82C55A
Reading Port C Status (Figures 15 and 16)
Applications of the 82C55A
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.
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.
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
ENABLE FLAG
ALTERNATE PORT C
PIN SIGNAL (MODE)
POSITION
INTE B
INTE A2
INTE A1
PC2
ACKB (Output Mode 1)
or STBB (Input Mode 1)
PC4
PC6
STBA (Input Mode 1 or
Mode 2)
ACKA (Output Mode 1 or
Mode 2)
FIGURE 17. INTERRUPT ENABLE FLAGS IN MODES 1 AND 2
INTERRUPT
REQUEST
PC3 PA0
PA1
HIGH SPEED
PRINTER
PA2
PA3
PA4
PA5
PA6
PA7
MODE 1
(OUTPUT)
HAMMER
RELAYS
PC7
PC6
PC5
DATA READY
ACK
PAPER FEED
FORWARD/REV.
PC4
82C55A
PB0
PB1
PB2
PB3
PB4
PB5
PB6
PB7
PAPER FEED
FORWARD/REV.
RIBBON
MODE 1
(OUTPUT)
CARRIAGE SEN.
PC1
PC2
DATA READY
ACK
PC0
INTERRUPT
REQUEST
CONTROL LOGIC
AND DRIVERS
FIGURE 18. PRINTER INTERFACE
14
HARRIS [ HARRIS CORPORATION ]
