Zone
Goal: Connection to real technology Content:
Zone types (Input, Output, In-Out, Mem)
Signal assignment
HMI texts
Diagnosability
1. Why are there zones?
In classic control projects, signals are used directly in the code. Their meaning arises implicitly through logic, comments or experience.
This leads to typical problems:
Signals are contextless
Meaning changes depending on where in the code
Diagnosis is fuzzy or manual
Safety and sequence aspects get mixed
The zone exists to give signals meaning, responsibility and context.
2. What is a zone?
A Zone is the formal representation of a technical fact within a sequence.
A zone:
encapsulates one or more signals
gives them functional meaning
is in the context of a specific sequence defined
is uniquely diagnosable
Everything used in a sequence must be a zone.
A zone is not:
just an input
just an output
just a memory bit
It is meaning + behavior + diagnosis.
3. Classification in the Selmo overall model
The zone is the link between logic and real technology:
Sequence – describes what is supposed to happen
Zone – describes with what it happens
Signal – is the physical implementation
Zones do not exist globally, but always:
within a sequence
with a clear functional role
4. Zone types
Selmo distinguishes four basic zone types:
Input zone
represents pure feedback
e.g. sensors, push buttons, status signals
no active influence
Output zone
represents pure actions
e.g. valves, lamps, motor enables
no feedback in the model
In-Out zone
combines action and feedback
e.g. cylinders, axes, grippers
typical source for sequence monitoring
Mem zone
pure logical memory
no physical counterpart
stores state or sequence information
The zone type defines structure, not behavior.
5. Zone and signal assignment
A zone is linked to real signals:
Inputs
Outputs
or both
The following applies:
a zone can contain multiple signals
a zone is not a PLC address
Signals can be exchanged without changing the model
The zone is stable – the technology is replaceable.
6. Zone in the system layer (bit-control context)
The behavior of a zone is not in the code, but in the system layer determined.
For each state of the sequence it is defined:
whether the zone is relevant
what behavior is expected
which deviations are allowed
This is done via bit-control operands (0, S, i).
Zone + state + operand = behavioral contract
The full meaning of the operands is described in the chapter Bit-Control .
7. Behavior of a zone in operation
A zone is at any time:
assessable
monitorable
diagnosable
Depending on the set operand:
S→ expected behavior / guidancei→ mandatory condition / protection0→ no meaning in the current state
A zone does not know a "debug state" or a special mode.
8. Zone in manual operation
In manual operation the zone immediately shows its state:
blue → zone meets the expectation of the current state
red → zone deviates from the expected behavior
The operator recognizes:
which zone is relevant
why a state is not fulfilled
where targeted intervention is necessary
The operator works not on actuators, but on the fulfillment of zone conditions.
9. Safety functions at zone level
Zones are carriers of safety and monitoring functions:
Interlock (
i) state-dependent sequence monitoringCMZ (Constantly Monitoring Zone) state-independent, permanent monitoring
Pair-Check monitoring contradictory signal states
further plausibility checks (e.g. processing-check)
Safety is a property of the zone, not of the code.
10. Zone and diagnosis
Each zone has:
a clear HMI text
an unambiguous meaning
a definable diagnostic reason
Diagnosis is created automatically:
from the current state
from the expected behavior
from the actual signal feedback
There is no diagnosis without a zone cause.
11. Distinction from other elements
To clarify:
A Zone is not a sequence → it does not describe a sequence
A Zone is not a signal → it gives signals meaning
A Zone is not a hardware zone → it does not encapsulate operating modes
A Zone is not a safety control → it models monitoring, but does not replace hardware safety
12. Typical modeling errors with zones
Common errors are:
Signals without a zone
collective zones without clear meaning
technical instead of functional zones
safety logic outside the zone
same zone for different tasks
A good rule of thumb:
If a zone cannot be explained, it is incorrectly modeled.
13. Summary
A zone is:
the smallest meaning-bearing unit in Selmo
the connection between logic and technology
fully monitorable and diagnosable
basis for safe operation and clear operation
Without zones there is no explainable machine.
Last updated
Was this helpful?