6
Digital operating mode (Modbus RTU)
6.1 Basic principles for Modbus RTU
The Modbus RTU is a master-slave bus system. It ena-
bles simple wiring of up to 32 control components
(slaves) which can be parameterised, written to and
read out centrally (e.g., switch cabinet) via a Modbus
master. This makes it easy to check the function of up to
32 volume flow controllers.
Fig. 19: Example: Modbus segment with one master
and 5 slaves
M Master
S Slave
Modbus allows a line structure (daisy chain) as a bus
structure. Due to circumstances on site, signal feedback
can occur at the end of a Modbus line. To filter this feed-
back, a 120 Ω terminal resistor is necessary at the end
of the bus line. The communication of the bus partici-
pants is based on a serial two-wire line (twisted-pair) fol-
lowing an EIA/TIA RS485 standard with master/slave
configuration. All Modbus participants must be given a
unique Modbus address (1 - 32) for system affiliation.
Slaves can also be third-party products.
6.2 Modbus RTU for XM0/XS0
In Modbus operation, no analogue control signal is
required. The setpoint value is transmitted digitally to
the control component via the Modbus. If connected, the
analogue setpoint value signal is ignored in this case.
For smooth data exchange in the on-site Modbus RTU
network, the communication parameters and user
address must be set for the Modbus interface. The
service tools GUIV3-M adjustment device, WINVAV2 PC
software or the internal display enable simple
addressing of the control components
‘Service tools functional overview’ on page 27
.
Various standardised Modbus registers are available for
setpoint value specification, actual value feedback or
status messages. Therefore, for example, the volume
flow rate setpoint value is specified with the Modbus
address by transferring the new setpoint value to
Modbus register 0. The volume flow rate actual value is
queried for each control component with the Modbus
address by reading out Modbus register 6. Further
Modbus registers:
Digital operating mode (Modbus RTU)
Modbus RTU for XM0/XS0
Control component Compact XM0 • XS0 for VAV terminal unit TVE
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