PLC Simulator
Modbus function reference

Modbus Function Code 01 — Read Coils

Read packed boolean output states

01
Load in Modbus sandbox

Data model

Coils (0xxxx reference family)

Access

Read packed boolean output states

Protocol quantity

1–2000 coils

Worked example

Read the PDU field by field.

Read 37 coil states beginning at protocol address 19. The response packs bits least-significant-bit first within each data byte.

Typical use: Polling commanded or writable boolean states such as run commands, solenoids and mode flags.

Example protocol data units

Request PDU

01 00 13 00 25

Normal response PDU

01 05 CD 6B 05 4E 01

PDU examples omit the Modbus TCP MBAP header and the Modbus RTU unit-address/CRC envelope. Multi-byte values are shown in network byte order.

Common implementation mistakes

  • Using a 00001 display reference as the transmitted address
  • Reading a coil when the required state is exposed as a discrete input
  • Decoding packed response bits in the wrong bit order

Run the reference

Turn a protocol table into evidence.

Compose the request publicly, then use Pro for advanced graded exceptions, saved fixtures, complete history and project sharing.

Open sandbox
03 Read holding06 Write register02 Illegal address