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14
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