Symbolic variables

Symbolic variables represent named ranges in the memory of the GE PLCs, which you can define without knowing the addresses of specific memory registers. You can also use them to access pre-defined tags in the PLC.

Important: Array and symbolic variable addressing is only supported in subscribing data from the PAC Rx3i and PAC Rx7i controllers. Because of protocol limitations, array or symbolic variable addressing is not supported from other GE controllers.

For example, if you know there is a tank pressure sensor connected to a PLC, you can access the data from its tag as "#TankPressure" rather than having to know the exact memory register address for that data.

Symbolic variables can be addressed with the following syntax:

#<var_name> <ItemSuffix>

(where "#" and <ItemSuffix> are optional).

• Prefix '#' - This identifies the item as a symbolic variable. If the PLC has a symbolic item with the same name as a register (such as R3000) then the '#' is required.

  If '#' is not provided, then the GESRTP Communication Driver will first try to resolve the item as an IO or memory register. If it is not a valid syntax for an IO or memory register, then the Communication Driver will create the Symbolic Item.

• <Item Suffix> - This identifies the data type, when the item is created in the Communication Driver. Combination of suffixes is not applicable.

  If <Item Suffix> is not supplied, the DA Server will create the item with delayed item validation. The data type at the time of item creation will be VT_EMPTY and the actual PLC data type will be determined later. For more information on data types and suffixes, see Suffixes to explicitly control I/O variables and data types for registers.

Examples:

#TankLevel ARRAY(S)

This creates an array named "TankLevel," with the canonical data type of VT_I2 (2 byte integer).

#R3000 W

This creates a symbolic variable named "R3000," with the canonical data type of VT_UI2 (2 byte unsigned integer).

TankLevel

This creates a symbolic variable item named "TankLevel." The data type will be determined when the Communication Driver attempts to access the item on the PLC.