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Theory of Operation
Chapter 3
AT-MIO-16D User Manual
3-18
© National Instruments Corporation
DIO-24 Circuitry I/O Connector
All digital I/O is transmitted through a 100-pin male connector. This 100-pin connector is
physically divided into two standard 50-pin female connectors using a cable assembly. The pin
assignments for the 50-pin DIO-24 I/O connector are compatible with standard 24-channel digital
I/O applications. All even pins on the 50-pin DIO-24 connector are attached to logic ground, and
pin 49 is connected to +5 V through a protection fuse (F4), which is often required to operate I/O
module mounting racks. See Chapter 2,
Configuration and Installation
, for additional information.
82C55A Programmable Peripheral Interface
The 82C55A PPI is the heart of the AT-MIO-16D DIO-24 circuitry. This chip has 24
programmable I/O pins that represent three 8-bit ports: PA, PB, and PC. Each port can be
programmed as an input or an output port. The 82C55A has three modes of operation: simple I/O
(Mode 0), strobed I/O (Mode 1), and bidirectional I/O (Mode 2). In Modes 1 and 2, the three 8-bit
ports are divided into two groups: Group A and Group B (two groups of twelve signals). One
8-bit configuration (or control) word determines the mode of operation for each group. The Group
A control bits configure Port A (A0 through A7) and the upper 4 bits (nibble) of Port C (C4
through C7). The Group B control bits configure Port B (B0 through B7) and the lower nibble of
Port C (C0 through C3). Modes 1 and 2 use handshaking signals from Port C to synchronize data
transfers. Refer to Chapter 4,
Programming
, or to Appendix F,
Oki MSM82C55A Data Sheet,
for
more detailed information.
82C55A Modes of Operation
The three basic modes of operation for the 82C55A are as follows:
¥
Mode 0 Ð Basic I/O
¥
Mode 1 Ð Strobed I/O
¥
Mode 2 Ð Bidirectional bus
The 82C55A also has a single bit set/reset feature for Port C. The 8-bit control word also
programs this function. For additional information, refer to Appendix F,
Oki MSM82C55A Data
Sheet
.
Mode 0
This mode can be used for simple input and output operations for each of the ports. No
handshaking is required; data is simply written to or read from a selected port.
Mode 0 has the following features:
¥
Two 8-bit ports (A and B) and two 4-bit ports (upper and lower nibble of Port C)
¥
Any port can be input or output
¥
Outputs are latched, but inputs are not latched
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