User`s guide

6-6
ground (C
leak,GND
) should be held below about 10-20pF to mitigate damage from
network ESD hits.
Surge Design Issues
Surge voltages encountered in industrial and residential environments as a result of
nearby switching transients and lightning can cause disturbances or failures to electronic
communications systems. Transient voltages and currents can couple capacitively or
magnetically to the twisted pair wiring of the link power system. Physical
construction of twisted pair wire allows transients to couple to the cable in a common-
mode fashion, i.e., both conductors of the wire pair see the same transient. Since link
power nodes float with respect to earth and communication occurs in a balanced
(symmetric) fashion, common-mode transients have minimal effect on transmission.
The single point of earth reference for a link power network segment is the LPI-10
module. When the LPI-10 module clamps an incident voltage transient, a residual
differential transient voltage may result across the twisted pair conductors. This
residual voltage is seen by the LPT-10 nodes. For small transient voltages found in
benign environments no additional protection for LPT-10 nodes is required. However,
additional protection is required to prevent higher energy transients from damaging
LPT-10 nodes. This section describes the recommended protection scheme necessary to
achieve immunity for link power nodes to various levels of transient voltages defined
by the 1993 IEC 801-5 EMC Surge immunity requirements draft.
Designing Systems for Surge Immunity
Adequate creepage and clearance distances must be built into each node's enclosure to
prevent surge discharges from the local LPT-10 transceiver logic ground to earth.
Unaided, LPT-10 transceivers are immune to 801-5 surge test level 1 (0.5kV). Level 2
(1.0kV) and level 3 (2.0kV) surge immunity can be attained (for revision C03 or later
LPT-10 transceivers) with the addition of a bi-directional transient voltage suppresser
(TVS). The TVS must be placed directly across Net_A and Net_B lines at each LPT-10
(not for use between either of the data lines and earth).
TVS
Differential
Protection
Figure 6.2 TVS Schematic Symbol