Rockwell Automation MAI32AD 32-Analogue Input Module Differential Input - Manual (Page 9)

MAI32*AD October 2005 – Issue 4 9
Triguard SC300E MAI32*AD 32-Channel A/I Module Differential Input
THEORY OF OPERATION
Analogue input module
The first part of the block diagram in Figure 2-5 shows the circuit used to process one of the 32
analogue inputs. The field input is connected across a ranging resistor which is selected in
accordance with the type of field input to be processed (current or voltage). The channel signal
is then distributed along four paths (A to D); paths A, B and C are the triplicated signal
processing circuit and path D feeds the metering test point.
In each path the signal is first passed through RF and low-pass filters before being applied to
one channel of a 32 channel multiplexer. For paths A, B and C a command from the
microcontroller in the Common Interface is isolated, decoded and used to select which input
channel will be switched to the output of the multiplexer. The multiplexer output is then buffered
and sent to an analogue to digital (ADC) converter which produces a serial data output coded
to represent the amplitude of the channel input signal. Factory set links connected to the ADC
enable the basic module to be configured for different +5Vdc or +10Vdc voltage ranges. The
ADC output is passed via an opto-isolator to the Common Interface for processing.
There is no ADC in path D, the buffered signal is applied directly to the metering test point on
the front panel. Channel selection is by means of the adjacent channel selector switches.
Power to the module is 5.4Vdc and 12Vdc supplied by each of the dual redundant SC300E
power supply units. Each line is divided into four branches to provide a supply to each of the
four paths. The dual inputs for both supplies (12V and 5.4V) in each branch are ORed via
diodes to provide the dual redundancy. After filtering and surge limitation the output rails VEE*
(nominally12V) and VCC* (nominally 5V) provide power to their respective paths. The A, B and
C 5V rails also supply power to the corresponding control circuits in the Common Interface.
Each of the A, B and C path supplies is also continuously monitored and the monitor outputs
A/PSUOK, B/PSUOK and C/PSUOK are sent via the Common Interface to the MPPs for
diagnostic purposes.
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.
The module contains a hardware identity circuit that enables system identification of the
module.
Links
Only user changeable link is identified as LK3 and located in the top right hand corner of the
module (see Figure 1-1). The link (LK3) enables the module to be set for 321 or 320 mode.
Modes of operation
The 321 or 320 mode sets the threshold that determines how much of the circuit can be
degraded while still preserving overall operation. Setting the 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.