Rockwell Automation MDO32BNS 32-Channel Digital Output Module - Manual (Page 9)

MDO32BNS October 2005 – Issue 7 9
Triguard SC300E MDO32BNS 32-Channel Digital Output Module
The output switch for each channel comprises six FETs connected in the series/parallel
network shown and providing 2 out of 3 majority voting between the three paths A, B and C.
A front panel LED at the switch output is lit when the switch is closed. Voltage and current
monitoring circuits are also connected to the switches to provide outputs for the Latent Fault
Detection (LFD) system, these are fed back to the microcontrollers via dedicated shift registers
and opto-isolators. The feedback information confirms that the output has switched to the
commanded state and that the line is not open or short circuit
Figure 2-4. FET ‘hex’ voter
Testing of the output switches using the LFD circuits is co-ordinated by the microcontrollers.
When all outputs are in the healthy condition and the microcontrollers confirm no faults present
the main processors will instruct each microcontroller in turn to switch its output alone to the
opposite state and confirm the correct operation of the output switch. This test also checks the
integrity of the output loops for open circuit conditions.
The module power requirements are served from two different sources. The power rails and
reference levels for the system-side circuits are derived from the 5.4V and 12V supplies from
the system chassis. The power rails and reference levels for the output-side circuits are derived
from the field supplies and also serve to confirm that the field circuits are in working order
An On/off Line Request switch on the front panel enables a request to be sent to the system
that the module be taken off-line for maintenance purposes or returned on-line.