Modes of operation of 8255

The 8255A PPI has three operating modes for its ports, selected by the control word (D7 = 1), plus a bit set/reset mode.

Mode 0 : Simple (basic) I/O

• Ports A, B and C (as two 4-bit halves) work as simple input or output; no handshake.
• Outputs are latched, inputs are not latched.
• Used for LEDs, switches and 7-segment displays. Example: control word 90H sets Port A as input and Port B, Port C as output.

Mode 1 : Strobed (handshake) I/O

• Only Port A and Port B can be used; each acts as an 8-bit I/O port with 3 handshake lines taken from Port C.
• Input port signals: STB (strobe from device), IBF (input buffer full), INTR (interrupt request).
• Output port signals: OBF (output buffer full), ACK (acknowledge from device), INTR.
• Port A uses PC3–PC5 (input) or PC3, PC6, PC7 (output); Port B uses PC0–PC2. Interrupt enable flip-flops INTE are set by BSR.
• Used for keyboards and printers.

Mode 2 : Bidirectional bus

• Only Port A; the same 8 lines work as input and output (bidirectional) with handshake signals PC3–PC7 (INTR, OBF, ACK, STB, IBF).
• Port B can then be used in mode 0 or mode 1. Used for two-way transfer with another processor or disk controller.

BSR mode (D7 = 0)

Sets or resets a single bit of Port C: D3–D1 select the bit, D0 = 1 sets and D0 = 0 resets.

 I/O mode control word:
 D7 D6 D5 D4 D3 D2 D1 D0
 1  [A mode] PA PCu [B]PB PCl
 D6D5: 00 mode0, 01 mode1, 1x mode2
 D4,D3,D1,D0: 1 = input, 0 = output ; D2: 0 mode0, 1 mode1