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responds by keying the carrier for a predetermined
interval.
The unit identifying times are:
• Master recognizes a five-second carrier
• Remote #1- recognizes a 10-second carrier
• Remote #2- recognizes a 15-second carrier
• Remote #3- recognizes a 20-second carrier
• Remote #4- recognizes a 25-second carrier
7.6.2 Encoded Carrier Tests
Encoded tests involve sending serial data mes-
sages back and forth between checkback units.
This method has two advantages over the keyed
carrier method: (1) it is usually faster and (2) you
can communicate with more units. The disadvan-
tage with this method is that it is more vulnerable
to noisy line conditions that may result in a failure
to send a good message. To prevent errors in the
received data, the unit sends a 16-bit CRC code
with each message. This error detecting code
enables the unit to detect bad messages and, in
some cases, repair them.
7.6.3 Primary & Fallback
Communications Modes
A UPLC™ first uses the primary communications
mode when performing a checkback test. It makes
several attempts to communicate, up to the num-
ber in the "Retries" setting. If all attempts fail, it
switches to the timed communication mode, if
"Fallback Timed Comm" is enabled and the
"Primary Comm Mode" is coded.
Once in timed communication mode, the unit
makes one final attempt to get through. If that
fails, it sets a major alarm. If it passes, no alarm is
issued.
May 2012
Chapter 7. Optional Testing Facilities
7.6.4 Communication Retries
To increase the communication robustness, the
checkback system can make repeated attempts to
get a message or signal through to another unit.
When the channel is noisy or weak, multiple
attempts are often successful. Setting a high retry
number, say 15, increases the likelihood of suc-
cess (slightly), but can cost a lot of time waiting
for a failure. We think that three (3) to five (5) is a
good compromise between speed and robustness.
7.7 Troubleshooting
You can identify and solve many checkback net-
work problems by examining the major and minor
alarms. If both high and low power checkback
tests are enabled, the major and minor alarms
work like this:
• If a unit fails only the low power test, it sets
the minor alarm
• If a unit fails the high power test, it sets the
major alarm
• If a unit fails the high power test and can
not receive its own messages, it sets both
major and minor alarms
Armed with this information, you can usually
identify a failed unit or line problem. the Network
Troubleshooting table lists all combinations for a
two-unit system and the probable situation.
To get the complete picture, you must wait long
enough for all remotes to initiate their own check-
back tests. For example, if the master has a major
alarm, but the remote has no alarms, you can not
determine if it failed to receive the command due
to a bad line or because its receiver failed. By
waiting for the remote to initiate a checkback, you
can tell if it is the line (major alarm) or the unit
(both alarms).
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Table of Contents
Troubleshooting
