Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 319)

Addressing
Appendix A
AĆ12
You select 1/2-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.
Figure A.9 illustrates the 1/2-slot addressing concept with a 32-point I/O
module. A 32-point I/O module (two 1/2-slot I/O groups) uses two input or
two output words of the image table. Module group 0 applies to the upper
16 points; module group 1 applies to the lower 16 points.
You can use 8-point and 16-point I/O modules with 1/2-slot addressing but
the rest of the bits are unused. They may be addressed through either of the
I/O module groups assigned to that chassis slot.