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I/O Block
connects the redundancy link. For the CIO block, it also con-
nects the station link to the module.
I/O modules can be inserted or removed without having to
power down the system. A captive mounting screw on the I/O
module secures it in place.
Hardware Keying
Each type of I/O module requires its own type of termination
base. Hardware keying using posts on the base and holes in
the back of the I/O module prevents an accidental mismatch of
I/O module and base. The block type and model number
shown on the front panel of an I/O module and base are the
same to easily identify that they are compatible.
I/O Redundancy
Redundancy allows backup electronics to read inputs and to
drive outputs in the event of a primary failure. As a back-
ground diagnostic task, the backup is continuously monitoring
its ability to read field inputs and to drive field outputs.
I/O redundancy is accomplished by using two I/O modules of
the same type, a wider base to mount and connect the two
modules, and a wider mounting column to attach the
redundant base (Fig. 3-15). In this arrangement, the field wir-
ing connects once at the base and is internally routed to both
modules. The redundancy feature allows a failed I/O module to
be replaced without affecting the operating module in the
redundant pair. Refer to Section 4 for further explanation of
redundancy.
Failure Mode (Outputs)
To insure known output conditions in the event of an I/O block
failure, output channels have two failure modes: default state
output and failure state output. The failure mode an output
channel enters depends on the type of failure. If Hnet commu-
nication or communication with the Harmony controller is lost,
block outputs enter their default states. This assumes, how-
ever, the I/O block is 100 percent operational. If critical I/O
block circuitry fails, block outputs will enter their failure
states.
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