Each controller canister contains a controller card, a battery, and an optional host interface card (HIC). You can add a second controller to a simplex configuration or replace a failed controller.
The figures show an example controller shelf before adding a second controller (one controller canister and a controller blank) and after adding a second controller (two controller canisters).
When you replace a failed controller canister, you must remove the battery and HIC, if one is installed, from the original controller canister, and install them in the replacement controller canister.
You can determine if you have a failed controller canister in two ways:
Attention LED