The 8255 is a 40 pin DIP (see in fig 6).It has three separately accessible ports. The ports are each 8-bit, and are named A, B, C. The individual ports of the 8255 can be programmed to be input or output, and can be changed dynamically. In addition, 8255 ports have handshaking capability, thereby allowing interface with devices that also have handshaking signals, such as printers.
·3 8-bit IO ports PA, PB, PC
·PA can be set for Modes 0, 1, 2. PB for 0,1 and PC for mode 0 and for BSR. Modes 1 and 2 are interrupt driven.
·PC has 2 4-bit parts: PC upper (PCU) and PC lower (PCL), each can be set independently for I or O. Each PC bit can be set/reset individually in BSR mode.
·PA and PCU are Group A (GA) and PB and PCL are Group B (GB)
·Address/data bus must be externally demux'd.
·Improved dc driving capability.
8255 block diagram along with the control busses