OZONE ANALYZER BMT 964 Manual, Rev. 04/2021
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Appendix G: MODBUS RTU Communication
MODBUS RTU is a widely used serial communication protocol. The physical interface used in
this implementation of the protocol is the common two-wire RS485 connection. MODBUS RTU
supports up to 32 nodes on the bus, with one master and several slaves. It is available as an op-
tion for all IP65 variants of the BMT 964. This implementation conforms to the following
MODBUS standards:
MODBUS Application Protocol Specification , V1.1b
MODBUS Over Serial Line Specification and Implementation Guide, V1.02
These documents can be found at http://modbus-ida.org/specs.php.
MODBUS enables a SCADA system to have direct access to all relevant parameters (concentra-
tion, pressure, cuvette status etc.) and all alarms of the BMT 964. 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.
Communication parameters can be set on the front panel, while the instrument is connected to
the bus, or via the RS-232 serial port with the BMT 964 Link program.
Physical Connection
The MODBUS interface is electrically isolated and has built in transient protection. A BMT 964
equipped with MODBUS comes with a male 5 pole IP67 M12 connector. The female counter-
part for the bus cable is also supplied. The cable should be of a shielded three conductor twisted-
pair type, OD 5-8 mm, e.g. Belden 3106A. With such a cable and proper bus termination (see
MODBUS standard) it is possible to transmit data at a speed of 19200 Baud over a distance of
1,000 m. Connecting the cable shield can improve noise immunity, but can also create problems,
if the cable connects distant locations. In this case large currents due to potential difference be-
tween different mains installations could flow, making it necessary to connect the cable shield
only at one point of the bus.
Electrical Connection:
Pin Function
1 Connect with 2
2 Connect with 1
3 0 V Common
4 Signal A (-)
5 Signal B (+)
It is important to connect pin 1 to pin 2, because this bridge will signal to the BMT 964, that
a MODBUS cable is connected (MODBUS auto detect). MODBUS communication parameters
will be automatically loaded, and communication can commence, as soon as this bridge is detect-
ed. Also, MODBUS parameters can now be set in the ‘Set I/O’ menu on the front panel.
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