8259A programmable interrupt controller pinout

DIP-28. Eight interrupt inputs (IRQ0 to IRQ7), priority logic and the vector for the 8088.

Takes eight interrupt requests, masks them, and passes the highest-priority one to the 8088 INTR pin. IRQ0 is the timer and IRQ1 the keyboard; IRQ2 to IRQ7 come from the I/O channel. It sits at port $20 (U2 on the 5150). The POST tests it early, so a dead chip gives the one-long-one-short beep. PCjr (ZM32): also at $20, but the IRQ use differs (serial, modem, vertical retrace, diskette).

Open the interactive 8259A pinout

Pins

Pin	Name	Type	Signal
1	/CS	ctrl	Chip select, active LOW. Enables /RD and /WR. /INTA works without it.
2	/WR	ctrl	Write strobe, active LOW. Loads the command words.
3	/RD	ctrl	Read strobe, active LOW. Reads IRR, ISR or the mask.
4	D7	data	Data bus bit 7.
5	D6	data	Data bus bit 6.
6	D5	data	Data bus bit 5.
7	D4	data	Data bus bit 4.
8	D3	data	Data bus bit 3.
9	D2	data	Data bus bit 2.
10	D1	data	Data bus bit 1.
11	D0	data	Data bus bit 0.
12	CAS0	ctrl	Cascade line 0. Master output, slave input. Unused with one 8259A.
13	CAS1	ctrl	Cascade line 1.
14	GND	ground	Ground.
15	CAS2	ctrl	Cascade line 2.
16	/SP/EN	ctrl	Slave program input (HIGH = master) or, in buffered mode, transceiver enable output.
17	INT	ctrl	Interrupt output, active HIGH, to the 8088 INTR pin (18).
18	IR0	ctrl	Interrupt request 0, rising edge or level. PC: system timer, 8253 OUT0. PCjr: timer (8253 OUT0).
19	IR1	ctrl	Interrupt request 1, rising edge or level. PC: keyboard. PCjr: I/O channel, reserved.
20	IR2	ctrl	Interrupt request 2, rising edge or level. PC: reserved (I/O channel B4). PCjr: I/O channel.
21	IR3	ctrl	Interrupt request 3, rising edge or level. PC: secondary asynchronous (COM2), SDLC or BSC (I/O channel B25). PCjr: serial port (RS-232C).
22	IR4	ctrl	Interrupt request 4, rising edge or level. PC: primary asynchronous (COM1) or SDLC (B24). PCjr: internal modem.
23	IR5	ctrl	Interrupt request 5, rising edge or level. PC: fixed disk on the XT (B23). PCjr: vertical retrace from the video gate array.
24	IR6	ctrl	Interrupt request 6, rising edge or level. PC: diskette controller (B22). PCjr: diskette adapter.
25	IR7	ctrl	Interrupt request 7, rising edge or level. PC: printer (B21). PCjr: I/O channel (parallel printer).
26	/INTA	ctrl	Interrupt acknowledge, active LOW, from the 8288. The pulses put the vector on the data bus. PCjr: there is no 8288; in minimum mode the 8088 makes /INTA itself (8088 pin 24).
27	A0	addr	Address bit 0. Selects between the command and status words.
28	VCC	power	+5 V supply.

8259A programmable interrupt controller pinout

Pins

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