Applies To:
  • CitectSCADA 1.xx, 2.xx, 3.xx, 4.xx, 5.xx, 6.00
  • CitectHMI 1.xx, 2.xx, 3.xx, 4.xx, 5.xx, 6.00
Summary:
When there are multiple devices on one SNMPII port and some of these devices are not responding, values on the devices that are operating correctly may show #COM intermittently.

Cause:

The port has the combined device traffic on it for:

o Reads of data from the on-line devices to update the displayed page.

o Every watchtime, attempts to communicate with the off-line devices.

Because these units are not responding, each request will "hang"

for a few seconds until it times out.

If there are several devices causing these temporary "hangs" then the reads from the on-line devices will be delayed so long that the display client will display #COM until it is able to read the PLC again.

 

Solution:
Increase [SNMPII]MaxPending

A suitable value for MaxPending is

2 * the number of units on the port with the most units

e.g. 10 devices on one port: [SNMPII]MaxPending=20

10 devices - one on each of 10 ports: Use the default value of 2

This will ensure that inaccessible devices will not block the port, and will provide reasonable performance because each device will be able to have two requests outstanding

NOTE: The default value of MaxPending is 2. Also MaxPending cannot exceed 256.

 
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