Cobranet® Input; Common Analogue Grounding Issues - Bosch LA3-VARI-B Installation Manual

VARI-directional array
4.2.7 CobraNet® input
The CobraNet® interface carries multiple channels of digital audio as well as all control and
monitoring data. If a VARI-CN CobraNet® interface module is fitted to the base unit, a single
CAT‑5 cable to the RJ‑45 socket on the card replaces both the normal RS‑485 and audio
connections.
4.2.8

Common analogue grounding issues

Using correct grounding procedures when connecting VARI line arrays gives several benefits:
Safety
The GND terminal of the IEC mains connector provides a direct low impedance path from the
metal parts of the chassis to ground. Always connect this terminal.
Reduction of RF emission
Although the electronics module in the VARI base unit is well shielded and external
connections are decoupled to prevent RF emission from the internal high speed digital
circuits, this protection will not work properly if the chassis is not connected to ground.
RF Immunity
RF currents induced in the signal cables by external RF fields are effectively shorted to
chassis-ground, provided that the cable screen (shield) is of sufficiently low impedance.
In addition to the mains ground, the VARI‑B/BH connector block has ground pins on several of
the other connectors. Each serves a dedicated purpose and care should be taken that they are
not connected to each other.
The GND pins on the line level audio inputs are connected internally directly to the chassis of
the VARI base unit. These GND pins are only for connecting the screens (shields) of the audio
cables.
The GND pins on the RS‑485 connectors are intended as a terminal for the network cable
screens (shields). Since the RS‑485 interface is optically isolated (to prevent ground loops),
this ground has no relation to the chassis ground. It should not be connected to any other
ground pin on the chassis.
Bosch Security Systems B.V. Installation manual
Installation | en 15
2018.06 | V1.3 |