User manual

PCI-DA12-8/16 Manual
Page 5-4
Manual MPCI-DA12-16.Dh
Programming the Digital I/O Circuit
The digital I/O circuit is comprised of an Intel 8255, a direction control latch, and four bi-directional
buffers with 10K pull-ups. Please refer to the 8255-5 specification in appendix A for a detailed
description of the PPI.
Address Assignment Operation
Base Address +20 Port A Read/Write
Base Address +21 Port B Read/Write
Base Address +22 Port C Read/Write
Base Address +23 Control Read/Write
Table 5-4: I/O Address Table for Digital
The circuit is designed to use the PPI in mode 0 wherein:
a. There are two 8-bit ports (A and B) and two 4-bit ports (C Hi and C Lo).
b. Any port can be configured as an input or an output.
c. Outputs are latched.
d. Inputs are not latched.
The 8-bit control register is used to set the mode and direction of the ports.
Bit Assignment Function
D0 Port C Lo (C0-C3) 1 = Input, 0 = Output
D1 Port B 1 = Input, 0 = Output
D2 Mode Selection 1 = Mode 1, 0 = Mode 0
D3 Port C Hi (C4-C7) 1 = Input, 0 = Output
D4 Port A 1 = Input, 0 = Output
D5,D6 Mode Selection 01 = Mode 1, 00 = Mode 0 1X = Mode 2
D7 Mode Set Flag&Tristate 1 = Active & Tristate
Table 5-5: Control Register Bit Assignments
Note
PPI Mode 1 cannot be used with this circuit without modification. Thus, bits D2, D5, and D6 should
always be set to "0". If your card has been modified to operate in Mode 1, then there is an Addendum
sheet in the front of this manual describing that modification. This circuit cannot be modified to
operate in PPI Mode.