Technical Documentation
BLR-CM Modbus
Rev. 05
2018-09
7
2.1.2
Line termination
The termination of the bus wire is strictly necessary, in order to prevent interferences and set up a stable connection.
For this reason, both wire ends of the bus must be terminated with a resistor. The value of the resistor must fit to the
cable impedance and is generally selected as 120 Ω. A resistor must be connected at each end of the bus-segment.
Some devices, particularly bus converters, include a termination resistor by default. Therefore, it is necessary to check
the user manuals of all implemented bus devices. If devices, with integrated termination resistor, are connected within
the bus segment, their termination resistors must be switched off. In case it is not possible to switch it off, the respective
devices must be placed at the ends of the bus! Consequently, it is only possible to use two devices with fix termination
resistors!
2.1.3
Line biasing
Without a bias voltage, the logic level in the data wires would be undefined while no data is transmitted. Additionally,
external influences may lead to interferences of the signal level. For this reason, line biasing is required.
This is done by two resistors, which must be in the range of 450 Ω…650 Ω. The first resistor is used as pull-up resistor
and is connected between the data wire + resp. D(+) and +5 V. The second resistor is used as pull-down resistor and is
connected between the data wire – resp. D(-) and 0 V. These resistors are required once per bus segment and their
position within the bus segments is freely selectable. However, it is recommend to select a position in the middle of the
bus wire. Before installing these resistors, it must be checked if the implemented devices already include integrated
bias resistors.
Attention: In case of the BLR-CM, a line bias voltage must be provided externally.
2.1.4
Communication indicator
The yellow LED on the backside of the device indicates an active transmission.
It only flashes, when the device is actually communicating with the bus
master.
The communication indicator LED is available for both connector variants.