Rockwell Automation Publication 1794-UM061B-EN-P - March 2020
27
About the FLEX I/O Diagnostic Input Module
Chapter
2
Unused Sensor Power Ports
You must connect dummy resistors to unused Sensor Power ports to mask the
diagnostic function. If external resistors are not used, the module's sensing
circuitry will not detect the intended voltage or current and not report that a
fault exists. The module’s fault and open-channel indicators light, and the
module’s open and module error bits are set, thus rendering fault detection of the
remaining channels useless. The recommended value of this dummy resistor is
20 k
(+10%), 1/8 W (or larger).
Open Contact Mechanical Switches and Relays
The module's sensing circuitry must detect a minimum current level to conclude
that an open circuit
does not
exist. Electronic input field devices typically have
sufficient leakage current to satisfy the minimum requirement. However, hard
contacts have no leakage current, so you must add dummy resistors in parallel to
the hard contacts to supply the minimum current that is needed for the module
to sense that an open circuit
does not
exist. The recommended resistor value is 20
k
(+10%), 1/8 W (or larger). Placement of the dummy resistor at the field
device also allows for monitoring of field wiring conditions. Connect the resistor
between sensor power and input signal, or between sensor power and common.
Sensor Power Internal Voltage Drop
The modules sensor power circuit exhibits an internal voltage drop. This voltage
drop can be as large as 2.2V for all operating conditions. You must subtract 2.2V
from the value of your external user power supply to determine the voltage that is
applied to power attached sensors. Make sure that this voltage meets sensor
requirements. Consult the data sheet for your sensor to determine what voltage is
necessary.
User Power
+
PTC Resistor
Off-Wire
Current Monitor
Fault if:
1 < 50
A
Sensor Power
Sensor
Input
Vsensor = (Ext Pwr) - 2.2V
Overcurrent causes PTC to open
Voltage Drop = 2.2V maximum
24V DC External
User Power Supply
User Power
-