UNAVAILABLE — Data item value is UNAVAILABLE (not an error)

Not an error code at all — it is a legitimate data value meaning the agent currently has no reading for that item. Treating it as a number is the most common MTConnect integration bug.

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Diagnosis

When you hit this

The machine is powered off or the adapter has not connected yet.
A data item declared in the device XML that the adapter never populates.
Immediately after agent startup, before the first adapter data arrives.

Resolution

How to fix it

Never coerce UNAVAILABLE to zero
A spindle speed of UNAVAILABLE is not zero rpm. Coercing it produces false idle time and corrupts every average computed downstream.
Treat it as absent
Exclude the item from that sample rather than substituting a value. Forge reports such tags as unresolved rather than inventing a reading.
Check the adapter if it persists
An item permanently UNAVAILABLE usually means the adapter never maps it, not that the machine lacks the capability.

Normalization

What Forge does about it

A request that fails on some points and succeeds on others still normalizes. The tags that arrived resolve to canonical fields as usual; the ones that did not come back are listed in unresolved_tags and excluded from coverage_pct, so a partial read reports as partial instead of quietly looking complete.