82C55A
Reading Port C Status (Figures 15 and 16)
In Mode 0, Port C transfers data to or from the peripheral device. When the 82C55A is programmed to function in Modes 1 or 2, Port C generates or accepts “hand shaking” signals with the peripheral device. Reading the contents of Port C allows the programmer to test or verify the “status” of each peripheral device and change the program flow accordingly.
There is not special instruction to read the status information from Port C. A normal read operation of Port C is executed to perform this function.
INTERRUPT |
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| ALTERNATE PORT C | ||||
ENABLE FLAG | POSITION |
| PIN SIGNAL (MODE) | ||||
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INTE B | PC2 |
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ACKB | |||||||
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| STBB (Input Mode 1) | ||||
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INTE A2 | PC4 |
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STBA (Input Mode 1 or | |||||||
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INTE A1 | PC6 |
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ACKA | |||||||
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FIGURE 17. INTERRUPT ENABLE FLAGS IN MODES 1 AND 2
Applications of the 82C55A
The 82C55A is a very powerful tool for interfacing peripheral equipment to the microcomputer system. It represents the optimum use of available pins and flexible enough to inter- face almost any I/O device without the need for additional external logic.
Each peripheral device in a microcomputer system usually has a “service routine” associated with it. The routine manages the software interface between the device and the CPU. The functional definition of the 82C55A is programmed by the I/O service routine and becomes an extension of the system software. By examining the I/O devices interface characteristics for both data transfer and timing, and matching this information to the examples and tables in the detailed operational description, a control word can easily be developed to initialize the 82C55A to exactly “fit” the application. Figures 18 through 24 present a few examples of typical applications of the 82C55A.
INTERRUPT
REQUEST |
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PC3 | PA0 |
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| PA1 |
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| PA2 |
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| PA3 |
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| PA4 |
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MODE 1 | PA5 |
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PA6 |
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(OUTPUT) |
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PA7 |
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| PC7 | DATA READY |
| PC6 | ACK |
| PC5 | PAPER FEED |
82C55A | PC4 | FORWARD/REV. |
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| PB0 |
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| PB1 |
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| PB2 |
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| PB3 |
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| PB4 |
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MODE 1 | PB5 |
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(OUTPUT) | PB6 |
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| PB7 |
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| PC1 | DATA READY |
PC0 | PC2 | ACK |
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INTERRUPT |
| CONTROL LOGIC |
REQUEST |
| AND DRIVERS |
HIGH SPEED
PRINTER
HAMMER
RELAYS
PAPER FEED
FORWARD/REV.
RIBBON 
CARRIAGE SEN.
FIGURE 18. PRINTER INTERFACE
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