Industrial Engineer · phase 5 of 16
Coils / Discrete Inputs
One-bit read-write and read-only data areas in the Modbus data model.
The Explain card
- Plain English
- In the Modbus data model, coils are single bits you can read and write, and discrete inputs are single bits you can only read. They are the on and off half of what a device exposes.
- Example
- A remote I/O module maps its outputs as coils at address zero and up, and its inputs as discrete inputs. The PLC writes a coil to close a relay and reads a discrete input to see a switch.
- Why it matters
- Modbus addresses are numbered inconsistently between vendors, with some starting at zero and some at one, and coil addresses live in a different space from register addresses. Reading the right number in the wrong table returns nonsense with no error.
- Hook
- The bits, in two tables, one of them read only.
Where it sits in the deck
Phase 5: PLC Industrial Communications
The protocols that let controllers, drives and operator panels talk, plus the addressing and description files that make them agree on what was said.
