For some DDE Servers, there is no mechanism or event to determine when a device is disconnected (down). For this type of DDE Server, you can configure a diagnostic (heartbeat) point for each physical device with which the DDE Server is communicating. The DDE Client Communications enabler can then read the point and set the device down if the read fails. The diagnostic point can be configured as any point type with the following address format:
Where <DDE_item_address> is the point address to read from the DDE Server. The scan rate of the point will be used as the heartbeat interval for the device.
In addition, Device heartbeat points can be configured to specify that a device is available based on the value of a specified point. Previously, a diagnostic point with the address $HP <addr> would cause the device to be marked unavailable whenever the address <addr> could not be polled.
Using the new hearbeat point syntax, the device can be marked unavailable whenever the address <addr> does not have a specific value. For example, if the point $HP_0 <addr> were configured, the device would only be available when <addr> could be read and contained the value 0.