ACS550-02/U2 User’s Manual 231
Embedded fieldbus
Modbus protocol technical data
Overview
The Modbus® protocol was introduced by Modicon, Inc. for use in control
environments featuring Modicon programmable controllers. Due to its ease of use
and implementation, this common PLC language was quickly adopted as a de-facto
standard for integration of a wide variety of master controllers and slave devices.
Modbus is a serial, asynchronous protocol. Transactions are half-duplex, featuring a
single Master controlling one or more Slaves. While RS232 can be used for point-to-
point communication between a single Master and a single Slave, a more common
implementation features a multi-drop RS485 network with a single Master controlling
multiple Slaves. The ACS550 features RS485 for its Modbus physical interface.
RTU
The Modbus specification defines two distinct transmission modes: ASCII and RTU.
The ACS550 supports RTU only.
Feature summary
The following Modbus function codes are supported by the ACS550.
Function Code (Hex) Description
Read Coil Status 0x01 Read discrete output status. For the ACS550, the individual bits
of the control word are mapped to Coils 1…16. Relay outputs are
mapped sequentially beginning with Coil 33 (e.g. RO1=Coil 33).
Read Discrete Input
Status
0x02 Read discrete inputs status. For the ACS550, the individual bits
of the status word are mapped to Inputs 1…16 or 1…32,
depending on the active profile. Terminal inputs are mapped
sequentially beginning with Input 33 (e.g. DI1=Input 33).
Read Multiple
Holding Registers
0x03 Read multiple holding registers. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
Read Multiple Input
Registers
0x04 Read multiple input registers. For the ACS550, the 2 analog input
channels are mapped as input registers 1 & 2.
Force Single Coil 0x05 Write a single discrete output. For the ACS550, the individual bits
of the control word are mapped to Coils 1…16. Relay outputs are
mapped sequentially beginning with Coil 33 (e.g. RO1=Coil 33).
Write Single
Holding Register
0x06 Write single holding register. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
Diagnostics 0x08 Perform Modbus diagnostics. Subcodes for Query (0x00),
Restart (0x01) & Listen Only (0x04) are supported.
Force Multiple Coils 0x0F Write multiple discrete outputs. For the ACS550, the individual
bits of the control word are mapped to Coils 1…16. Relay outputs
are mapped sequentially beginning with Coil 33 (e.g. RO1=Coil
33).
Write Multiple
Holding Registers
0x10 Write multiple holding registers. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
Read/Write Multiple
Holding Registers
0x17 This function combines functions 0x03 and 0x10 into a single
command.
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