ABB SCU200 - SCU200 Communication Interfaces; Modbus Protocol Fundamentals

78 SCU200 INSITE ENERGY MANAGEMENT SYSTEM
7.SCU200 communication interfaces
7.1.Modbus TCP/RTU readings
• Introducing MODBUS protocol
The Modbus serial line protocol is a Master-Slaves protocol. This means that only one master and one or
more slave nodes (max. 247) can be connected to the same serial bus. A Modbus communication is always
initiated by the master and there is only one transaction at the same time. For further information: www.
modbus.org If you intend to use Modbus, you should only use ASCII characters in the Web UI. Unicode
characters will not be displayed in Modbus.
Address PDU Frame Error Check
Address Field Function Code Data CRC
1 byte 1 byte 0 - 252 bytes 2 bytes CRCL, CRCH
ADU Application Data
PDU Protocol Data Unit
Stopbit 1
Address Field contains the slave address
Function Code: indicates what kind of action to perform
Data contains request and response parameters
CRC
contains the value generated by the cyclic redundancy check (standard
CRC-16 defined by CCITT)
The maximum size for a Modbus RTU frame is 256 bytes.
In RTU mode, message frames are separated by a silent interval of at least 3.5-character times. The entire
message frame must be transmitted as a continuous string of characters. If a silent interval of more than
1.5-character times occurs between two characters, the message frame is declared as incomplete and
should be discarded by the receiver.
Modbus Data Encoding Modbus uses a big-endian allocation for addresses and data items. This means
that, when a numerical quantity larger than a single byte is transmitted, the most significant byte is sent
first. Example: 1234h → first 12h then 34h.
• Physical Interface RS-485
To communicate with the SCU200 from an upper system, all devices (masters & slaves) must have
the same data rate and data format. These settings are defined over the Web UI, as described in the
dedicated chapter.
Parameter Values Default Values
Data rate 2400, 4800, 9600, 19200, 38400, 57600, 115200 Bit/s 19200 Bit/s
Data format even parity, odd parity, without parity even parity
• Control unit’s MODBUS-ID
You can connect up to 247 control units to one Modbus RTU line. Each control unit must have a unique
Modbus ID (address).
• Function Code
• Read operation on registers with access code “R” or “RW” is defined by function 03h “Read Holding
Registers”.
• Write operation on registers with access code “W” or “RW” is defined by function 06h “Write Single
Register”.
Do not apply functions other than those specified.