ABB S800 Product Manual page 76

Fieldbus Communication Interface
Figure 29. Dimension for typical MTU for CI840 or TB840
S800M I/O modules may be exchanged during system operation. A module locking
mechanism disconnects ModuleBus and process signals safely from the module. 
A newly inserted I/O module is automatically put into operation if the system
identifies the module being correct type and without faults.
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Section 2 Product Description
3BSE015969-600 A