Rockwell Automation 61C544A RTD Module - Manual (Page 11)

2Ć3
2.2 Electrical Description
The module provides sixteen channels of precise RTD temperature
information to AutoMax and AutoMate systems. Each channel
consists of a constant current source and a pair of high impedance
measurement inputs. See figure 2.2. The current sources are
powered by an onĆboard supply that is isolated from the system
common. No external userĆsupplied power supplies are required.
Thesixteen RTD inputs sharethesamecommon. This analog
common has 2500 VAC of isolation from thesystem common.
The sixteen pairs of measurement inputs are multiplexed to an
instrumentation amplifier which drives a fourteenĆbit A/D converter.
This configuration measures the voltage drop across the selected
RTD element.
The data register (registers 0 to 15) of a configured channel is
updated when the processor converts and averages channel input
samples over 16.6 ms for a 60 Hz line or 20 ms for a 50 Hz line. In
this manner, line frequency noise at the input of the A/D converter is
integrated out of the result that is provided to the system. This
overĆsampling of the analog inputs also simplifies the filter
requirements. A single pole analog filter is located on each
measurement input line.
The module also contains positive and zero references for A/D
converter calibration. These references are checked at the end of
each scan of the configured channels and the calibration coefficients
are updated accordingly. All nonĆzero references are derived from
the A/D converter's buriedĆzener reference. The reference for the
constant current sources is likewise derived from the A/D converter.
This provides a ratiometric style of conversion. No potentiometer
adjustments are required to maintain the specified accuracy.
The onĆboard processor averages the raw A/D converter data,
linearizes the average using the calibration coefficients, and then
performs a table lookĆup based upon the specified alpha value of the
RTD. This value will then either be written to the appropriate channel
data register (registers 0Ć15) or will become part of the running
average specified in register 26. The running average value is stored
in 0.1_C or 0.1_F units in the appropriate channel data register.
A broken wirefrom theRTD is indicated through an out of range
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 low/high values have been exceeded.
All read operations on the module's memory by the onĆboard
processor include a parity test. Failure of the parity test results in a
module 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 executesuccessfully beforethemodulecan go into therun
mode.