iso1685DP_D00272_03_M_XXEN/08.2017
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Device communication via the BMS bus
10. Device communication via the BMS bus
10.1 RS-485 interface with BMS protocol
The RS-485 interface, galvanically isolated from the device electronics, serves as a physi-
cal transmission medium for the BMS protocol. When an ISOMETER® or other bus-capable
devices are interconnected via the BMS bus in a network, the BMS bus must be terminat-
ed at both ends with a 120 resistor. For this purpose, the device is equipped with the
terminating switch RS-485 Term. (ON/OFF).
An RS-485 network that is not terminated is likely to become unstable and may result in
malfunctions. Only the first and last device in one line may be terminated. Hence, stub
feeders in the network must not be terminated. The length of the stub feeders is restrict-
ed to 1 meter.
10. 2 Topology RS-485 network
maximum length of the RS-485 network: 1200 m
1. device … device last device
stub feeder
max. 1 m
The optimum topology for an RS-485 network is a daisy-chain connection. In this connec-
tion, device 1 is connected to device 2, device 2 to device 3, device 3 to device n etc. The
RS-485 network represents a continuous path without branches.
Correct arrangement
Three examples for correct arrangement:
Wrong arrangement
Three examples for wrong arrangement:
A1 A2E KE
21 22 2431 32 34k l kT ITA B S
RS-485
Ter m .
o on
CAN 1
CAN 2
I2+ I2- I1+ I1-
K3 K2
11 12 14
K1
I2+ I2- I1+ I1-
A B S
k I kT IT
31 32 34 21 22 24 11 12 14
E KE
A1 A2
A B S
RS-485
Te r m .
o on
CAN 2
A B S
A B
A B
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