Rockwell Automation 6008-SI IBM PC I/O SCNNR 6008-SI - Manual (Page 11)

Chapter 2
I/O Scanner Concepts
2-3
We can divide I/O, and therefore I/O modules, into discrete and intelligent
modules.
Discrete I/O is characterized by one terminal (or point) per I/O image table
bit. Your program handles discrete I/O through I/O image tables, where
each input or output terminal corresponds to one of the 1024 input and
1024 output image table bits (64 x 16 bits = 1024 bits.)
The input image table is an area of memory that monitors the terminals of
discrete input modules. When an input switch is closed, the corresponding
bit is set (1). The output image table is an area of memory that controls
output terminals of output modules. After a bit is set to 1, the
corresponding switch is closed or the terminal is energized.
A standard–density module is a discrete input or output module that has 4,
6, or typically 8 input or output terminals. A high–density module is a
discrete input or output module that has 16 input or output terminals. A
quad–density module is a discrete input or output module that has 32 input
or output terminals.
Intelligent I/O is characterized by the transmission of one or more 16–bit
words in a particular format to or from an I/O module. A block transfer
(BT) is the transmission of data to or from an intelligent I/O module. A
BT read or read BT transfers information (typically analog input and status
data) from the module to the host; a BT write or write BT transfers data
(typically analog output and configuration data) from the host to the
module.
You should become familiar with these terms used to describe the I/O
subsystem.
Input image table: An area of memory that monitors input terminals of
input modules. When an input switch is closed, its corresponding input bit
is set. 64 16–bit words (1024 points) are available.
Output image table: An area of memory that controls output switches of
output modules. When a bit is set, its corresponding output is energized.
64 16–bit words (1024 points) are available.
Discrete I/O: I/O characterized by one terminal per image table bit
(terminal and point are the same).
Standard density module: Discrete I/O module having four, six, or
typically eight input or output points.
High density module: Discrete I/O module having 16 input or output
points.
Terms