Important pin signals of 8085

The 8085 is a 40-pin IC. Its signals are grouped as follows.

1. Address and data bus

• A15-A8: higher-order address bus, output, unidirectional.
• AD7-AD0: lower-order address bus multiplexed with the data bus. Address during T1, data in T2-T3.
• ALE (Address Latch Enable): output pulse in T1, to latch the low byte of the address in an external latch.

2. Control and status signals

• RD: active low, memory/I/O read.
• WR: active low, memory/I/O write.
• IO/M: high = I/O operation, low = memory operation.
• S1, S0: status signals; 00 = halt, 01 = write, 10 = read, 11 = opcode fetch.
• READY: input; if low, the CPU waits (wait states) for a slow device.

3. Interrupt signals

• TRAP: non-maskable, highest priority, edge and level triggered; vector 0024H.
• RST7.5, RST6.5, RST5.5: maskable, vectored interrupts (3CH, 34H, 2CH).
• INTR: general-purpose maskable interrupt; non-vectored, device supplies the instruction.
• INTA: output, acknowledges INTR.

4. DMA signals

• HOLD: input, DMA controller asks for the buses.
• HLDA: output, CPU acknowledges and releases the buses.

5. Serial I/O

• SID: serial input data (read by RIM).
• SOD: serial output data (set by SIM).

6. Reset and clock

• RESET IN: input, active low; resets PC to 0000H and clears interrupt enable.
• RESET OUT: output, resets peripheral devices.
• X1, X2: connections for crystal, RC or LC; internal clock = f/2.
• CLK OUT: system clock output.

7. Power supply

• Vcc (+5 V) and Vss (ground).