Rockwell Automation T8403 Trusted 24V dc Digital input Module - 40 Channel - Manual (Page 12)

Trusted
TM
Module T8403
Issue 15 Jun 13 PD-T8403 12
The module biases each input channel by means of a 5kΩ (typical for 24V dc digital inputs) termination
resistor to the 0V reference of the module input circuitry. In the absence of any line fault, the zener
diodes used for line monitoring purposes form a voltage divider with this 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, illustrated
in the previous threshold table, default threshold values used for non line monitored inputs are as
follows (in raw units)
Default = -4096, -2305, -1971, 3046, 3072, 7168, 8192, 14848, 15872, 19456
1.6. Housekeeping
The input module automatically performs local measurements of several on-board signals that can be
used for detailed fault finding and verification of module operating characteristics. Measurements are
made within each slice’s HIU and FIU.
1.7. Fault Detection and Testing
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 Σ∆ 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 Σ∆ 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.
1.8. Sequence of Events Characteristics
The input module automatically measures the field-input voltage, compares the value to the
configurable thresholds, and determines the state of the field input. An event occurs when the input
transitions from one state to another. When an input changes state, the on-board real-time clock value
is recorded. When the TMR Processor next reads data from the input module, the input state and real-
time clock values are retrieved. The TMR Processor uses this data to log the input state change into
the system Sequence of Events (SOE) log. The user may configure each input to be included in the
system SOE log. Full details of SOE are contained in PD-8013 – Trusted
TM
SOE And Process
Historian.