Modbus coils

What a coil is

A coil holds one bit: on or off. The reference notation writes coils as 0xxxx, so 00001 is the first coil, sent on the wire as address 0. Coils are separate from holding registers: address 0 of the coils and address 0 of the holding registers are different values.

Reading coils: function 01

Function 01 (0x01) reads a block of coils. One request can read at most 2000 coils (0x07D0). The answer packs the bits into bytes, least significant bit first: the first coil is bit 0 of the first data byte. Unused bits in the last byte are zero.

Writing coils: functions 05 and 15

• Function 05 (0x05) writes one coil. The value is FF 00 for on and 00 00 for off. Other values are not valid.
• Function 15 (0x0F) writes several coils. One request can write at most 1968 coils (0x07B0).

Discrete inputs are read-only coils

Discrete inputs (1xxxx) use the same bit layout as coils but a master can only read them, with function 02. A device sensor or limit switch usually appears there, while a command to the device usually sits in a coil.

Common problems

• Exception 02 on a coil read: the address or the block end is outside the coils the device has. Coils and holding registers can have different ranges.
• The coil writes, but the device does not react: some devices lock coils while in a certain mode, or expect the write to go to a different unit ID.
• The bits look shifted: the first coil is the lowest bit of the first byte, not the highest. Check the bit order before blaming the device.

Test it in Busscope

Poll a coil block in a Busscope Master window and set the display format to binary to see each bit. The Modbus TCP tester page shows how to write a single coil and confirm the answer.