Serial interfaces
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IRDH275_D00122_02_M_XXEN/10.2016
7.2 RS485 interface with BMS protocol (IRDH275B)
The RS485 interface galvanically isolated from the device electronics and cur-
rent output serves as a physical transmission medium for the BMS protocol. If
several IRDH275B or other bus-capable devices are interconnected in a net-
work via the BMS bus, the BMS bus must be terminated at both ends with a
120 Ω resistor.
An RS485 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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