ABB ACS255 - 10. Serial communications; Modbus RTU Communications; Modbus Telegram Structure

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10. Serial communications
10.1. RJ45 Connector Pin Assignment
ACS255 has an RJ45 connector on the front of the control panel. This connector allows the user to set up a drive network via a wired connection.
The connector contains multiple interfaces for different communication protocols.
 PC and peripheral connection only
 Modbus RTU
 CANBus
The Remote keypad connection is always available, and can be used simultaneously with other interfaces, however only one other interface may
be used, e.g. If Modbus RTU is in use, CAN is disabled.
The electrical signal arrangement of the RJ45 connector is shown as follows:
CAN-
CAN+
0 Volt
Remote Keypad / PC Connection -
Remote Keypad / PC Connection +
+24 Volt Remote Keypad Power Supply
RS 485- Modbus RTU
RS 485+ Modbus RTU
10.2. Modbus RTU Communications
10.2.1. Modbus Telegram Structure
The ACS255 supports Master / Slave Modbus RTU communications, using the 03 Read Holding Registers and 06 Write Single Holding Register
commands. Many Master devices treat the first Register address as Register 0; therefore it may be necessary to convert the Register Numbers
detailed in section 10.2.4 by subtracting 1 to obtain the correct Register address. The telegram structure is as follows.
Command 03 – Read Holding Registers
Master Telegram
Length
Slave Response
Length
Slave Address
1
Byte
Slave Address
1
Byte
Function Code (03)
1
Byte
Function Code (03)
1
Byte
1
st
Register Address
2
Bytes
Byte Count
1
Byte
No. Of Registers
2
Bytes
1
st
Register Value
2
Bytes
CRC Checksum
2
Bytes
2
nd
Register Value
2
Bytes
Etc...
CRC Checksum
2
Bytes
Command 06 – Write Single Holding Register
Master Telegram
Length
Slave Response
Length
Slave Address
1
Byte
Slave Address
1
Byte
Function Code (06)
1
Byte
Function Code (06)
1
Byte
Register Address
2
Bytes
Register Address
2
Bytes
Value
2
Bytes
Register Value
2
Bytes
CRC Checksum
2
Bytes
CRC Checksum
2
Bytes