PLC Scanned Device Tag

Help Contents

Introduction

The PLC Scanned Device Tag PLC Device Tag Icon represents a logical link to a device where the data is:

A device can be any of the following:

Configuration Properties

The PLC Scanned Device Tag supports the configuration properties of the PLC Device Tag plus these properties:

Property Type Version Description
EnableDirectWrite UINT 2.3 Whether data can be written directly to the PLC device.
TagScanInterval TIME 3.15 The interval between each scan of the I/O Data Point Tags directly associated with the PLC device. See Interval and Offset for information on how Intervals and Offsets define the time of activities.
TagScanOffset TIME 3.15 The offset added to the TagScanInterval to get the actual tag scan time. See Interval and Offset for information on how Intervals and Offsets define the time of activities.
OfflineTagUpdateInterval TIME 3.84 How often I/O Data Point Tags associated with the PLC Device are updated when offline.

EnableDirectWrite Property

The EnableDirectWrite configuration property controls access to these tag commands:

Note: A PLC tag supports DirectWriteByte, or DirectWriteWord, but not both.

When EnableDirectWrite is FALSE access to the DirectWrite tag commands is blocked. This prevents any user, or program, sending direct write requests to the PLC from this tag.

When EnableDirectWrite is TRUE access to the DirectWrite tag commands is allowed. A user can send write requests to the PLC from this tag if the user has the Direct Write User Privilege.

TagScanInterval and TagScanOffset Properties

The TagScanInterval property defines how often Fernhill SCADA reads the value of I/O Data Point Tags associated with this PLC Device.

Tag values are read from the PLC Device only when both of the following apply:

See Interval and Offset for information on how Intervals and Offsets define the time of activities.

OfflineTagUpdateInterval Property

The OfflineTagUpdateInterval property defines how often Fernhill SCADA updates the value of I/O Data Point Tags when the PLC Device is offline.

Offline updates occur under any of these conditions:

Offline Updates can help to avoid gaps in historic data if the PLC Device is offline, or out of service, for a long time.

To disable Offline Updates, set OfflineTagUpdateInterval to T#0s.

Data Properties

The PLC Scanned Device Tag supports the data properties of the PLC Device Tag plus these properties:

Property Type Version Description
TagScanTimer TimerStatus 3.15 The status of the tag scan timer

Tag Commands

The PLC Scanned Device Tag supports the tag commands of the PLC Device Tag plus these commands:

Tag Command Version Description
Refresh 3.15 Requests an immediate refresh of all of the I/O Data Point Tags associated with the PLC Device.
DirectWriteBit 3.15 Writes a bit value to an address in the PLC Device.
DirectWriteByte 3.15 Writes an array of 8-bit bytes to an address in the PLC Device.
DirectWriteWord 3.15 Writes an array of 16-bit words to an address in the PLC Device.

Relationships to Other Tag Types

The PLC Scanned Device Tag builds on the features of these tag types:

The PLC Scanned Device Tag is used as a building block for these tag types:

Further Information

Core Tag

To learn about properties common to all tags.

Alarm Conditions

To learn about abnormal conditions in SCADA applications.

Glossary

For the meaning of terms used in Fernhill SCADA.