Module Operation; Module Field Calibration - Allen-Bradley Compact 1769-IR6 User Manual

1-8 Overview
Input
CHN0
EXC0
SENSE0
RTN0
1
2
3
4
5
Publication 1769-UM005A-EN-P

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.
VA2 VA1
EXC
Current
Multiplexer
A-GND
Vref
VA3
Channels 1 through 5 same as
channel 0 above.
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.
VA1
VA1
AIN+1
A/D MCU
AIN+2
AIN-
VREF
VA2
Channel Select
VA3
TXD
ASIC
VS1
DC/DC
Power Supply
VA1
VS2
S-GND
A-GND