Rockwell Automation 1746-NI8 SLC 500 Analog Input Modules User Manual - Manual (Page 30)

3–9 Installation and Wiring
Publication 1746Ć6.8 - April 1997
Wiring Guidelines
!
ATTENTION: To prevent shock hazard, care should
be taken when wiring the module to analog signal
sources. Before wiring any analog module, disconnect
power from the SLC 500 system and from any other
source to the analog module.
Follow the guidelines below when planning your system wiring.
•
To limit noise, keep signal wires as far away as possible from
power and load lines.
•
To ensure proper operation and high immunity to electrical noise,
always use Belden 8761 (shielded, twisted pair) or equivalent
wire.
•
Connect the shield drain wire to the shield terminal on the
1746-NI8 module. Up to four shield wires may be connected to
each shield connection screw terminal that provides a connection
to chassis ground.
•
Tighten terminal screws using a flat or cross-head screwdriver.
Each screw should be turned tight enough to immobilize the
wire’s end. Excessive tightening can strip the terminal screw.
The torque applied to each screw should not exceed 5 lb-in
(0.565 Nm) for each terminal.
•
Follow system grounding and wiring guidelines found in your
SLC 500 Modular Hardware Style Installation and Operation
Manual, publication 1747-6.2.
Transducer Source Impedance
If the source impedance of the input device and associated cabling is
too high, it will affect the accuracy of the channel data word. Source
impedance of 500 ohms will produce up to 0.05% of module error
over and above the specified accuracy of the module.
You can compensate for device impedance error by implementing
the following equation in your ladder program:
V
S
+ V
measured
ƪ
(R
s
) R
in
)
R
in
ƫ
where: Vs = input device voltage
R
s
= input device source impedance
R
in
= 1746-NI8 input impedance (1 M
Ω
)
Input Devices