Calculated Variables

System Model > Variable Tags > Calculated Variables

A calculated variable allows you to:

generate a tag value at runtime that is the result of a Cicode expression
call an internal Cicode function as a variable tag.

Note: Calculated variables are evaluated on the I/O server only when a subscription to the calculated variable exists.

To configure a calculated variable, you use a variable tag that addresses an I/O device with its protocol set to "CICODE". If the variable tag address is a valid Cicode expression, the Cicode I/O device will calculate the result of the expression and present it as the tag value.

Note: If the expression in the Address field reads from or writes to another tag in the system, the calculated variable is added towards the point count.

Example 1

The Citect SCADA Example Project includes a number of tags that support the Steel Mill page. These tags include two tags that identify the length and width of the coil that is currently loaded on the uncoiling station.

Tag Name	I/O Device	Address	Data Type	Units
Entry_CoilLength	SteelMillDisk	R10	REAL	meters
Entry_CoilWidth	SteelMillDisk	R13	REAL	millimeters

Using a calculated variable, you could determine how many square meters of steel will be produced by the coil that is currently uncoiling. To do this, you could create the following calculated variable on an I/O device called "CicodeIODevice".

Tag Name	I/O Device	Address	Data Type	Units
Entry_CoilArea	CicodeIODevice	Entry_CoilLength * (Entry_CoilWidth)/1000	REAL	m2

You could then use the value generated by this tag at runtime to estimate the production potential of the current coil.

Example 2

Using a calculated variable, you can call a Cicode function for diagnostics and determine the state of an I/O device.

Tag Name	I/O Device	Address	Data Type
IOServer_Mixer_PM800_State	CicodeIODevice	IODeviceInfo("IOServer_Mixer_PM800", 10)	STRING

In the example above, a CICODE function has been used in a calculated variable to determine the state of the specified mixer (for example, online or offline).

Note: Cicode functions used for calculated variables need to be non-blocking functions.

Example 3

Using a calculated variable, you can trigger an alarm when an I/O device goes offline. In this case, you will need to:

create a variable tag with a calculated variable to determine the state of an I/O device
create an advanced alarm with an expression that will trigger an alarm when the expression evaluates to TRUE.

Tag Name	I/O Device	Address	Data Type
IOServer_Mixer_PM800_State	Mixer	IODeviceInfo("IOServer_Mixer_PM800", 10)	STRING

Alarm Tag	I/O Device	Expression
IOServer_Mixer_PM800_Offline	Mixer	(StrToInt(IOServer_Mixer_PM800_State) = 16) OR (StrToInt(IOServer_Mixer_PM800_State) = 32)

Alarm Tag

I/O Device

Expression

IOServer_Mixer_PM800_Offline

Mixer

(StrToInt(IOServer_Mixer_PM800_State) = 16)

(StrToInt(IOServer_Mixer_PM800_State) = 32)

In the example above, the variable tag calculates the state of the mixer. The alarm will be triggered if the tag is calculated to be 16 (offline) or 32 (disabled).

Example 4

Using a calculated variable, you can perform mathematical calculations. In this example, the variable tag calculates the quotient of two integers and in the event of the divisor being 0 returns an error code.

Tag Name	I/O Device	Address	Data Type
Divide_Tag_Value	CicodeIODevice	Divide(4,9)	REAL

REAL FUNCTION Divide( INT iDividend, INT iDivisor)

REAL iResult;

INT iError;

Errset(1);

iResult = iDividend / iDivisor

iError = isError();

Errset(0);

RETURN iResult;

END