Download 1775-6.4.1, PLC-3 Family Programmable Controller, Programming

Chapter 14
Addressing Memory and Monitoring
Controller Status
14.2.4
I/O Communication Retry
Counts (Status File 3)
A communication retry is a re-transmission of data that occurs when the
original transmission is unsuccessful. If the I/O adapter does not respond or
sends invalid data, when the scanner communicates to the I/O adapter, the
scanner executes a retry.
You can track the retry count by monitoring status file 3 (Figure 14.14)
Figure 14.14
I/O Communication Retry Counts (Status File 3)
17
16
15
14
13
12
11
10
07
06
05
04
03
02
word 0
01
00
Adapter #
0
Starting
I/O group
0
word 1
2
word 2
4
word 3
word 4
word 5
word 6
word 7
6
1
0
2
4
6
word n
To find the word that stores the retry count for an I/O adapter:
Adapter # in decimal x 4 =
the first of the words that stores the retry
counts for the adapter. The particular
word depends on the starting I/O group.
For example, the retry counts for adapter 10 are in words 40 to 43 (S3:40 to S3:43).
Important: The processor does not create status file 3 at power-up. You
must create it in memory by using the create command.
If you find that the retry count is high for an I/O channel, there could be a
loose connection or a noise problem with the twinaxial cable (cat. no.
1770-CD) that connects from the scanner’s terminal arm to the I/O adapter.
Retries could also result from improper installation of I/O terminator
resistors along the I/O channel. Refer to the PLC-3 Family Controller
Installation an Operation Manual (publication 1775-6.7.1) for detailed
information.
Upon executing a retry, if the I/O adapter responds properly, normal
operation continues. If the I/O adapter does not respond properly, the
scanner continues to execute retries until:
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