Allen-Bradley Compact 1769-IR6, User Manual

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Publication 1769-UM005A-EN-P
1-8 Overview
Module Operation
As shown in the block diagram below, each input channel of the module
consists of an RTD/resistance connection that accepts excitation current; a
sense connection that detects lead wire resistance; and a return
connection. The signals are multiplexed to an A/D converter that reads
the RTD or resistance value and the lead wire resistance.
From the readings taken by the converter, the module returns an accurate
temperature or resistance to the controller user program through the
microprocessor. The module uses two bidirectional serial ports for
communication, each using an optocoupler for isolation. A third
optocoupler is used to reset the microprocessor if the module detects a
loss of communication.
Module Field Calibration
The input module performs autocalibration when a channel is initially
enabled. Autocalibration compensates for offset and gain drift of the A/D
converter caused by temperature change within the module. An internal,
high-precision, low drift voltage and system ground reference is used for
this purpose. In addition, you can program the module to perform a
calibration cycle once every 5 minutes. See Selecting Enable/Disable
Cyclic Autocalibration (Word 6, Bit 0) on page 4-19 for information on
configuring the module to perform periodic calibration.
VA2
A-GND
VA1 VA1
AIN+1
AIN+2
AIN-
VREF Vref
VA3
VA1
TXD
VA1
VA2
VA3
A-GND
VS2
VS1
S-GND
1
2
3
4
5
Input
EXC0
SENSE0
RTN0
CHN0
Multiplexer
EXC
Current
A/D MCU ASIC
BUS
Channel Select
DC/DC
Power Supply
Channels 1 through 5 same as
channel 0 above.