When configuring a module to access diagnostic data, you should take into consideration the following characteristics with respect to run-time behavior:
Configuring a dynamic reference parameter to reference a device diagnostic parameter is not supported, because a run-time connection to the device diagnostic parameter cannot be established.
Controller
A module can reference a diagnostic parameter in a device connected to the controller itself; a module cannot reference a diagnostic parameter in a device connected to another controller. An attempt to reference a device connected to another controller results in a download warning.
When a module is initially downloaded, the controller attempts to resolve any references to diagnostic parameters. During this time, the value of the .CST field of an external reference to a diagnostic parameter is COMM_STATUS_MODULE_NOT_CONFIGURED. Because a reference to a diagnostic parameter is not considered copy-critical, this is also the case for a partial download.
ProfessionalPLUS workstations and Application Stations
A module running on a ProfessionalPLUS workstation or an Application Station cannot reference a diagnostic parameter in a device. A module can only reference a diagnostic parameter in a device connected to the node where the module is executing, which means that only a controller can support a diagnostic parameter reference.
OPCWatchit
OPCWatchit can display the value of a diagnostic parameter by using a marshalling module. OPCWatchit does not support direct access to the diagnostic parameter.
History data collection
Historical data cannot be set up to reference a diagnostic parameter directly. However, you can set up historical data collection to reference a diagnostic parameter configured as an external reference parameter in a module. Note that this results in collecting historical data that is module-based rather than data that is device-based. Assigning the node for history collection does not collect historical data for device diagnostics of a device connected to the node. Instead, you must assign the area of module that has a reference to a device diagnostic parameter.
Redundancy
An external reference to a diagnostic parameter supports redundancy. When a module contains an external reference to a diagnostic parameter, the external reference continues to read the diagnostic parameter after a switchover at a controller, fieldbus, or Profibus DP card. The last value read is held.
Performance
A maximum of one parameter per second is read on each port. To minimize the performance impact on other controller functions, there is a delay of one second between reads on any given port. Therefore, throughput cannot exceed one read per second per port. The actual time of the delay is dependent on the number of devices on the bus and the number of parameters assigned per device.
Capacity
Attempting to read more than 250 diagnostic parameters on a port may result in bad status and unpredictable and slower update rates. A maximum of 32 ports having diagnostic parameters are supported on each controller. This 32 port count is any of the three bus technologies, (Profibus, Devicenet and Fieldbus) or any combination of the three.