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8255 PIO

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24 BIT DIGITAL I/O PIO DETAILS

PROGRAMMING INFORMATION

The 8255 I/O PIO occupies (4) consecutive 8 bit I/O locations starting at BASE+0 and going to BASE+3.

The structure of the addressing is as follows:

The following is an overview of each of the three available modes:

MODE 0 (Basic Input/Output):



MODE 1 (Strobed Input/Output):



MODE 2 ( Strobed Bi-directional Bus I/O):


CONTROL WORD to define operation mode: (This word is written to base+3)

D7 = 1  Designation of mode set flag

D6  D5  Mode selection of Group A
0   0  = mode 0
0   1  = mode 1
1   x  = mode 2

D4  Input/Output selection of Port A
0 = Output
1 = Input

D3  Input/Output selection of high order 4 bits of Port C
0 = Output
1 = Input

D2  Mode selection of Group B
0  = mode 0
1  = mode 1

D1  Input/Output selection of Port B
0 = Output
1 = Input

D0  Input/Output selection of low order 4 bits of Port C
0 = Output
1 = Input

CONTROL WORD to set and reset bits in In/Out Port C (This word is written to base+3)

D7 = 0  Bit set/reset flag 0 = Active.

D6 D5 D4 Don't care.

D3 D2 D1 Bit selection
0  0  0  PC0
0  0  1  PC1
0  1  0  PC2
0  1  1  PC3
1  0  0  PC4
1  0  1  PC5
1  1  0  PC6
1  1  1  PC7

D0 Bit set/reset selection
0 = Reset
1 = Set


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