Device group-specific system items

System Item Name (Type)

Type/Access Rights

Description

Values

$SYS$UpdateInterval

DWord/ReadWrite

Accesses the currently set update interval. It is the current update interval of the device group in milliseconds. A client can poke new values into this item.

The value of zero indicates that non-system items on that topic are not updated (data for these items are not acquired from the device).

RANGE: 0…604800000

0: Topic inactive, no items are updated. Data acquisition is stopped.

Greater than 0: Expected updated interval for the set of all items in the device group.

$SYS$MaxInterval

DWord/Read

It provides the rolling average in milliseconds of the last 10 actual update durations for the Communication Driver to send and receive all the items in the same topic. This item is read-only.

Not supported by some Communication Driver.

$SYS$WriteComplete

Integer/ReadWrite

Accesses the state of pending write activities on the corresponding device group. On device group creation (adding items to an OPC group), the value of this system item is initially 1, indicating all write activities are complete – no pokes are pending.

If values are poked into any items of the device group, the value of this item changes to 0, indicating write activity is currently in progress.

If the server has completed all write activities, the value of this item changes to 1 if all pokes were successful or to -1 if at least one poke has failed.

If the value of this item is not zero, the client can poke 1 or -1 to it (poke a 1 to clear errors, or a -1 to test a client reaction on write errors).

If the value of this item is zero, it cannot be poked.

RANGE: -1, 0, 1

1: Write complete (no writes are pending – initial state).

0: Writes are pending.

-1: Writes completed with errors.

$SYS$ReadComplete

Integer/ReadWrite

Accesses the state of initial reads on all items in the corresponding device group.

The value is 1 if all active items in a device group have been read at least once.

If at least one item in the device group is activated, this item changes to 0. It changes to 1 if all items have been read successfully, or to -1 if at least one item has a non-good quality.

Poking a 0 to this item resets the internal read states of all items in this device group. This resets this item to 0. If all items are read again after this poke, this item changes back to 1 or -1.

RANGE: -1, 0, 1

1: Read complete (all values have been read).

0: Not all values have been read.

-1: All values have been read but some have a non-good quality.

$SYS$ItemCount

DWord/Read

Accesses the number of items in the corresponding device group. This item is read-only.

RANGE: 0…2147483647

>=0: Number of items.

$SYS$ActiveItemCount

DWord/Read

Accesses the number of active items in the corresponding device group. This item is read-only.

RANGE: 0…2147483647

>=0: Number of active items.

$SYS$ErrorCount

DWord/Read

Accesses the number of all items (active and inactive) that have errors (non-good OPC quality) in the corresponding topic.

If the communications status of a device group is bad, all items have errors. This item is read-only.

RANGE: 0…2147483647

>=0: Number of all items (active and inactive) with errors.

$SYS$PollNow

Boolean/ReadWrite

Poking a 1 to this item forces all items in the corresponding device group to be read immediately (all messages in this device group become due). This is useful if you want to force to get the newest values from the device, regardless of its update interval. This also works on device groups with a zero update interval (manual protocol triggering).

Not supported by some Communication Driver.