Communications Settings

Sampling rate on reception

The lower the value of this parameter, the better the response times for synchronous access will be. However, this parameter only affects CPU load on the PC on which OFS is installed and has no impact on the network bandwidth usage.

Typically, the sampling rate on reception should be set to 20ms.

Minimum group update rate

OFS manages communications with devices based on the group update rate. This rate is the limit below which OFS will not accept OPC client requests.

To maximize performance, it is recommended to set this rate according to the following rules:

Rate <= Fastest Group rate / 2, and

Rate is the greatest common divisor of the group rates (provided this doesn’t violate the previous rule)

For example:

Group1UpdateRate = 400 ms

Group2UpdateRate = 800 ms

Group3UpdateRate = 2000 ms

Therefore, the recommended group update rate is 200 ms, (the greatest common divisor of the three group rates, which is also <= 400 /2).

Note: If your group rate is too high compared to your system capabilities then OFS will display a message "Polling rate overrun for …" in OFS diagnostic window when "verbose" mode is set (Menu Misc/verbose).
The Quality of some of the items will then become uncertain (depending on Communication Overrun behavior). This quality will be propagated up to the Plant SCADA system, and therefore should be avoided.
Therefore a general rule is to set the OPC client’s refresh rate to twice the calculated "OFS group scan time".

The OFS group scan time can be calculated with the formula:

TS=group average time (2) * group number request (3) / max pending request used (1).

The different parameters of the equation are provided by the OFS server in "Varman" and "Network" windows.

QuickSetActive

This parameter is new in OFS v3.35. It causes the value of an item to be read immediately after activation instead of waiting for the next cycle. This is useful, for example, in reducing the delay for first display of variables in a popup. Therefore, it should be set active.

Communication Overrun behavior

This parameter allows deciding whether the overrun in OFS should be notified to Plant SCADA as a "Bad quality" or if the server should instead try to auto-adapt its own polling rate. Set this parameter to "rate adapt" to avoid some false "bad quality" when the load on the OFS server is high.