Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 310)

Addressing
Appendix A
AĆ3
The processor addresses two I/O module slots as one I/O group.
Each physical 2-slot I/O group is represented by a word in the input
image table and a word in the output image table. Each input terminal
corresponds to a bit in the input image table word and each output terminal
corresponds to a bit in the output image table word.
The maximum number of bits available for one 2-slot I/O group is 32: 16
bits in the input image table word and 16 bits in the output image table
word. The type of discrete I/O module you install, either 8-point (standard
density) or 16-point (high-density, used in complementary mode)
determines the number of bits in the words that are used.
You select 2-slot addressing by setting two switches in the I/O chassis
backplane switch assembly. See your scanner’s or adapter’s users’ manual
for the specific switches and their settings.
Using 8ĆPoint I/O Modules
I/O modules generally provide eight input terminals or eight output
terminals. Figure A.2 illustrates the 2-slot I/O group concept with two
8-point input modules. Figure A.3 illustrates the 2-slot I/O group concept
with an 8-point input and an 8-point output module.
A.2.1
2ĆSlot Addressing