This document discusses programmable peripheral interface devices that use handshake signals for data input and output. It focuses on the 8255 programmable interface, which has 3 ports (A, B, C) that can be programmed for different I/O schemes. Port C can be used as individual bits or two 4-bit ports. The 8255 supports bit set/reset of port C and 3 modes - simple I/O, I/O with handshake, and bidirectional I/O. Modes 1 and 2 use control signals on port C like STB, IBF, ACK for handshake-based input/output with ports A and B.

1. Programmable Devices with
Handshake Signals

2. Data Input with HandshakeMPU
Programmable
Interfacing
Device
Peripheral
Such as
KeyboardRD
STB (Strobe)
IBF
INTR

3. Data Output with HandshakeMPU
Programmable
Interfacing
Device
Peripheral
Such as
PrinterWR ACK
OBF
INTR

4. PROGRAMMABLE PERIPHERAL
INTERFACE – 8255

5. 8255
• Programmable parallel I/O device
• Can be programmed to transfer data in
various schemes from simple I/O to INTR I/O
• Has 24 I/O pins grouped into 3 ports, PORT A,
PORT B and PORT C
– Port C is used as either individual bits or in two 4-
bit ports : CUPPER and CLOWER

6. Block Diagram

7. • Address (A0-A1):This is used to select the ports
CS A1 A0 Select
0 0 0 PA
0 0 1 PB
0 1 0 PC
0 1 1
Control
reg.
1 X X
8255 Is not
selected

8. Modes of 8255
1. The Bit Set/Reset (BSR)mode
– Set or Reset bits of Port C
2. I/O mode
1. Mode 0 – Simple I/O mode
2. Mode 1 – I/O with Handshake
3. Mode 2 – Bidirectional Data transfer for Port A

11. BSR (Bit Set/Reset) Mode
BSR Control Word

12. Mode 0 : Simple Input or Output
• In this mode, ports A and B are used as two
simple 8 bit ports and port C as two 4 bit ports
• Each port can be programmed to function as
simply an input port or an output port.
• The input/output features in Mode 0 are
– Output ports are latched
– Input ports are not latched

13. Mode 1: Input or Output with
handshake
• Two ports (A and B) function as 8 bit I/O ports
They can be configured either as input or
output ports
• Each port uses three lines from port C as
handshake signals. The remaining two lines of
port C can be used for simple I/O functions
• Input and output data are latched
• Interrupt logic is supported

14. Mode 1: Input Control Signals
• Ports A and B are configured as input ports
• Port A uses the upper three signals PC3 ,PC4
and PC5 as handshake signals
• Port B uses the lower three signals PC2, PC1
and PC0 as handshake signals
• STB
• IBF
• INTR
• INTE

16. Mode 1: Output Control Signals
• OBF
• ACK
• INTR
• INTE

18. Mode 2: Bidirectional Data Transfer
• The single 8-bit port in group A is available
• This 8-bit port is bidirectional
• The 5-bit control port C (PC3-PC7) is used for
generating / accepting handshake signals
• Inputs and outputs are both latched