Rockwell Automation T7411 User Manual
ICS Regent+Plus
®
PD-7031
Monitored Digital Input Modules
24 VDC
(T7411)
Issue 1,
Monitored digital input modules provide input sensing for 16
field input devices. With a line monitor device installed at the
field switch, monitored digital input
switch status and field wiring open and short circuits. Input
and line status are reported back to the controller for use in
application program logic.
Features
modules detect input
March, 06
·
Sixteen input points.
·
Fault tolerant operation when connected in parallel with
another module of the same type.
·
Hot-replaceable.
•
Complete, automatic testing of all input circuits.
•
Automatic line monitoring detects open and short field wiring
faults.
·
Individual front panel indicators on each module show the
module’s active/fault and power status, as well as input status
and line fault status for each point.
·
2500 volt minimum electrical isolation between field and logic
circuits.
·
TÜV certified, Risk Class 5.
The input module circuits are completely and automatically
tested, providing a fail-safe interface for safety-critical inputs.
Two or three monitored digital input modules can be
connected in parallel to obtain fault tolerant input sensing.
In these fault tolerant configurations, a failed module can be
removed and replaced without interrupting the input signals.
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Monitored Digital Input Modules
The module requires connection to a field power supply
voltage, ranging from 15 to 80 VDC. This field power is
internally regulated by the module to 24 VDC to power the
field inputs —
common in safety system applications.
(T7411)
providing safe, low-voltage inputs signals
Module Operation
A block diagram of a typical monitored digital input module is
shown in Figure 1.
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Figure 1. Block Diagram of the Monitored Digital Input Module.
A field power supply is connected to the monitored input
module. The field power is internally regulated to 24 Vdc to
power the input module’s field interface circuits. For each
input ci
dropping resistor to the input screw terminal which is wired to
the field input switch. The other side of the field switch is
rcuit, the regulated power passes through an internal
Industrial Control Services
Monitored Digital Input Modules
(T7411)
wired back to the field power supply return. Optionally a line
monitor device can be connected across the input switch in the
field (as shown in Figure 1).
The input module monitors the voltage of the input circuit
after the internal dropping resistor, comparing it to a
reference voltage generated by the D/A converter inside the
module. The comparator generates an on or off state
depending on which voltage signal is greater than the other.
The field-side FPGA controls and monitors the reference
voltage signal and reads the status of all 16 input
comparators. This information is stored and sent through
optical isolation to the logic-side FPGA. The logic-side FPGA
interfaces the input data to the I/O Safetybus and drives the
module’s front panel status LEDs.
Testing and Diagnostics
Standard I/O Module Testing
The processor modules send triplicated read data requests to
the input module over the I/O Safetybus. The processors’
addressing data and data read requests are voted by the
module (preventing I/O Safetybus failures upstream from the
module from affecting its ability to be read). The voted result
is then passed to the I/O bus interface logic.
After receiving the voted data read request, the I/O bus
interface logic sends its input data to the module’s three bus
drivers. Each of the three bus drivers is independently
controlled — preventing failures in a single driver from being
propagated into the rest of the system.
The bus drivers then move the data onto the I/O Safetybus
which, in turn, passes it to the processors.
Each module’s voter circuits are periodically tested by the
processor modules. Discrepant data are sent through one of
three legs of the I/O Safetybus to determine whether the
module’s voter is able to outvote the incorrect data. A failure
to return the correct majority-voted result t
o the processors
produces an I/O module error indication at the processor
modules and a module fault indication at the I/O module.
PD-7031
Mar-06
3
Monitored Digital Input Modules
Each type of module has a unique identification code that is
read by the controller. This code lets the controller know
which type of module is installed in each I/O chassis slot and
address that module and its points specifically. The processor
modules periodically check each module’s identification code
to determine whether the type of module installed matches
type of module
loaded when the system was started. If a module is removed,
or is replaced with a module of a different type, the processor
modules will indicate I/O module errors.
Loopback logic tests periodically write data to the module and
then read it back to determine whether the module’s I/O bus
interface logic is functioning correctly.
(T7411)
indicated in the I/O configuration that was
Input Circuit Testing
The field interface circuits of the monitored input module are
completely tested to detect stuck-on or stuck-off i
nput circuit
faults and optionally identify input wiring open and short
circuits. The automatic execution and evaluation of the input
testing is controlled by the triplicated Regent processor
modules.
During normal operations, the internal reference voltage is
set to three levels and the comparator output is read. These
three reference voltage levels represent thresholds that
determine whether the input signal is on or off and if the field
wiring is open circuit or short circuit. The logic-side FPGA
dete
rmines input status and line fault status based on the
comparator data for the three voltage levels. The FPGA data
is provided to the I/O processors for application processing.
On a background basis, the I/O processors test the input
circuit comparators and logic circuits for stuck-on and stuck
off failure modes. During testing the D/A converter generates
two reference voltages outside the normal operating range of
the field input voltages to test that the comparator output can
turn-off and turn-on. The
I/O processors read the resulting
input status, line fault status, and reference voltage readings
for the test cycle to determine if there are faults in the input
circuits or the common data paths.
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Industrial Control Services
Monitored Digital Input Modules
Note:
(T7411)
If the I/O processors detect a faulted input circuit, an I/O
module fault is indicated at the processor modules and the
Fault LED on the face of the input module is turned on.
Input Circuit Test Interval
The Regent processor modules schedule testing of the input
circuits on a background basis. The test interval
for these
circuits may range from a few seconds to several minutes,
depending on the application program scan time and the size
of the I/O configuration. The equation below can be used to
estimate the test interval for monitored digital inputs.
TI = 172 * IOU
where:
TI
IOU
TSCAN
=
=
QTY
=
* TSCAN + 2
QTY
Test interval, seconds
Quantity of I/O Units in the system (1 to 16)
The application program scan time, seconds
For example, for a system with 8 I/O units and an application
scan time of 60 milliseconds, the
TI = 172 * IOU
TI = 84.6 seconds
* TSCAN + 2 = 172 * 8 * 0.060 + 2
QTY
test interval would be:
In this system, all of the monitored input modules would be
tested for stuck-on and stuck-off faults approximately every 85
seconds. This test interval can be used in reliability and
availability calculations to select the fault tolerant input
configuration that meet the application’s safety requirements.
PD-7031
Mar-06
Due to I/O processor fault filtering algorithms, it may take up
to four test intervals to report a failed input module as a
permanent fault
Front Panel Indicators
Figure 2 shows the physical features of the monitored digital
input module. The front panel of each module contains
fault/active and power indicators for the module as well as
input status indicators for each channel.
Active/Fault Status Indicator
This green and red LED pair indicates the overall health of
the module and its field circuits. During normal operation the
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Monitored Digital Input Modules
(T7411)
green ACTIVE indicator flashes at the
controller's scan rate.
If a module fault occurs the red FAULT indicator turns on
and the green indicator turns off.
Power Status Indicator
The POWER GOOD LED indicates the presence of field
voltage at the module’s field power input terminals and the
overall health of the module’s field power regulator circuits.
Input Status Indicators
Input status indicators show contact and line status for each
point.
A yellow contact status indicator is turned on when the
contact is closed or the line has shorted.
A red line status indicator is turned on when the field wiring
is open circuit or short circuit. If line monitoring is not
desired, this LED can be disabled (see Disable Fault LED,
page 17 for more details).
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Monitored Digital Input Modules
(T7411)
PD-7031
Mar-06
Figure 2. Monitored Digital Input Module.
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