Cable Length; Cable Termination - Bosch LA3-VARI-B Installation Manual

VARI-directional array
connected with the AB and YZ terminals interchanged (see previous figure). Because all slave
devices share the same bus, the outputs (YZ) of these units are only enabled during
transmission of data from the unit to the master. The implemented network protocol sets the
output of all other units in a high impedance state during this period to prevent conflicts.
5.5.2

Cable length

The maximum cable length over which the network connection will operate reliably depends
on the cable type and the baud rate used. With good quality cable, a safe maximum figure is
2000 m. If the distance is significantly longer than this, a network repeater will be required.
Refer to section RS-485 network, page 14.
5.5.3

Cable termination

According to the official RS‑485 standard, the network should be terminated with 120 ohm
resistors at both ends of each pair, while the length of the stub lines should not exceed 7 m.
The RS‑485 transceivers, which are implemented in the VARI, are slew rate limited which
minimizes reflections from open cable lengths. This fact, in combination with the relative low
baud‑rate, makes the network very tolerant to stub lengths or improper termination.
Star‑wiring of multiple VARI slaves to the master should be avoided. If star-wiring is needed,
use a multi-port full duplex hub or multiple full duplex RS‑485 repeaters.
Bosch Security Systems B.V. Installation manual
Connector and wiring details | en
2018.06 | V1.3 |
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