6.5 Page 1: 82C55 Digital I/O Circuit

This section is included as a reference to the page 1 82C55-like digital I/O registers. More info on the behavior of these digital I/O signals can be found later in this document.

Page 1, Base + 12 Read/Write Digital I/O Port A

A7-0 Port A data

Page 1, Base + 13 Read/Write Digital I/O Port B

B7-0 Port B data

Page 1, Base + 14 Read/Write Digital I/O Port C

Page 1, Base + 15 Read/Write Digital I/O Control Register

1	Bit 7 must be set to 1. This indicates port configure mode in the 8255 (as opposed to bit set mode which is not supported).
ModeA-C	Mode configuration bits. These must be set to 0.
DIRA, DIRB, DIRCH, DIRCL	Direction control bits. On ports A and b, all the bits in each port must be the same direction. On port C, the upper half C7 – C4 can have a different direction than the lower half C3 – C0.
0	Output
1	Input