Siemens SIMATIC NET Specifikace stáhnout pdf (Strana 17)

Chapter 1. First-time Users 7

Signal conditioning is driver-dependent. Consequently, one driver may support one

type of scaling, but not another.

See Also

Specifying Signal Conditioning in Database Manager

What is an Overrun?

In 7.x drivers, 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 your driver is poorly configured for its environment and, as a

result, your data is not updating at the specified poll rate.

Any of the following conditions can cause overruns:

• Bandwidth saturation on your network.

• Turnaround delays in the PLC.

• Overflowing message queues in the process hardware.

To understand how overruns occur, you need a some knowledge about 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

Assume that at this setting, the driver can poll 5 datablocks per second. If you configure 10

datablocks, all 10 try to poll at the same time. In the first second, the first 5 datablocks poll,

leaving 6 to 10 still pending. The following figure shows what happens next.

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Siemens SIMATIC NET Specifikace Strana 17