MXIC – Manual Cross Interlock
6.4 MXIC – Manual Cross Interlock
MXIC stands for Manual Cross Interlock. It is the Selmo standard feature that secures manual actions in manual mode.
MXIC is not a control element and not a button. It is a rule-based protection logicthat decides whether a manual movement is allowed or must be prevented.
Why MXIC exists
In manual operation the operator can selectively influence zones to create states or eliminate deviations.
Without additional protective logic a risk arises:
Movements could be executed even though prerequisites are missing
technical interlocks would be implicitly expected
safety and logic would be mixed
MXIC exists to prevent exactly that.
MXIC ensures that manual movements are only allowed when they fit the current model state.
Basic principle of MXIC
MXIC acts:
exclusively at Manual operation
the interface between operator action and Zone
independent of the automatic sequence
For every manual action MXIC checks:
which zone is being addressed
which state is currently active
whether an authorization is defined for this combination
The result is unambiguous:
allowed → movement is executed
not allowed → movement is prevented
How MXIC is defined
MXIC is defined model-based, not programmed.
The definition is made:
as the intersection of button and Zone
with clearly formulated conditions
analogous to the mindset of the system layer
In doing so it is specified:
which button affects which zone
under which conditions this is allowed
when a movement must be prevented
MXIC is a formal rule – not an exception in the code.
MXIC in the context of state
MXIC always considers:
the current state of the sequence
the significance of the zone in this state
overarching monitors (e.g. CMZ)
This means:
no movement outside the model context
no implicit authorizations
no bypass of the logic
Even in manual operation the state remains the reference.
Diagnosis instead of movement
If a manual movement is not permissible is:
the movement will be not executed
there is no partial movement
instead a clear diagnosis is displayed
points to the affected zone
explaining why the movement was prevented
describes the contradictory state
makes the missing condition visible
MXIC replaces dangerous movements with explainable information.
Distinction from other mechanisms
To clarify:
MXIC ≠ Interlock (
i) →iprotects the sequence, MXIC protects manual actionsMXIC ≠ CMZ → CMZ acts permanently, MXIC only in manual mode
MXIC ≠ safety hardware → MXIC complements but does not replace a safe shutdown
MXIC is:
a logical protective function
part of the model
completely deterministic
Typical misconceptions about MXIC
Common misunderstandings are:
❌ “MXIC is just a key lock”
❌ “MXIC is optional”
❌ “MXIC replaces safety logic”
❌ “In manual mode MXIC can be bypassed”
All of these assumptions are false.
MXIC is an integral part of a safe manual operation.
Summary
MXIC:
protects against unauthorized manual movements
checks every manual action based on the model
prevents movement instead of correcting it
produces diagnosis instead of risk
MXIC ensures that the human in manual operation remains within the model.
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