Rockwell Automation T8424 Trusted TMR 120Vac Digital Input Module - Manual (Page 9)

Trusted TM Module T8424
Issue 8 Sep 11 PD-T8424 9
1.5. Line Monitoring Thresholds
The T8424 Input module provides thresholds on the AC input amplitude that are in accordance with
the requirements of IEC61131-2. (sec 5.2.3) Amplitudes above 82VAC are reported as ON (state 4),
and amplitudes below 26VAC are reported as OFF (state 2). The range between these two values
provides the hysteresis.
States
Input Voltage
(RMS) 82V+/-10% 26V+/-10%
2
4
Figure 3 Input Monitoring Thresholds
Line monitoring is not provided by the module. Open field signals will be reported as OFF. Shorted
field loops will blow the excitation fuse and be reported as OFF. Open wires between the FTA and the
module are detected and the signal state is reported as state ‘0’, with the channel LED flashing red.
Note that the Threshold template which may be entered in the System.INI configuration is not used by
this module and has no effect on the state discrimination.
1.6. Housekeeping
The input module automatically performs local measurements of several on-board signals that can be
used for detailed troubleshooting and verification of module operating characteristics. Measurements
are made within each slice’s HIU and FIU
1.7. Fault Detection and Testing
Extensive diagnostics provide the automatic detection of module faults. The TMR architecture of the
output module and the diagnostics performed ensure the validity of all critical circuits. Using the TMR
architecture provides a Fault Tolerant method to withstand the first fault occurrence on the module and
continue normal output controls without interruption in the system or process. Faults are reported to
the user through the Healthy status indicators on the front panel of the module and through the
information reported to the TMR Processor. Under normal operations all three Healthy indicators are
green. When a fault occurs, one of the Healthy indicators will be flashing red. It is recommended that
this condition is investigated and if the cause is within the module, it should be replaced.
From the IMB to the field connector, the input module contains extensive fault detection and integrity
testing. As an input device, all testing is performed in a non-interfering mode. Data input from the
IMB is stored in redundant error-detecting RAM on each slice portion of the HIU. Received data is
voted on by each slice. All data transmissions include a confirmation response from the receiver.
Between the HIU and FIU, there are optically isolated links for data and magnetic isolation for power.
The data link is synchronised and monitored for communication errors. Both the FIU and HIU have
onboard temperature sensors to characterise temperature-related problems.
The power supplies for both the HIU and FIU boards are redundant, fully instrumented and testable.
Together, these assemblies form a Power Integrity Sub System.