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Operation Theorem • 23
4.1.4 Digital I/O Port Programming
Users can write the digital output value to or read back the digital signal
level from the PPI ports by using the software library. Here we define the
port name in Table 4.1. These port names are used both in software library
and all through this manual. The programming for PCIDIO 24H/48H
and PCIDIO 96H are fully compatible.
Connector
Numbers
CN1 CN2 CN3 CN4
P1A P2A P3A P4A
Port P1B P2B P3B P4B
Names P1C P2C P3C P4C
P1CTRL P2CTRL P3CTRL P4CTRL
Table 4.1 I/O Port Names
There are four ports on every 8255 PPI, including port A,B,C and the control
port. PA, PB and PC could be written or read but the control port is write
only. Refer to chapter 5 for details about programming of DIO ports.
4.1.5 Control Word
The control word written in the control port is used to setup PA, PB and PC
as input or output port. Figure 4.1 shows the format of the control word.
Table 4.2 shows the 16 possible control words and the respective I/O
configurations .
X X X ? ?
?
X
?
1/0 : Input/Output of Port C low nibble
D7 D6 D5 D4 D3 D2 D1 D0
1/0 : Input/Output of Port B
1/0 : Input/Output of Port C high nibble
1/0 : Input/Output of Port A
X : don't care
Figure 4.1 Control Word Format