Installation guide

12 en | Installation with RS-485, AMC and Access PE Access Control by Example
| 1.0 | 2009.10 Introductory Guide Bosch Security Systems
3.4.2 Shielding data cables and avoiding ground loops
Cables with cores that carry data have a conducting wrapper, accompanied by a naked
grounding wire, between the cores and the outer plastic casing. When the naked wire is
grounded properly this wrapper “shields” the cores from electrical interference. Without
shielding, the integrity of the data signals is threatened.
A common installation error (particularly in cases where the reader-end and the controller-end
of the cable are handled by different persons) is to ground the shielding at both ends. If the
two grounds are not of identical potential, there is a possibility of current flow through the
shielding, which can disrupt the signals in unpredictable ways, cause malfunctions in the
access control hardware and even masquerade as software errors. This phenomenon is
known as a ground loop.
Figure 3.3 Avoiding a ground loop
Figure 3.1 A diode
Figure 3.2 Position of the protective diode
(Note the Key: 1: Door opener, 1A: Protective diode, 5: Access controller
colored ring) The colored ring corresponds to the crossbar in the diode symbol.
5: Access controller 6: Shielding around cable 2: Reader
CAUTION!
To avoid ground loops, be sure to ground cable shields only ONCE.
CAUTION!
Follow carefully the grounding instructions for the reader and other sensitive components.
Failure to ground components correctly can result in damage to those components and to
malfunction of the access control hardware, which can masquerade as a software error.