iSMA-B-FCU /FCU Hardware
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consumption with thermal valves and external devices cannot exceed 7 VA (~0.3
A).
iSMA-B-FCU-LL is equipped with a 24 V AC, 7 VA power supply output to supply the external
equipment like sensors and actuators. This power supply uses a separate 24 V AC transformer.
The external separate power supply terminal connection is labeled L2, N2.
2.3 Connecting the communication bus (RS485)
Figure 7 RS485 connection
2.3.1 RS485 grounding and shielding
This device can be exposed to electromagnetic field. Electromagnetic radiation can induce
electrical noise into both power and signal lines, as well as direct radiation into the FCU device with
negative results for the system. Appropriate grounding, shielding and other protective steps should
be taken at the stage of installation to prevent these effects. These protective steps include
grounding the control cabinet and the cable shield, installing protective elements for
electromagnetic switching devices, correct wiring, as well as proper choice of cable types and their
cross sections.
2.3.2 RS 485 network termination
Transmission line effects often present a problem for data communication networks. These
problems include reflections and signal attenuation.
To eliminate the presence of reflections of signal from the end of the cable, the cable must be
terminated at both ends with a resistor across the line adequate to its characteristic impedance.
Both ends must be terminated since propagation is bidirectional. In case of an RS485 twisted
pair cable this termination is typically 120 Ω.
