1. 8255 Programmable Peripheral Interface

2. 8255 It is a programmable peripheral Interface with three i/o ports
A 8 bits
B 8 bits
C 4 bits upper and 4 bits lower

3. Pin diagram 8255

6. CS (Chip Select) RD (Read) WR (Write) D0-D7 (Data Bus) A0,A1 (Address)
8 bit i/o ports
PA, PB, PC
(Port A,B,C)
CS (Chip Select)
A low signal at this pin enables the chip
RD (Read)
Read enable
WR (Write)
Write enable
Bi-directional data lines
D0-D7 (Data Bus)
A0,A1 (Address)
Address select

7. 8255 The 8255 can work in 2 modes that are I/O mode and BSR mode
i/o mode is further divided in to
1. Mode 0 (Basic I/O)
2. Mode 1 (Strobe I/O)
3. Mode 2 (Bi - Conditional Bus)

8. Control Bytes for I/O mode1
Control Bytes for I/O mode
Group B
Port C (lower)
1 = input
0 = output
Port B
1 = input
0 = output
Mode selection
00 = mode 0
01 = mode 1
1x = mode 2
Group A
Port C (upper)
1 = input
0 = output
Port A
1 = input
0 = output
Mode selection
00 = mode 0
01 = mode 1
1x = mode 2