MOS 8520 / 8520A CIA pinout
DIP-40. 1581 serial bus, fast-serial shift register, LEDs, device number, disk-change and motor lines
Pin-compatible with the 6526 (Commodore part 318029, also used in the Amiga and as an alternative in the 1571). In the 1581, port B carries the serial bus (DATA in/out, CLK in/out, ATN ACK, fast-serial direction, /WPRT, ATN IN) and PA5/PA6 drive the power and activity LEDs. Timer A clocks the fast-serial shift register out on SP with the clock on CNT. A bad U5 gives a drive that never finishes power-up (Ray Carlsen).
Open the interactive 8520 pinoutPins
| Pin | Name | Type | Signal |
|---|---|---|---|
| 1 | VSS | ground | Ground. |
| 2 | PA0 | io | Port A bit 0. |
| 3 | PA1 | io | Port A bit 1. |
| 4 | PA2 | io | Port A bit 2. |
| 5 | PA3 | io | Port A bit 3. |
| 6 | PA4 | io | Port A bit 4. |
| 7 | PA5 | io | Port A bit 5. 1581: power LED. |
| 8 | PA6 | io | Port A bit 6. 1581: activity LED. |
| 9 | PA7 | io | Port A bit 7. 1581: /DISK CHANGE. |
| 10 | PB0 | io | Port B bit 0. 1581: serial DATA in. |
| 11 | PB1 | io | Port B bit 1. 1581: serial DATA out. |
| 12 | PB2 | io | Port B bit 2. 1581: serial CLK in. |
| 13 | PB3 | io | Port B bit 3. 1581: serial CLK out. |
| 14 | PB4 | io | Port B bit 4. 1581: ATN acknowledge. |
| 15 | PB5 | io | Port B bit 5. 1581: fast serial direction. |
| 16 | PB6 | io | Port B bit 6. 1581: /WPRT. |
| 17 | PB7 | io | Port B bit 7. 1581: ATN IN (label beside PB7 on schematic 252380 sheet 3). UNVERIFIED |
| 18 | /PC | ctrl | Handshake: a LOW pulse after a port B read or write. |
| 19 | TOD | clock | Time-of-day / event counter input. 1581: no clock here; it is strapped by J1 and R5 (2.7 kΩ to +5 V) to tell the DOS a WD1770 from a WD1772 (schematic 252380 sheet 3). UNVERIFIED |
| 20 | VCC | power | +5 V supply. |
| 21 | /IRQ | ctrl | Interrupt request, open drain. |
| 22 | R/W | ctrl | HIGH = read from the CIA, LOW = write. |
| 23 | /CS | ctrl | Chip select, LOW while φ2 is HIGH. |
| 24 | /FLAG | ctrl | Negative-edge interrupt input. |
| 25 | φ2 | clock | Phase 2 clock in. |
| 26 | DB7 | data | Data bus bit 7. |
| 27 | DB6 | data | Data bus bit 6. |
| 28 | DB5 | data | Data bus bit 5. |
| 29 | DB4 | data | Data bus bit 4. |
| 30 | DB3 | data | Data bus bit 3. |
| 31 | DB2 | data | Data bus bit 2. |
| 32 | DB1 | data | Data bus bit 1. |
| 33 | DB0 | data | Data bus bit 0. |
| 34 | /RES | ctrl | Reset. LOW sets every register to its reset value: ports become inputs, timer latches go to all ones, the control and other registers to zero. |
| 35 | RS3 | addr | Register select bit 3. |
| 36 | RS2 | addr | Register select bit 2. |
| 37 | RS1 | addr | Register select bit 1. |
| 38 | RS0 | addr | Register select bit 0. |
| 39 | SP | serial | Serial port: fast-serial data in and out. |
| 40 | CNT | clock | Count / serial clock: Timer A baud clock in output mode. |
Fitted in
- Commodore 1581 disk drive U5 ($4000-$400F)