82C55A datasheet(14/26 Pages) HARRIS

14 / 26 page

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.

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

ENABLE FLAG

POSITION

ALTERNATE PORT C

PIN SIGNAL (MODE)

INTE B

PC2

ACKB (Output Mode 1)

or STBB (Input Mode 1)

INTE A2

PC4

STBA (Input Mode 1 or

Mode 2)

INTE A1

PC6

ACKA (Output Mode 1 or

Mode 2)

FIGURE 17. INTERRUPT ENABLE FLAGS IN MODES 1 AND 2

FIGURE 18. PRINTER INTERFACE

PA0

PA1

PA2

PA3

PA4

PA5

PA6

PA7

PC7

PC6

PC5

PC4

PB0

PB1

PB2

PB3

PB4

PB5

PB6

PB7

PC1

PC2

DATA READY

ACK

PAPER FEED

FORWARD/REV.

DATA READY

ACK

PAPER FEED

FORWARD/REV.

RIBBON

CARRIAGE SEN.

MODE 1

(OUTPUT)

82C55A

MODE 1

(OUTPUT)

CONTROL LOGIC

AND DRIVERS

INTERRUPT

REQUEST

PC0

INTERRUPT

REQUEST

PC3

HAMMER

RELAYS

HIGH SPEED

PRINTER

82C55A

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