Rockwell Automation MHB44IND Pulse Input and Analogue Output Module - Manual (Page 9)

MHB44IND October 2005 – Issue 2 9
Triguard SC300E MHB44IND Pulse Input and Analogue Output Module
THEORY OF OPERATION
The module has four isolated 1Hz to 35kHz pulse-input frequency measuring channels, and
four 0-22mA current output channels using a common 24V supply via termination card
TPH44AIC. The circuits are triplicated except for the final combining of the analogue output
stages into a single voted output.
Common functions
Power supplies and monitoring
The 5V and 12V dual-redundant supplies from the backplane are diode-OR’ed to the three
channels plus an auxiliary section. The analogue output section is isolated by dc/dc converters
providing +5V and ±15V. All the supplies and fuses are monitored by the CI.
An On/Off Line Request switch on the front panel enables a request to be sent to the MPPs
that the module be taken off-line for maintenance purposes or returned on-line.
Mode settings
The module contains a hardware identity circuit that enables system identification of the
module, and seven configuration jumper links (JP1to JP4 and LK1 to LK3). The jumper links
JP1, JP2, JP3 and JP4 must be set to the Off position. The operation of and settings links LK1
to LK3 is described below:
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 3 is always set to HW.
Latent fault detection (LFD) on outputs
The voted analogue voltage outputs from each channel are compared in turn with the D-to-A
outputs. The LFD scheme employed is to change each channel’s output in turn by 0.4V for
1ms, first negatively then positively, leaving the other channels unchanged. At each change,
the discrepancy between every channel and the common output is tested by a window
comparator. If a difference is detected on the deviated channel and the other two are 'same'
then no latent fault is detected and the Health LED is illuminated.