MDO16GNS October 2005 – Issue 2
9
Triguard
SC300E MDO16GNS 16-Channel D/O Module 48Vdc
The data streams are then applied to the 16-bit output shift registers. The serial bits are
clocked successively into the shift registers until the 16th bit has been received. The registers
then latch the data and present it on the 16-bit busses (CHANA, CHANB and CHANC) to the
output switches. The data is continuously refreshed in this manner and updated at each scan
of the SC300E system.
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 outputs 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.
Testing of the output switches by the LFD circuits is co-ordinated by the microcontrollers on
the common interface. When all outputs are in the healthy condition and the microcontrollers
confirm no faults present, the SC300E processors (main processors) will instruct each
microcontroller in turn to switch its output alone to the opposite state and confirm the correct
operation of its individual output switch.
The module power requirements are served from two different sources. The power for the
output-side circuits is derived from the chassis 12V supply via dc/dc converters. All of the
supplies are monitored. Electronic fuse circuits sense the current in each output path and cut
off the upper FET if the current exceeds a predetermined limit.
An On/Off Line Request switch on the front panel enables a request to be sent to the SC300E
system that the module be taken off-line for maintenance purposes or returned on-line. The
module contains a hardware identity circuit that enables system identification of the module,
and three manual links 1, 2 and 3. Link 1 allows the module to be set up for 321 or 320 mode
operation which sets the threshold that determines how much of the circuit can be degraded
while still preserving overall operation. 320 mode means that the system will continue to
function with two out of three serviceable circuits. If the number falls to one out of three the last
read data is maintained. In 321 mode the system will continue to function with one out of three
serviceable circuits. If that fails the last read data is maintained and the module is taken off-
line.
Link 2 (HLV/GTZ) determines whether, in the event of a failure due to 321/320 action, the last
read values are held (HLV) or are set to zero (GTZ). Link 2 is only active however, if Link 3
(ICCB/HW) is set to HW. If Link 3 is set to ICCB Link 2 is ignored and its function determined
by software.
