Allen-Bradley E Series Installation Instructions Manual page 13

Determine Module Slot Location
The figure Chassis Slot Numbering shows chassis slot numbering in a 4-slot
chassis. Slot 0 is the first slot and is always the leftmost slot in the rack (the
first slot to the right of the power supply). You can use any size ControlLogix
chassis and install the module in any slot. You can also install multiple
1756-CNB and 1756-CNBR modules in the same chassis. You can install as
many modules as your power supply can accommodate (such as the number
for which the power supply is rated).
If you plan to install a redundant system, you must place the primary
IMPORTANT
and redundant modules in the same corresponding slot in their
respective chassis. For example, if you place a 1756-CNBR module
in slot 3 (from the left) in the primary chassis, you must also place
a 1756-CNBR module in slot 3 in the redundant chassis.
Chassis Slot Numbering
Slot 0
Power Supply
ControlLogix ControlNet Interface Module 13
Slot 2
Slot 1 Slot 3
Publication
Chassis
1756-IN604A-EN-P - February 2006