Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 35)

Assembly and Installation
Chapter 2
2Ć22
or indicating instrument if used (typically 250 ohms) must be considered
when calculating the voltage drop in the loop.
For example, the required voltage for an input loop that contained the PID
module, manual control station, and current transmitter that requires at
least 18V dc would be 18V +2 (250 ohms) (.020 amps)= 18V +10V =
28V dc. Up to 2 V dc could be allowed for voltage drops in the cables
when using a 30 V dc source such as the +15V dc analog power supply
with the module output configured for current operation and additional
compliance.
If the input loop were to contain a chart recorder and an indicating
instrument (250 ohms each) in addition to the basic loop in the previous
example, the power supply would have to provide a least 38V dc plus an
amount equal to the voltage drop of the cables. A separate power supply
would have to be used.
Power Supply Current
The +15V dc analog power supply must provide 100mA to the PID
module’s analog circuits. This includes 20mA that the module provides to
each output current loop. If the +15V dc supply is providing 20mA to
each input current loop, a 2-loop control system will require 140mA. This
is within the specification for the Allen-Bradley 1770-P1 power supply
rated at 150mA at +15V dc.
To minimize the effects of electromagnetic interference (EMI), group
analog modules together in the I/O chassis whenever possible. Avoid
placing PID modules close to AC modules or high voltage DC modules.
It is important to use shielded cable when wiring analog signals. Belden
8761 or equivalent cable has an insulated twisted pair of conductors
covered by a foil shield. To reduce the effect of EMI induced noise along
the cable, the shield must be properly grounded at one end, only. The
recommended grounding point for each cable is identified in the wiring
diagram (Figure 2.5-Figure 2.8 and Figure 2.10). One end of the shield
should be cut short and taped to insulate it from any electrical contact.
The other end should be tied to a common point at earth ground.
Optional Instrument Grounding Bus
An alternate grounding system can be used to separate analog signal
ground from the grounding of equipment such as motor starters and are
Installation Practices