iso1685P_D00003_01_M_XXEN / 12.2016
26
Device communication via the BMS bus
8. Device communication via the BMS bus
8.1 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 Ω 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.
maximum length of the RS-485 network: 1200 m
1. device
… device
last device
stub feeder
max. 1 m
A1 A2
E KE
21 22 24
31 32 34
k l kT IT
A B S
RS-485
Term.
off o
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
Term.
off o
CAN 2
A B S
CAN 2
