Technical reference and worked-example guide
Modbus function 15 write-multiple-coils guide: field reference
Direct answer
The reader can construct and inspect a bounded request, calculate byte count and bit order, validate the echoed response and diagnose exception or application-state mismatches.
Written for pLC programmers and commissioning technicians writing several discrete outputs with Modbus function code 15 (0x0F).
Definition
Client and server roles, unit identity, starting address, quantity, byte count, least-significant-bit packing, unused bits, response echo and application permission.
Signal path
Operator or control intent through client mapping and request frame into server coil references, physical output ownership and independent process feedback.
Worked example
A known pattern written across byte boundaries and read back with the expected address, quantity, bit order and permitted physical response.
Limits
Off-by-one addressing, wrong unit, reversed bit order, incorrect byte count, read-only range, illegal quantity, exception, timeout, partial application and stale display.
Common mistake
A transport, unit, function, address, quantity, byte-count, packed-data, server-map or application-response mismatch.
Verification
The request constrained, access-controlled and retested on the exact server without assuming that a successful protocol response proves safe motion.
How are coils packed in Modbus function code 15?
Coils are packed from the addressed sequence into data bytes, with the first coil in the least-significant bit of the first byte. Unused bits in the final byte are set according to the specification.
What should a successful function 15 response contain?
The normal response echoes the starting address and quantity written. Read-back and independent process evidence are still needed because an accepted request does not prove the intended physical outcome.