Using Read Commands to Retrieve Data

In some cases, the value for an I/O point can only be retrieved using a read command. For example, a particular IO point may not support spontaneous events or general interrogation.

Under these circumstances, there are two ways you can read from an IO point:

• the TagRead function

• the IEC870IP driver's polling engine

In both cases, you will need to configure Read Tags to successfully retrieve data from field devices. Where a read tag has been used, the driver will send a read command to request data.

The TagRead function

The TagRead function will send a read command to the specified IO point and process the response before responding to replying to the function call.

The polling engine

The IEC870IP driver has an integrated polling engine that allows you to retrieve values for any IO point using regular read command requests sent directly to the device.

The rate at which requests are executed is defined by the subscription rate of the tag. If more than one tag is using the same item, the polling rate will be determined by the highest subscription rate.

The polling engine consists of two main components:

• poll item buckets - which contain the variables that need to be scanned, grouped according to their scan rate.

• the polling manager - which creates the poll item buckets as required, and notifies them if read requests need to be executed.

At start up, all variables are stored in the "not polled" bucket (scan rate is -1). When a subscription is received, the polling manager assigns variables to the poll item buckets according to their scan rate, creating the buckets as required.

If the subscription changes for a tag, the polling rate for any affected item is changed as well and the variables are moved to an appropriate bucket. If the subscription for a variable ends, it is placed back in the "not polled" bucket.

Note: The polling interval used by the polling manager can be configured using the parameter Polling Parameters. The default value is 100ms.