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Frequently Asked Questions
What does Modbus actually define?
How to read and write numbered registers.
Nothing about what the numbers mean.
A register holds a value and no context.
The integrator supplies the interpretation.
Manufacturers publish a register map for it.
That is the entire protocol.
Why is it on so much equipment?
Because it is simple and unencumbered.
It is old and trivial to implement.
Drives, meters, generators and chillers carry it.
So does industrial plant with no building interest.
Often it is the only integration route available.
That makes it the common denominator.
What work does the integrator have to do?
Map every point by hand.
Address, data type and byte order for each.
Scaling factors and whether values are signed.
No discovery or self-description exists.
Two devices can express the same value differently.
Only the documentation resolves it.
What is missing compared with a building protocol?
Alarming, scheduling and trend definitions.
There is no standard for any of them.
Each has to be built in the supervisor.
Object descriptions do not exist.
Nothing can be browsed.
That work does not disappear, it moves.
Why does the register map matter years later?
It is the only key to the data.
A drifted value cannot be diagnosed without it.
A replaced device may map differently.
Without the map the points are meaningless numbers.
It belongs in the handover documentation.
Not in an integrator's laptop.
Serial or TCP?
Both are common and they differ practically.
Serial suits field devices on a daisy chain.
It carries the bus rules that go with that.
TCP suits equipment already on a network.
Addressing and security differ between them.
Gateways commonly convert one to the other.
How does it usually sit in a building system?
As an ingest layer for everything else.
Equipment speaking nothing else is read over Modbus.
A gateway presents it in the site standard.
That keeps the supervisor consistent.
The mapping lives in the gateway.
Document it as carefully as the map itself.