4.23Formula engine

The Formula Engine calculates a new KNX value from an arithmetic expression over one or more KNX values, and writes the result to another group address. Use it for scaling, unit conversion and simple calculations. This page lists every field of the dialog and the syntax the gateway accepts.

#What it does

Each formula has variables (each tied to a KNX group address), one expression and one output group address. Whenever a variable that triggers the computation receives a new value, the gateway calculates the expression and writes the result to the output group. The result is only written to KNX: the engine publishes no System Object.

#Before you enable it

• The service is Disabled by default. Turn Enabled on and deploy the project (see Deploying to a gateway). Each formula also has its own Enabled switch.
• The gateway reads and writes the bus through its KNX connection, so the KNX engine must be working.

#Settings

#Formula Engine dialog

The list has Add, Clone, Remove, Up and Down, and shows each formula with (Enabled) or (Disabled). Import and Export in the header save and load all formulas as formula-engine.json; importing replaces the whole list. Cancel and Save Changes only close the dialog.

#Formula Details

#Variables

Add Variable adds a row. Each variable has:

#Syntax

There are no comparison operators, no conditions such as a > 5 ? 1 : 0, no text values and no constants such as pi. For conditions use Logic Studio or the Rule gateway. An error in the expression, such as an unknown name, is logged by the gateway and nothing is written.

#How it works on the gateway

• At start the gateway fills each variable according to Value At Startup: it reads the group once, uses 0 or 1, or waits for a telegram.
• A formula is calculated only when every variable has a value. Until then nothing is written.
• When a write or response telegram arrives for a variable's group, the gateway stores the value. If Trigger Computation is on for that variable, it calculates the expression and writes the result to Output Group with the chosen data type.
• Do not use the output group of a formula as one of its own variables. The gateway sees its own writes as incoming telegrams, so the formula would trigger itself.

#Example

Convert a KNX temperature in degrees Celsius to degrees Fahrenheit:

1. Enable the Formula Engine and add a formula: Name Outdoor temperature F, Expression a * 9 / 5 + 32.
2. Set Output Sending Behaviour On result change, Output Group 2/2/10, Type DPT 9.001 - Temperature / 2 byte float.
3. Add a variable: Variable a, Name Outdoor temperature C, KNX Group 2/2/5, Type DPT 9.001, Value At Startup Read current value, Trigger Computation on.
4. Deploy. A reading of 21.5 on 2/2/5 writes 70.7 to 2/2/10.

A formula with two variables, for example the total power of two meters, uses the expression a + b with variables a and b and a separate group for each.

#Tips and common problems

• Nothing is written: the engine or the formula is disabled, the Output Group is empty, or a variable has no value yet. Choose Read current value or 0 until first telegram for variables that do not change often.
• The result is wrong by a factor: check the Type of each variable. The gateway decodes the telegram with that type.
• The output keeps repeating: On new value reception writes after every incoming telegram. Use On result change unless the receiver needs the repeats.

#See also

• Logic Studio
• Rule gateway
• Automation
• KNX engine