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EXAMPLE 4:
Looping occurs between both the Link and
CobraNet connections.
8.6.7 AVOIDING STUBS AND CORRECT 120 OHM TERMINATION
A long stub or branch off of an EIA-485 bus can cause a significant impedance mismatch and thus reflections
and degradation of the computer control signals. Stubs of several feet / 1 m or more are considered "long."
Avoid using stubs. If a stub is necessary, keep it as short as possible to avoid communication problems.
Rule-of-thumb
You can avoid stub and termination issues by connecting no more than two EIA-485 cables at any one
module. Turn on the termination switch only for the module with one EIA-485 cable connected to it. This
will, by definition, be the one at the end of the cabling.
EXAMPLE:
This installation may have problems with the
EIA-485 branch from DSAi "A" down to "B". If
in close physical proximity, this cable can be
replaced with a Link cable connection from "A"
to "B". If this is a long distance, connect "C" to
"B" instead of "A", as shown by the dotted line.
For either condition, the end of line termination
is turned on only for "D". This is because "D" is
at the opposite end of the daisy-chained EIA-485
cabling from the PC.
43
PC
DSA PILOT
DSA "A"
ETHERNET
SWITCH
DSA "B"
CM1
SLink
485
PLink
CM1
SLink
485
PLink
CM1
SLink
485
PLink
CM1
SLink
485
PLink
DSA "C"
DSA "D"
CobraNet
Source
Figure 8.6.6c Example 4
PC
DSA PILOT
DSA "A"
DSA "B"
CM1
SLink
485
PLink
CM1
SLink
485
PLink
CM1
SLink
485
PLink
CM1
SLink
485
PLink
DSA "C"
DSA "D"
Figure 8.6.7 Example