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Control units VCS
Connection to the Master System (ModBus Standard)
Modbus RTU Slave (BMS)
e VCS control unit enables integration of the centralized
BMS (Building Management System) using the Modbus
communication bus. Using a suitably integrated system, it
is possible to control the air-handling device operating state.
Specifications of the variables (data points) used for integra-
tion are described in the section Description of Pre-defined
Variables in the Modbus Network. e final functionality,
monitoring and control options will be dependent on the inte-
grator used to ensure connection to the master BMS system.
Generally
Modbus is a worldwide recognized standard defined by
Modbus Organization, Inc.
e Modbus Organization is a group of independent suppliers
of automation devices. e Modbus Organization administers
and develops communication systems for distributed auto-
mation systems. It also provides information to obtain and
share information about the protocols, their application and
certification to simplify implementation by users with the aim
to reduce expenses for communication.
For detailed information on the Modbus RTU protocol, refer
to www.modbus.org.
Trademarks and Copyright
e trademarks used in this document are listed on the back
page, including their owners. ese trademarks are quoted
from Siemens materials.
Modbus RTU Protocol
e Modbus communication protocol works using the Master/
Slave principle. e Slave function is used for the VCS control
unit communication with the master system. us, the VCS
control unit behaves like a Slave during communication and
expects requests from the Master (master system).
ere are two variants of the Modbus protocol serial transfer
mode. e VCS control unit uses the RTU (Remote Terminal
Unit) mode. erefore, a unique Modbus RTU identification
is very important.
e RS 485 standard is used to transfer the data through a
half-duplex twin-lead.
e transfer lead (cable) must be selected depending on this
standard and other conditions. For more information, refer
to www.modbus.org
Correct wiring must be carried out to ensure proper operation
– 115/230 V AC must be strictly isolated from 24 V SELV AC.
ere are three terminals, "+", "-" and reference voltage ter-
minal "REF", available in the control unit to connect the VCS
control unit to the bus.
To ensure correct functioning of the bus, the first and last
device on the bus must be fitted with a terminal resistor.
erefore, it is possible to perform soware settings of the
terminal resistor in the VCS control unit for the Modbus Slave
(see the Data Points in the VCS Installation and Operating
Instructions).
It is also necessary to set other communication parameters
in the VCS control unit data points (section – Communication
with Modbus RTU Master System).
Default values of the data points for the Slave Modbus RTU:
• Slave address – 1
• Baud rate – 9600 Bd
• Parity - None
• Number of stop bits – 2
• Response time limit – 5 s
For more information, refer to the List of Data Points Note:
Once these data points have been set, the device must be re-
started. Aer the restart, the device is ready to communicate
in accordance with the set parameters.
Modbus Registers
e Modbus registers are divided according to their properties.
For a general description and explanation of the registers,
refer to the table below:
Data types used to transfer the values:
16-bit for real values (Unsigned Word)
16-bit for status values (Signed Word)
1-bit for status values like 0=Off (switched off) and 1=On
(switched on)
1-bit for alarm values like 0=Alarm and 1=Normal (OK)
Divisor (Multiplier):
e so-called Divisor (Multiplier) is used for values which need
the decimal numbers to be transferred. For example, for tem-
peratures the value Divisor (Multiplier) of 10 is used to enable
a one decimal place value to be transferred.
For example, a temperature of 23.2°C is transferred by the
Modbus as 232. e number must be divided by 10 to get the
correct value. For information indicating this fact, refer to the
note for a particular data point.
ModbusType Reference Description
Coil Status 0xxxx
Read/Write Discrete Out-
puts or Coils.
Input Status 1xxxx Read Discrete Inputs.
Input Register 3xxxx Read Input Registers.
Holding Reg-
ister
4xxxx
Read/Write Output or
Holding Registers.
Table 8 – Modbus registers
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