Rockwell Automation Current Input Module - Manual (Page 11)

2Ć3
2.2 Electrical Description
The module provides sixteen channels of 4Ć20 mA analog current
information to AutoMax andAutoMate systems. Each channel
consists of a pair of high impedance measurement inputs. The
sixteen current inputs share the same common. This analog common
has 2500 VAC of isolation from the system common. See figure 2.2.
An external userĆsuppliedpower supply is requiredfor the 4Ć20 mA
transmitters. Terminal boardassemblies (M/N 61C549 and61C550)
have terminals available for both a primary power supply anda
backup power supply. The backup supply is optional. The two power
supplies are diodeĆisolated as shown in figure 2.2.
The sixteen pairs of inputs are multiplexedto an instrumentation
amplifier which drives a thirteenĆbit plus sign bit A/D converter.
The data register (registers 0 to 15) of a configured channel is
updatedwhen the onĆboardprocessor converts andaverages
together approximately 166 samples (60 Hz) or 200 samples (50 Hz)
of the channel input. In this manner, line frequency noise at the input
of the A/D converter is integratedout of the result that is providedto
the system. This overĆsampling of the analog inputs also simplifies
the filter requirements. A single pole analog filter is locatedon each
measurement input line. The line frequency filter can be disabled on
a perĆchannel basis via register 31 for a faster response time.
The module also contains three voltage references for A/D converter
calibration. These references are checkedprior to each scan of the
configuredchannels, andthe calibration coefficients are updated
accordingly. All nonĆzero references are derived from the A/D
converter's buriedĆzener reference. No potentiometer adjustments
are requiredto maintain the specifiedaccuracy.
The onĆboardprocessor averages the raw A/D converter data,
linearizes the average using the calibration coefficients, andthen
adjusts the reported output value per the userĆselectable scaling
values. If scaling values are not used, the reported output value will
be in the range 0 to 4095. This value will then either be written to the
appropriate channel data register (registers 0Ć15), or it will become
part of the running average specifiedin register 26. If a running
average is specifiedin register 26, the reportedoutput value in the
channel data register is the averaged data value.
The module can also be configured, on a perĆchannel basis, to
provide square root extraction on the input signal. This provides flow
information directly from differential pressure inputs.
A broken wire from a transmitter is indicated through an outĆofĆrange
status bit in register 20. The module also provides high and low limit
registers which can be set by the user. Status registers then indicate
when the desired high/low values have been exceeded.
All read operations on the module's memory include a parity test.
Failure of the parity test results in a system shutdown. Processor
modules can read all locations within the slot address but can only
write to registers 23 through 31.
The module will execute a full set of powerĆup diagnostics which
must execute successfully before the module can go into the run
mode.