What They Are
Modbus coils are single-bit read/write points. They are typically used for commands, enables, relay outputs, and other binary states that an external system can both read and write.
In practical Modbus work, coils are where binary command intent often shows up. Start and stop bits, mode enables, relay controls, and simple on-off outputs are all commonly represented as coils because the protocol treats them as writable bit-level points rather than as 16-bit register values.
How They Differ From Other Modbus Data Families
| Data Family | Access | Typical Use |
|---|---|---|
| Coils | Read/write | Commands, relays, enable bits |
| Discrete Inputs | Read-only | Status contacts and alarm states |
| Holding Registers | Read/write | Setpoints and 16-bit values |
| Input Registers | Read-only | Measurements and counters |
Common Function Codes
| Function Code | Operation |
|---|---|
01 | Read coils |
05 | Write single coil |
15 | Write multiple coils |
Why Coil Writes Fail
| Failure Pattern | What Usually Happened | Practical Result |
|---|---|---|
| Point is not actually a coil | The register map labeled a status bit loosely | Reads may work elsewhere, but writes fail |
| Write is blocked by device logic | The point is writable in theory, but interlocks or mode state prevent it | Command tests look inconsistent |
| Addressing convention is off by one | The tool and map use different numbering styles | The wrong bit is written or no action occurs |
| Multi-write assumption is wrong | Device expects single-coil writes or narrower behavior | Batch writes fail while single writes pass |
Commissioning Notes
Good coil commissioning proves more than read visibility. It should confirm the exact addressed bit, the write path the device supports, and one known-good state change that can be observed safely in the field. That keeps a command point from being mistaken for a read-only status bit or a protected internal state.