OZONE ANALYZER BMT 965 Manual, Rev. 01/2024
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Appendix H: MODBUS
The MODBUS protocol is an open, standardized communication protocol which provides a means
to monitor and interrogate electronic devices. The device requesting information is the MODBUS
client. The devices sending the information are the so-called serves. There can be several servers
in a MODBUS network, each one is defined by its own address.
MODBUS enables a SCADA system to have direct access to all relevant parameters (e.g. concen-
tration, pressure, cuvette status) and all alarms of the BMT 965. It is also possible to set some
parameters like Low and High Alarm Limits and to execute a zero command. Additionally, Modbus
diagnostic information can be requested.
MODBUS RTU
MODBUS RTU (Remote Terminal Unit) is a binary transmission protocol for serial interfaces
(RS-232 or RS-485). The BMT 965 C and BMT 965 OG are available with a MODBUS RTU
option that implements the common two-wire RS-485 interface with up to 32 nodes. The operation
conforms to the document:
MODBUS Application Protocol Specification, V1.1b
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MODBUS Over Serial Line Specification and Implementation Guide, V1.02
5
Note: Parallel use of MODBUS RTU and other communication interfaces is allowed.
Installation: A BMT 965 with MODBUS RTU option is equipped with a male 5 pole IP67 M12
connector and the female counterpart for the bus cable is part of the delivered accessories.
The RS-485 interface provides galvanic isolation including transient protection. A shielded three
conductor twisted-pair type should be used as bus cable, with an outer diameter (OD) 5 to 8 mm,
e.g. Belden 3106A. The maximum cable length that can be connected is determined by various
factors (transmission rate, cable type, number of connected devices). With a high-quality cable and
proper bus termination (see MODBUS specification) the cable length can be up to 1,000 m, at a
transmission rate of 19,200 bit/s.
Connecting the cable shield can be improve noise immunity. If the cable connects distant locations
with a potential difference in their mains installation, then unwanted potential equalization current
may flow through the shield, making it necessary to connect the cable shield only at one point of
the bus.
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www.modbus.org
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