Technical reference and worked-example guide
Modbus function 01 coil-reading guide: field reference
Direct answer
The reader can construct a valid request, interpret packed response bits, handle address notation and prove that a returned Boolean has the intended application meaning.
Written for pLC, HMI and integration learners reading discrete coil state and distinguishing protocol bits from physical outputs and feedback.
Definition
Client, server, transport, unit identity, starting coil address, quantity, zero- or one-based notation, packed-bit order, freshness, quality and physical meaning.
Signal path
Application request through transport and server map to packed response bits, Boolean tags, consuming logic and independent equipment feedback.
Worked example
One known off and on condition read repeatedly with correct address, bit position and application interpretation.
Limits
First and last address, excessive quantity, exception, timeout, short response, stale cache, restarted server and physical feedback disagreement.
Common mistake
An identity, function, address-base, quantity, packing, freshness, mapping, logic or physical-feedback mismatch.
Verification
The exact coil map and response checked against current device documentation and a known equipment state.
What does Modbus function code 01 read?
It reads one or more coil bits from a server; the device documentation defines whether a bit is a command, status, internal state or something else.
Does a true Modbus coil prove an output is energized?
No. It proves a returned Boolean at the protocol boundary. Separate I/O and physical feedback are needed to prove equipment state.