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Alarming From a PLC or ModBus Device

Elite PLC

The Elite PLC is a dedicated alarm notification system to large PLC or SCADA systems. It continuously monitors up to 72 registers from a single device or spread across a network of devices.

The Elite PLC can also be split up between physical I/O that is connected directly to the Elite and I/O from a PLC. Physical I/O can be added in groups of 8 analog or digital inputs and 4 relay outputs - up to a maximum of 48 physical I/O.

The most common application for the Elite PLC, is to provide the following capabilities:

  • provide a dedicated hardware alarm notification system for PLC systems,
  • alarm notification that the PLC system has stopped communicating,
  • monitor some physical I/O that is not part of the PLC system,
  • interface to SCADA system for all PLC and physical I/O and
  • remote access to the PLC to change setpoints.

Different than alarm software

The Elite PLC is a dedicated hardware device, not software running on a PC. Since reliability of an alarm system is critical, a dedicated hardware platform provides significantly more up time, like 100%, versus a PC which is prone to lock-ups.

The Elite PLC does not provide thousands of alarm points, but will provide the highest reliability system for up to 72 of the most critical conditions.

Highly flexible settings allow notifying designated personnel as needed:

  • only one time per alarm,
  • every time any alarm occurs,
  • again for the same alarm if it still exists after a defined period of time, and
  • call in to get any/all channels in alarm.

Connection to the Elite PLC

The Elite PLC, EPLC, is physically connected to the PLC via a single serial RS-232 or RS-485 connection, making for a quick and clean installation. The communication between the EPLC and the PLC uses the Modbus RTU protocol with the EPLC as the Modbus Master or Slave.


The EPLC is programmed through the front panel, locally connected PC or remotely connected PC by:

  • Selecting a channel in the EPLC
  • Select the type of data to read/write. e.g. read coil, read input status, read register, read holding, write coil, write holding
  • Select the Modbus ID for that data
  • Select the register number to read/write

Reading and writing registers

For analog values (read holding and read register), the EPLC allows the user to specify a channel name, independent low and high limits, engineering units, additional scaling if needed, automatic calculation of Totalized values, phone lists for alarm notification, and delay times until the value is considered in alarm.

For digital values (read coil and read input status), the EPLC allows the user to specify a channel name, the normal condition (open or closed), automatic computation of cycles and run-times, phone lists for alarm notification and delay times until the value is considered in alarm.

For writing analog values (write holding), the EPLC allows the user to enter in a value in engineering units from the front panel or over the phone. The value is then written to the PLC and read back to confirm that the PLC has received the correct value.

For writing digital values (write coil), the EPLC allows the user to enter the state of the coil (1 or 0) from the front panel or over the phone. Again, the value written to the PLC is read back to confirm proper operation.

Modbus Master and Modbus Slave

The EPLC has 3 serial port and a modem that can be used to communicate with different devices all at the same time. This allows the EPLC to combine PLC data with physical I/O and present all this information to a SCADA system.

 Port 1Port 2Port 3Modem
Modbus Master   
Modbus Slave
Support radio modem 

Internal system monitoring

In addition to monitoring the 72 registers in the PLC or SCADA system, the EPLC also monitors and alarms on:

  • Communication to the PLC or SCADA
  • Loss of primary power
  • Low backup battery voltage
  • 24VDC supply for physical I/O
  • Internal temperature

Local User Interface

The EPLC allows the user to see values in the PLC and to make changes to them.

The EPLC has a keypad and display that are used to display the current status of channels, alarm call status and for programming.

When a channel goes into alarm, the channel that is in alarm is automatically displayed showing the channel name, the current value and the alarm violated. Additional information, including the Modbus ID and register number are obtained by pressing a key.

If multiple channels are in alarm, then the screen automatically switches between each of those channels.

If no channels are in alarm, the user can request the status of any channel by pressing the Status key and then Prev or Next to cycle through all the channels in the system.


To assist in programming, the EPLC has pre-formatted reports showing:

  • The current status of all monitored channels
  • The current programming of each channel
  • The register map of all channels
  • View a Sample Report

To assist with diagnosis of alarm events, the EPLC records the last 1000 events. These can be automatically printed to an attached printer as they occur, printed on demand or retrieved remotely by dialing in. These date and time-stamped events include:

  • Transitions into and out of alarm
  • Alarm notification calls
  • Alarm acknowledgments
  • Parameter changes
  • Call ins and outs

Alarm Notification

The EPLC provides 16 separate lists of numbers for alarm notification, allowing the appropriate personnel for a particular alarm to be notified.

Alarm notification can be:

  • voice based to phones or cells
  • text to alphapagers
  • numeric to pagers
  • fax reports
  • automatically call different people based on time of day