Technical reference and worked-example guide
Modbus function code 04 input-register guide: field reference
Direct answer
The reader can construct and inspect a function 04 transaction, separate protocol offset from documentation notation and validate returned words against a known device state.
Written for pLC programmers and technicians reading measurement or status registers from Modbus servers.
Definition
Client, server unit identity, function 04, starting address, quantity, protocol data unit, response byte count, register notation and data type.
Signal path
Documented measurement through zero-based protocol offset, request frame, server lookup, returned words, byte or word order, scaling and engineering value.
Worked example
A bounded valid request returns the documented number of registers and a value matching a known input condition.
Limits
First and last valid address, excessive quantity, illegal address, timeout, exception, stale data, signed value and floating-point word order.
Common mistake
A transport, identity, function, offset, quantity, exception, byte-order, data-type, scaling or freshness mismatch.
Verification
The request verified with a current map, captured exchange and independent process evidence.
What does Modbus function code 04 do?
It reads one or more input registers from a Modbus server using a starting protocol address and quantity.
Are Modbus 3xxxx labels sent in the request?
No. Documentation labels are conventions; the request carries a zero-based register offset defined by the device map.