Channel Status Leds (Green); Invalid Channel Configuration; Open- And Short-Circuit Detection - Allen-Bradley SLC 500 1746-NR4 User Manual

Chapter 7
Module Diagnostics and Troubleshooting
7–4

Channel Status LEDs (Green)

The channel LED is used to indicate channel status and related error
information contained in the channel status word. This includes conditions
such as:
normal operation
channel–related configuration errors
broken input circuit errors such as open– or short–circuit (RTDs only)
out–of–range errors
All channel errors are recoverable errors and after corrective action, normal
operation resumes.

Invalid Channel Configuration

Whenever a channel's configuration word is improperly defined, the channel
LED blinks and bit 15 of the channel status word is set. Configuration errors
occur for the following invalid combinations:
Input type is a 10 Ω Copper RTD and the excitation current is set for 0.5
mA, which is not allowed.
Scaling select bits 13 and 14 are set to 11, which is invalid.
Broken Input select bits 6 and 7 are set to 11, which is invalid.
Scaling select bits 13 and 14 are set to 01 or 10 and scaling
limit words=0
Data format bits are set to 11 (proportional counts), the scaling–select
bits are set to 01 or 10 and the lower limit user–set scale word is
greater than or equal to the upper limit user–set scale word.
Open– and Short–Circuit Detection
An open– or short–circuit test is performed on all enabled channels on each
scan. Whenever an open–circuit or short–circuit condition occurs (see
possible causes listed below), the channel LED blinks and bit 13 of the
channel status word is set.
Possible causes of an open or short circuit include:
The RTD or potentiometer may be broken.
A RTD or potentiometer wire may be loose or cut.
The RTD or potentiometer may not have been installed on the configured
channel.
The RTD may be internally shorted.
The RTD may be installed incorrectly.
If an open– or short–circuit is detected, the channel data word reflects input
data as defined by the broken input configuration bits (6 and 7) in the
channel configuration word.
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