Group Properties
NOTE: The following fields and descriptions apply to both the Group Properties Viewer and the Group template.
Group Name
Specifies the name of the selected group. Any application requesting data from the I/O driver uses this name to access items in the group. Each group that the driver communicates with should have a unique name across all servers, groups, and items.
Valid Entries
Up to 30 alphanumeric characters including underscores ( _ ) and hyphens ( - ).
Group Enable Check Box
When selected, the driver is enabled to poll the selected group. If you clear the check box, the selected group and its associated items are not polled.
You may want to disable one or more groups in the following situations:
- You are swapping hardware for repair or maintenance and do not want to display errors.
- You do not need to collect data from the items within a group and you want to reduce the communications load.
- You want to isolate a group for debugging.
Description
Specifies user-defined comments about the selected group. Such descriptions can be very helpful when you go back to look at old configuration or report files, or when you need to modify an existing configuration that you did not create. The more detailed and specific the information you enter in this field, the easier it will be to identify the group at a later date.
Valid Entries
Up to 40 alphanumeric characters and symbols.
OPC Group Communications Settings
I/O Type
Specifies the communications method the driver uses to obtain data from the OPC server.
Valid Entries
|
The I/O Type... |
Enables the driver to... |
|
Synchronous |
Poll the OPC server at regular intervals, according to the specified poll rate. |
|
Asynchronous |
Update items at regular intervals (when the asynchronous watchdog is enabled) or on an exception basis when data has exceeded the user-supplied percent deadband. We recommend using asynchronous I/O with the asynchronous watchdog disabled whenever possible because it provides the least amount of communications overhead. |
Data Source
Specifies where the OPC server will obtain data from when the OPC Client issues a read request.
Valid Entries
|
The data source... |
Returns... |
|
Cache |
The last value read by the OPC server. The server updates the cache based on the group’s update rate. |
|
Device |
The latest values from the process hardware. Reading directly from a device can cause undue stress on your system. We recommend this selection only for diagnostic and troubleshooting purposes. |
Update Rate
Specifies the rate at which the OPC server polls for data from the process hardware and updates its internal cache. The rate you enter applies to all items in the current group.
Valid Entries
Accepts the time format: days:hours:minutes:seconds.milliseconds, within the range of 0.1 to 6:23:59:59.9. Entering 0 forces the OPC Server to run at its maximum rate and increases CPU usage.
In general, you need to balance CPU usage against an acceptable update rate. For example, if CPU usage is 90% and you never want the CPU usage to exceed 50%, enter a longer (larger) value here. However, if changing the update value causes updates to be too slow for your application, you will need to shorten the update rate to find the acceptable minimum value.
% Deadband
Specifies the maximum fluctuation in this group’s items that can be tolerated before reporting the change (an exception) to the OPC Client. Enter this value as a percentage of the full EGU range of the group’s items.
Valid Entries
0 to 100
Example
Assume the full EGU range is 0 to 100 and the maximum fluctuation (the deadband) is 5%. Using these values, if the current EGU value is 10, the new value must exceed 15 or fall below 5 before the OPC server reports an exception. However, if the full EGU range is 0 to 200, the new value must exceed 20 or fall below 0 before an exception is reported.
The OPC server uses the following formula to determine whether an exception has occurred:
absolute value(last cached value – current) > Percent Deadband * (High EGU – Low EGU)
When this formula is true, the server reports an exception and updates its cache. When the formula is false, it takes no action.
NOTE: The exact implementation of this feature differs from server to server. Refer to your server’s documentation to learn the EGU range used by the server.
Client Communications Settings
Poll Rate
Specifies the rate at which the driver polls the OPC server for the selected group’s items. The time range for poll rates is different for synchronous and asynchronous I/O types.
Valid Entries
Accepts the time format: days:hours:minutes:seconds:subseconds. For synchronous, the range is from 00:00:00:00.00 to 6:23:59:59.9. For asynchronous, the range is from 00:00:00:30.0 to 6:23:59:59.9. For more information, see Zero and Subsecond Poll Rates.
Enable Asynchronous Watchdog
When selected, the OPC Client refreshes the selected group’s items at the interval specified by the Poll Rate. This check box is available only when you select Asynchronous from the I/O Type list.
The asynchronous watchdog does not override the item’s access time. Consequently, when the access time expires, the watchdog stops.
Async Timeout
Specifies how much time the OPC Client waits for an asynchronous update from the OPC server. When a timeout occurs, the OPC Client sets the quality of all items waiting for data to Uncertain, Last Usable because the quality of the data is no longer known. This setting is available only when you select Asynchronous from the I/O Type list.
By default, the driver waits 10 seconds (10000ms).
Valid Entries
250 to 2,147,483,647 milliseconds (over 24 days)
