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204 Siemens Simatic S7 Driver Online Help

Understanding Overruns

An overrun occurs when the driver attempts to read more data from the process

hardware than it can update within the specified poll rate. A high number of overruns

indicates that your driver is poorly configured for its environment and, as a result,

your data is not updating at the specified poll rate.

To understand how overruns occur requires some knowledge of how the driver reads

data. When a 7.x driver requires data for a datablock, it places a read request into its

read queue. When the driver processes this request, it sends a message to the process

hardware for data.

The driver reads (polls) each datablock according to its poll rate. Each time the poll

rate expires, the driver creates a read request for that datablock. If a read request for a

datablock is still pending in the read queue, or the datablock is waiting for a response

when the driver issues another read request, an overrun occurs. The driver discards

the second message, since a request for the same data is already pending.

Example

Poll Rate = 1 second

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