Modbus Protocol Technical Data; Rtu; Feature Summary - ABB ACS550 User Manual

ACS550 User's Manual

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.
Read Coil Status
Read Discrete Input
Status
Read Multiple
Holding Registers
Read Multiple Input
Registers
Force Single Coil
Write Single
Holding Register
Diagnostics
Force Multiple Coils 0x0F
Write Multiple
Holding Registers
Read/Write Multiple
Holding Registers
Function Code (Hex)
0x01
0x02
0x03
0x04
0x05
0x06
0x08
0x10
0x17
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 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. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
Read multiple input registers. For the ACS550, the 2 analog input
channels are mapped as input registers 1 & 2.
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. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
Perform Modbus diagnostics. Subcodes for Query (0x00),
Restart (0x01) & Listen Only (0x04) are supported.
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. For the ACS550, the entire
parameter set is mapped as holding registers, as well as
command, status and reference values.
This function combines functions 0x03 and 0x10 into a single
command.
Description
Embedded fieldbus
201