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Kepware Knowledge Base: Solution

Remaining Channel Devices Are Slow or Fail

Last Update: 11/7/2018

When communicating with serial devices through a communication port via a multi-drop connection, the server will poll each device individually. Consequently, if one of these devices is removed or fails, the server must timeout its requests to that device on all scan cycles. To resolve this issue, users can do one of the following:

1. Disable communications to the lost device.

  • To disable communications to a device, write 0 to its _Enabled Tag. A hidden tag group named _System will be created automatically for every device added to the server. This group contains tags that are linked to device-specific memory/parameters in the server. One of these tags (called _Enabled) is a Boolean tag that indicates when data collection is enabled or disabled. Writing 1 to this tag will enable data collection. This is the default setting. Writing 0 to this tag will disable data collection.

    Note: Writing to the _Enabled tags will modify the project. To control how the application handles this on shutdown, refer to the setting at Tools | Options | General | Exit Options.
  • Instead of writing to the _Enabled Tag, users can manually disable communications through the Device Properties. To do so, open the General tab and then unselect Enable data collection. When finished, click OK.

2. Enable auto-demotion.
Auto-demotion temporarily places unresponsive devices in an off-scan state for a specified time period, allowing the driver to continue optimizing communications with the other devices on the channel. To configure the auto-demotion parameters, click Device Properties | Auto-Demotion .

Note 1:
Disabled data collection is recommended for devices that are offline for a definite period of time. This is due to the following:

  • When data collection is disabled, the server will not attempt to communicate to the disabled device. This is the most efficient scenario. When using auto-demotion, however, the server will occasionally attempt to access the device. This will cause delays in data collection for the remaining devices.
  • When data collection is disabled, only one message per disabled device will be received in the server application's Event Log. When using auto-demotion, however, an event will be logged every time a device is automatically promoted or demoted. This may cause other event messages to be less noticeable, and will also cause the Event Log to fill up quickly (thus reducing its total time span).

Note 2: Auto-demotion is recommended for devices that are offline sporadically (such as with some wireless networks). Since the server will periodically check the offline devices, it will put them back on scan when once they come back online.

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