Pin Diagram of Intel 8085 Microprocessor

There are 40 pins in Intel 8085 microprocessor. The photo image is shown in figure and pin diagram is shown in figure.

Intel 8085 microprocessor

Pin diagram of Intel 8085 microprocessor

The signals are classified into the following categories.

S.No.	Group
1	Address bus
2	Multiplexed Address / Data bus
3	Control and Status signals
4	Peripheral initiated signals
5	Clock signals
6	Reset signals
7	Interrupt signals
8	Serial I/O signals
9	Power supply and Ground signals

The signal diagram is shown in figure below.

Signal diagram of Intel 8085 microprocessor

Pin number	Signal name	Description
Address Bus
21 – 28	A8 – A15	Higher order byte (8 bits) of address bus. The address bus is unidirectional (one-way, ie. microprocessor to memory).
Multiplexed Address / Data bus
12 -19	AD0 – AD7	Lower order byte (8 bits) address bus (A0 - A7). And Data bus (8 bits) (D0 – D7).The data bus is bidirectional (two-wayie. microprocessor to memory and memory to microprocessor).
Control and Status signals
30	ALE	ALE - Address Latch Enable. Used for de-multiplexing the Address / Data bus. i.e. separating the lower order address lines (A0 – A7) and data lines (D0 – D7) from (AD0 – AD7).
29	S0	The microprocessor indicates its status (memory write, memory read, I/O write, I/O read, Opcode fetch, Halt, Hold, Reset) through these three output pins. IO/M’ will be ‘1’ for I/O operation and ‘0’ for memory operation.
33	S1
34	IO / M' (M - bar)
32	RD’	This is an active low signal. It will be 0 during memory read and I/O read operation.
33	WR’	This is an active low signal. It will be 0 during memory write and I/O write operation.
Peripheral initiated signals
35	READY	This pin is used for interfacing slow devices with the microprocessor.
39	HOLD	DMA (Direct Memory Access) signals: HOLD (hold request) HLDA (hold acknowledge)
38	HLDA
Clock signals
1, 2	X1, X2	Used for connecting the crystal for generating the clock signal.
37	CLK OUT	Clock signal for the peripheral devices.
Reset signals
36	RESETIN’	Used to apply reset signal to the microprocessor. When this pin goes low, the microprocessor is reset and begins executing the instruction from the memory location 0000. All the registers inside the microprocessor is reset to zero.
3	RESET OUT	Used to reset the peripheral devices.
Interrupt signals
6	TRAP	Interrupt request input signals to the microprocessor.
7	RST 7.5	Interrupt request input signals to the microprocessor.
8	RST 6.5
9	RST 5.5
10	INTR
11	INTA’	Interrupt Acknowledge signal from the microprocessor to the peripheral devices.
Serial I/O signals
5	SID	Serial Input Data. Used to receive serial data bits.
4	SOD	Serial Output Data. Used to transmit serial data bits.
Power supply and Ground signals
40	VCC	Power supply pins. +5V to VCC GND to VSS
20	VSS