Serial interfaces
53
TGH1361en/11.2009
6.2 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
network 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
result in malfunctions. Only the first and the last device in one line may be
terminated. 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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