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Alarm function (ALE command) tutorial
Alicat flow and pressure devices have an optional alarm functionality that helps simplify control systems such as PLCs or manned stations. An alarm is transmitted as an analog signal though the communications connector on the device.
- In systems where an Alicat is not connected to a central system, the alarm pin can be wired to a simple bulb and used to indicate a deviation from the flow or pressure setpoint.
- When connected to a PLC, the alarm output can be set to trigger different stages of a controlled process or notify a central system when the process falls outside of tolerances.
The alarm is preconfigured at the factory, and the ALE serial command can be used to change the alarm settings in the field. The following article gives a brief introduction to the ALE serial command and provides a breakdown of the command structure for setting an alarm.
The ALE serial command format
Before setting an alarm, it is important to determine the parameters you want to work with and how you want the alarm to behave.
- The deviceID is the standard ASCII A-Z unit ID each device is assigned. This is ‘A’ by default.
- The SetExpression is the full expression for when the alarm should trigger.
- The ClearExpression is the full expression for when the alarm should stop. This is written such that the alarm will stop when the clear expression is false.
The SetExpression and ClearExpression are both formatted using Reverse Polish Notation.
- s indicates that the following Value is in reference to a “statistic” or parameter value of the device
- c indicates that the following Value is a constant
- Units indicates the engineering units selected for the statistic
- Operator specifies the alarm trigger
Using the alarm command (basic steps)
1. Connect your device and open the serial terminal app
Plug the device in and connect the communication cable to your PC, then open the serial terminal app.
If you don’t already have a serial terminal application, download Alicat’s pre-configured application here: https://www.alicat.com/support/software-drivers/.
2. Decide which of the two alarms to set
- 0 = first alarm
- 1 = second alarm
3. Determine which value should be compared to for the alarm
Not all parameters are available on all devices. See table below for available parameters.
Select Available Parameters
|Gauge Pressure (if available)
|Volumetric Totalized Flow
|Mass Totalized Flow
4. Determine the engineering units you would like to use for this value
See the table below for available engineering units. If default, skip this step.
Select Available engineering units
5. Determine the operator
Do you want the alarm to trigger when the parameter is greater than, less than, or equal to a specific value? See the available operators table below.
|Not Equal to
|Less Than or Equal to
|Greater Than or Equal to
While using a Boolean is completely optional, it presents more logic options. An expression with a Boolean operator follows a similar format as the SetExpression and ClearExpression described above.
See the valid Boolean operators in the table below.
Available Boolean Operators
|Both True (And)
|Either True (Or)
|Only One True (Xor)