Application note
Digital Output Channel Monitors
Digital output current
The current in each output channel is continuously measured, and can be read from a programmed controllers or displayed in VIGO.
The user can read this value into a controller and check if the power consumption of the connected device is within the expected limits.
For limit checks and safety monitors, please refer to: Minimum Current Monitor / Maximum Current Monitor.
(See also: OutCurrent.)
Minimum current monitor
Monitors if the connected device is defective, not connected, wire broken etc.
Minimum current monitor is especially useful for devices where a malfunction does not directly influence the process e.g. a signal lamp.
The module itself, based on the configuration values, performs the monitoring.
When the output is switched ON and the Start current grace time has elapsed, the actual current is compared with the specified Minimum current limit.
If the current is outside the limit, an alarm for under load can be generated. During an under load condition the output stays in the ON state.
Programming in the controller is only needed if the alarm needs special handling or the alarm condition needs to change the control scheme.
(See also: MinMaxCurrent.)
Maximum current monitor
Monitors if the connected device draws more current than expected.
Maximum current monitor is especially useful for devices where a sudden raise in current needs the output to switch OFF immediately, e.g.
when an automatic door reaches the end position or is obstructed in its movement. This can often save the cost and complexity of a position feedback system.
The monitoring is performed by the module itself, based on the configuration values.
When the output is switched ON and the Start current grace time has elapsed, the actual current is compared with the specified Maximum current limit.
If the current is higher than the limit, the output is immediately switched OFF and an alarm for overload can be generated.
(See also: MinMaxCurrent.)
Start current grace time
When an output is switched ON, the Start current grace time must elapse before the minimum and maximum current monitors are enabled.
This timer allows the current consumption to stabilise before the automatic current monitoring is started.
(See also: MinMaxCurTime.)
Overload protection
When the output is switched ON the digital output channel allows an initial current of 2A for a maximum of 2 seconds.
If the initial current is higher than 2A the output is switched OFF.
After the initial current period, 1A is the maximum allowable continuous load current.
When the start current grace time has elapsed, the output will be switched OFF if the current is higher than 1A or higher than defined as the Maximum current monitor limit.
(See also: MinMaxCurTime, MinMaxCurrent.)
Output status feedback
Using Output status feedback, it is possible for the controller to read back the physical status of the output signal.
If the output module switches OFF the output (e.g. due to overload), the controller will still regard the output flag as high, although the physical output is low.
The Output status feedback flag can therefore be used to directly test the state of the physical output
Output status flag passes through a low-pass filter, to eliminate noise on the input signal.
(See also: FlagReg.)
Output cycle counter
Counts every time the physical output is changed from a low to high signal.
For a connected device that gets increasingly worn by ON/OFF operations (e.g. valves), this counter can be readout, compared to a limit and if exceeded, can generate a request for preventive maintenance.
If the output is used for e.g. dosing, the difference between the counter value after dosing has finished and the counter value before dosing was started, is the number of dosing strokes used.
(See also: Counter.)
Accumulated output active time counter
Counts the number of seconds the physical output is in active state.
For a connected device that gets increasingly worn when switched ON (e.g. pumps), this counter can be read by the controller, compared with a limit and if exceeded, can generate a request for preventive maintenance.If the output is used for e.g. dosing; the difference between the counter values when dosing has finished and the counter value before dosing was started, is the amount of dosing time used.
(See also: OperatingTime.)
Alarming
The alarm system consists of two levels: Active alarm and Historic alarm.
When an alarm condition is active, it is shown both in the active and historic alarm list. As soon as the alarm condition disappears, the alarm is removed from the active alarm list, but remains in the historic list, until acknowledged. This ensures that the cause of an alarm is always available, even if the alarm condition disappeared before action was taken.
Alarms can be detected automatically in a controller by using a “When Error Then” statement, or by polling the specific alarm condition in the channel.
When a controller reads the alarm status register, any alarm is automatically acknowledged and removed from the alarm list. However, if the alarm condition is still active, the corresponding bit is immediately set again. Reading from an alarm byte automatically acknowledges all alarms in that byte. If more alarm bits are expected to be active, the alarm byte has to be read back to the controller as a byte and each bit individually examined for appropriate action.
(See also: Channel Error, Channel Config.)
System Watchdog
One common watchdog for the entire module is provided. If enabled, the watchdog monitors communication to the output module.
If no communication has been received within the specified watchdog time, all outputs n the module will be switched off.
Activating the watchdog ensures that the outputs will go to a fail-safe state when either the controller is out of order or the P-NET communication fails.
(See also: WDPreset, WDtimer, DeviceConfig.)
Notification
When an event happens in the output channel, a notification can be given to the controller. For each digital output channel, notification can be selected to show a change in: Output flag, Control flag, Output status flag and Actual error. Each notification can be individually enabled or disabled.
(See also: Channel Config)
