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

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.


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.


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.