General Diagnostic Features; Section 1.2 System Overview; System Operation; Module Operation - Spectrum Controls 1769 Series User Manual

Section 1.2
System Overview

1.1.5 General Diagnostic Features

The module contains a diagnostic LED that helps you identify the source of
problems that may occur during power-up or during normal channel operation.
The LED indicates both status and power. Power-up and channel diagnostics are
explained in Chapter 4, Diagnostics and Troubleshooting.
The modules communicate to the controller through the bus interface. The
modules also receive 5 and 24V dc power through the bus interface.

1.2.1 System Operation

At power-up, the module performs a check of its internal circuits, memory, and
basic functions. During this time, the module status LED remains off. If no
faults are found during power-up diagnostics, the module status LED is turned
on.
After power-up checks are complete, the module waits for valid channel
configuration data. If an invalid configuration is detected, the module generates
a configuration error. Once a channel is properly configured and enabled, it
continuously converts the thermocouple or millivolt input to a value within the
range selected for that channel.
Each time a channel is read by the input module, that data value is tested by the
module for an over-range, under-range, open-circuit, or "input data not valid"
condition. If such a condition is detected, a unique bit is set in the channel status
word. The channel status word is described in Input Data File on page 3-11.
Using the module image table, the controller reads the two's complement binary
converted thermocouple or millivolt data from the module. This typically occurs
at the end of the program scan or when commanded by the control program. If
the controller and the module determine that the data transfer has been made
without error, the data is used in the control program.

1.2.2 Module Operation

When the module receives a differential input from an analog device, the
module's circuitry multiplexes the input into an A/D converter. The converter
reads the signal and converts it as required for the type of input. The module also
continuously samples the CJC sensors and compensates for temperature changes
at the terminal block cold junction, between the thermocouple wire and the input
channel. See the block diagram below.
Chapter 1: Module Overview
User's Manual Pub. 0300244-02 Rev. A
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