Rockwell Automation T8423 Trusted TMR 35 - 120 Vdc Digital Input Module - Manual (Page 14)

Trusted
TM
Module T8423
Issue 15 Apr 10 PD-T8423 14
When a module is inserted into the standby slot in a line-monitored application, then the field
terminations for all the input channels are paralleled, resulting in a drop in the input voltages for the
active module.
The module biases each input channel by means of a 60kM (typical for 120V dc digital inputs)
termination resistor to the 0V reference of the module input circuitry. In the absence of any line fault,
the resistors used for line monitoring purposes form a voltage divider with the termination resistor in
the module. Typically, the values are chosen so that the open contact voltage at the input terminal is
about
1 /
3
of the field supply; with the user contact closed the voltage is about 2 /
3
of the field supply.
Default threshold values used for non line monitored inputs are as follows (in raw units)
Default = -640, -256, 2432, 3712, 11136, 12416, 18560, 19840
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.
Module replacement activities depend on the type of spare module configuration chosen when the
system was configured and installed. The module may be configured with a dedicated Companion Slot
or with a SmartSlot for a spare replacement module.
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-correcting 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 a series of optically isolated links for data and power. The data
link is synchronised and monitored for variance. Both the FIU and HIU have onboard temperature
sensors to characterise temperature-related problems. Each FIU is also fitted with a condensation
sensor.
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.
The module field input is connected to a single bit ADC known as the input circuit. These circuits,
one per channel on each slice, produce a digital output which naturally transitions between on and off.
Any failure in the circuit causes the output to saturate to stuck-on or stuck-off which is automatically
detected. As the conversion process is dynamic and not gated like traditional ADCs, failures are
rapidly diagnosed and located.
By using the circuit, the analogue path in the module is short and does not involve many
components. This results in analogue failures being contained to a single channel on a single slice
instead of causing a group of eight or more inputs to fail.