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.
Alarm expression format
deviceID ALE Alarm# SetExpression ClearExpression
- 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.
SetExpression and ClearExpression format
sValue:UnitscValueOperator
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: /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
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.
Available Operators
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.
SetExpression and ClearExpression format with Boolean operator
sValue:UnitscValueOperatorsValue:UnitscValueOperatorBoolean
See the valid Boolean operators in the table below.
Available Boolean Operators
Example alarm expressions:
Example 1: Set Expression
Example 1: Set Expression
s2:10c105.0>
- s2 indicates that the “statistic” variable is number 2. The Select Available Parameters table above shows this corresponds to absolute pressure.
- :10 indicates that the engineering units selected are value 10. From the Select Available Engineering Units table above, this corresponds to PSI.
- c105.0 indicates that the constant value is 105.0
- > is the operator used
In this case, the alarm will “set” (trigger or turn on) when the absolute pressure > 105 PSIA.
Example 2: Clear Expression
s2:10c95.0>=
- s2 indicates that the “statistic” variable is number 2. The Select Available Parameters table above shows this corresponds to absolute pressure.
- :10 indicates that the engineering units selected are value 10. From the Select Available Engineering Units table above, this corresponds to PSI.
- c95.0 indicates that the constant value is 95.0
- >= is the operator used
In this case, the alarm will “clear” (cancel or turn off) when the pressure IS NOT ≥ 95.0 PSIA.
Example 3.1: Full Command
A ALE 0 s2:10c105.0> s2:10c95.0>=
- A indicates that this alarm should be added to the device with unit ID A
- ALE is the alarm set command
- 0 indicates setting the first alarm
- s2:10c105.0> is the SetExpression stating that the alarm should trigger (or “set”) when the pressure > 105.0 PSIA, as in example 1.
- s2:10c95.0>= is the ClearExpression stating that the alarm should stop (or “clear”) when the pressure is NOT ≥ 95.0 PSIA.
After sending this command, if the pressure increases to the point where it exceeds 105.0 PSIA, the alarm will trigger and the device will output a voltage or open drain signal (depending on the physical configuration). If the pressure then decreases to 104.0 PSIA, the alarm will still be on. Until the pressure drops below 95.0 PSIA, the alarm will still be triggered and the device will output the voltage or open drain signal.
Example 3.2 Full Command
C ALE 0 s13c95> s13c95>
- C ALE 0 sets the first alarm on the device with unit ID C.
- s13 indicates the 13th statistic (valve drive) in its default engineering units (%).
- c95> indicates a constant value of 95. The device will trigger if the statistic is greater than this value.
Since the clear expression is identical to the set expression, the device will only trigger when the set expression is true (similar to the old-style alarm). This command will trigger the alarm when the valve drive > 95% and clear when it isn’t.
Example 4: Command with &
In situations where your system needs to be firmly outside of the set expression conditions, giving the clear expression wider bounds can give an alarm where it will remain active even when just beyond the set expression.
D ALE 1 s2:10c51>s2:10c99<& s2:10c49>s2:10c101<&
- s2:10c51>s2:10c99<& sets the alarm when absolute pressure > 51 PSIA and < 99 PSIA.
- s2:10c49>s2:10c101<& clears the alarm when absolute pressure is outside of the range 49 to 101 PSIA.
Together than means that if the pressure increases from 40 to 60 PSIA and decreases again to 40 PSIA, the alarm will trigger after the pressure exceeds 51 PSIA and clear once it equals 49 PSIA. Similarly if the pressure decreases from 120 to 90 PSIA and increases back to 120 PSIA, it will trigger the alarm once the pressure drops below 99 PSIA and clear it once the pressure increases to at least 101 PSIA.
Example 5: Command with |
E ALE 0 s2:10c49<s2:10c101>| s2:10c51<s2:10c99>|
This will command the device with unit ID E to set up the first alarm as follows:
- Trigger when absolute pressure < 49 PSIA OR > 101 PSIA.
- Clear when absolute pressure is NOT < 51 PSIA OR > 99 PSIA.
This will clear if the value is between or equal to 51-99 PSIA.
Example 6: Command with ^
F ALE 1 s5:12c5>s6:4c700>^ s5:12c5>s6:4c700>^
This will command the device with unit ID F to set up the second alarm as follows:
- Trigger when mass flow > 5 SCCM OR gauge pressure > 700 kPaG, but not both.
- Clear when mass flow is NOT > 5 SCCM OR gauge pressure is NOT > 700 kPaG, but not both.
This will clear when either mass flow ≤ 5 SCCM OR when gauge pressure ≤ 700 kPaG.
Example 7: Command with !
G ALE 0 s2:10c14.8<! 1
This will command the device with unit ID G to set up the first alarm as follows:
- Trigger when absolute pressure is NOT < 14.8 PSIA.
- Never clear.
This is a good alarm to set if the device is always used at atmospheric pressure. It will trigger an alarm to warn of a potential blown sensor when the device thinks atmospheric pressure is above 14.8 PSIA. The alarm will only be cleared by repowering the device or sending another ALE command to the device.