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IRDH275BM-7_D00123_06_M_XXEN/07.2018
6. Serial interfaces
6.1 RS-485 interface with BMS protocol
The RS-485 interface which is galvanically isolated from the device electronics
and current output serves as a physical transmission medium for the BMS pro-
tocol. If several ISOMETER®s or other bus-capable devices are interconnected
in a network via the BMS bus, the BMS bus must be terminated at both ends
with a 120 Ω resistor.
An RS-485 network that is not terminated, is likely to get instable and may re-
sult in malfunctions. Only the first and the last device in one line may be ter-
minated. Devices in between must not be terminated with 120 Ω. Hence, stub
feeders in the network must not be terminated. The length of the stub feeders
is restricted to 1 meter.
AB AB AB
1st device ... device last device
120 W
0.4 W
120 W
0.4 W
Stub feeder
maximum 1 m
Maximum length of the RS485 bus: 1200 m
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