist.uwaterloo.ca/~schepers/MJK/cia.html and https://www.c64-wiki.com/wiki/CIA.

Settings on Philips/Fluke PM3055 60 MHz analog oscilloscope:

• 2 V / DIV

• See individual pictures for time settings

Signals have been measured on a working Commodore 64 assy 250466 in idle mode (blue screen).

Pin #1
VSS/GND

Pin #2
PA0

I/O-Port A Bit0 Bidirectional parallell port

Pin #3
PA1

I/O-Port A Bit1 Bidirectional parallell port

Pin #4
PA2

I/O-Port A Bit2 Bidirectional parallell port

Pin #5
PA3

I/O-Port A Bit3 Bidirectional parallell port

Ground.

Pin #6
PA4

I/O-Port A Bit4 Bidirectional parallell port

Pin #7
PA5

I/O-Port A Bit5 Bidirectional parallell port

Pin #8
PA6

I/O-Port A Bit6 Bidirectional parallell port

Pin #9
PA7

I/O-Port A Bit7 Bidirectional parallell port

Pin #10
PB0

I/O-Port B Bit0 Bidirectional parallell port

Pin #11
PB1

I/O-Port B Bit1 Bidirectional parallell port

Pin #12
PB2

I/O-Port B Bit2 Bidirectional parallell port

Pin #13
PB3

I/O-Port B Bit3 Bidirectional parallell port

Pin #14
PB4

I/O-Port B Bit4 Bidirectional parallell port

Pin #15
PB5

I/O-Port B Bit5 Bidirectional parallell port

Pin #16
PB6

I/O-Port B Bit6 Bidirectional parallell port

Pin #17
PB7

I/O-Port B Bit7 Bidirectional parallell port

Pin #18
/PC

Port control. Indicates availability of data on port B or both ports.

Note that there are some small periods where the signal is low.

Pin #19
TOD

Time of day. A TTL signal carrying the mains frequency (derived from the 9VAC) of 50Hz (PAL) / 60Hz (NTSC) is applied here to trigger the realtime clock.

Pin #20
VCC

Supply voltage (+5V DC).

Pin #21
/IRQ

Interrupt ReQuest. Becomes LOW when it matches a set bit in the interrupt control register on occurrence of the corresponding event. In the C64, this line is connected to the CPU's /IRQ pin (CIA #1) or to the /NMI pin (CIA #2), respectively.

Pin #22
R/W

Read/-Write. 0=read on data bus, 1=write on data bus.

Pin #23
/CS

Chip select. 0=coupled to data bus, 1=tri-state.

Chip Select - low level means active CIA.

Note that there are some small periods where the signal is low.

Pin #24
/FLAG

Negative edge IRQ input, can be used as handshake for either parallel port.

Pin #25
ø 2

Phi 2. System clock signal. All data bus action takes place only when ø2=1.

Pin #26
D7

Databus line D7

Pin #27
D6

Databus line D6

Pin #28
D5

Databus line D5

Pin #29
D4

Databus line D3

Pin #30
D3

Databus line D4

Pin #31
D2

Databus line D2

Pin #32
D1

Databus line D0

Pin #33
D0

Databus line D1

Pin #34
/RES

Reset input, low signal initiates CIA.

Pin #35
RS3

Register select #3. These four pins select one of the CIA's internal registers.

Pin #36
RS2

Register select #2. These four pins select one of the CIA's internal registers.

Pin #37
RS1

Register select #1. These four pins select one of the CIA's internal registers.

Pin #38
RS0

Register select #0. These four pins select one of the CIA's internal registers.

Pin #39
SP

Serial Port - bidirectional, internal shift register converts CPU parallel data into serial data and vice-versa.

Pin #40
CNT

Count - Internal timers can count pulses to this input. Can be used for frequency dependent operations.

Banner picture credits: Xato