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I/O Addressing
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 4096 input and
4096 output image table bits (256 x 16 bits = 4096 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 to 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 transfers information (typically analog input and status data)
from the module to the host; a BT write transfers data (typically analog
output and configuration data) from the host to the module.
You assign each adapter an I/O rack number (0 – 31) by setting switches
on the adapter. A logical rack may be a single chassis; or two to four
chassis may comprise one rack number; or a single chassis can be
addressed as two racks. It is not necessary to assign rack numbers
sequentially: for instance, you could have a full rack 0, half a rack in rack
3, and a quarter rack in groups 6-7 of rack 7.
For addressing purposes, each rack is equivalent to a block of 8 I/O groups
in the I/O image table. Groups within a rack are numbered 0 through 7.
An I/O group is two 16-bit words, one from the output image table and one
from the input image table, with the same address. Refer to the I/O
Concepts Manual, publication 6008-6.5.1, for more information. In most
applications, only the input word or only the output word is used in any
given I/O group.
Chapter 1
Understanding I/O Scanner Concepts
1-3
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