8255 block diagram

                            

It has been divided in two section: Group A and Group B. Each of 12 bits.

Group A: PA0-PA7 & PC4-PC7(port C upper)

Group B: PB0-PB7 & PC0-PC3(por C lower)

Each group can be programmed separately.

The GA and GB control block receives commands from r/w control logic to accept bit pattern from CPU.

GA(Group A) controls GA ports and GB controls GB ports.

The function of port a and port b is decided by the control bit pattern available in GA and GB control register.

Function of port a and b are also dependent on the mode of operation.

Port C can be programmed separately for port c upper and port c lower.

The port C function is also dependent on the mode of operation.

It can be used as simple i/o, handshaking signals and status signal inputs.