Establishing Grounding Connections with the I/O Base
Electrical ground connections for the processors are created with the I/O base on which the processor is mounted. Electrical ground connections are established during the assembly process, as follows:
When you snap together the I/O base and the processor, an electrical ground connection is established by touching the ground contact spring on the I/O base to the ground contact plate on the processor.

1 ground contact spring on I/O base
2 ground contact plate on the processor
For an I/O base unit with a fixed ground nut, an electrical ground connection is established when you perform the following tasks:
Insert the male-female standoff into the fixed ground nut on the I/O base.
Connect a ground screw to the standoff through the top of the processor.
Refer to the assembly topic for instructions on how to install the male-female standoff and ground screw.
NOTE: A ground screw and male-female standoff can be used only with I/O base modules that include the fixed ground nut.