Device Group-Specific System Items

$SYS$UpdateInterval

Not used.

$SYS$MaxInterval

Not used.

$SYS$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 active 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.

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).

RANGE: 0, 1,