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Alarming From a PLC or ModBus Device


The Scout PLC is available in 2 models for interfacing to a PLC or other Modbus device.

An SPLC12, which can read/write 12 registers, and the SPLC24, which can read/write 24 registers.

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

  • alarm notification from a PLC that is performing some control function,
  • local user interface into the PLC,
  • alarm notification that the PLC has stopped communicating, and
  • remote access to the PLC to change setpoints.

Connection to the PLC

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


The SPLC is programmed through the front panel by:

  • Selecting a channel in the SPLC
  • 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 SPLC allows the user to specify low and high limits, engineering units, additional scaling if needed, automatic calculation of daily High, Low and Totalized values and delay times until the value is considered in alarm.

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

For writing analog values (write holding), the SPLC 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 SPLC 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.

Internal system monitoring

In addition to monitoring the 12 or 24 registers in the PLC, the SPLC also monitors and alarms on:

  • Communication to the PLC
  • Loss of primary power
  • Low backup battery voltage

Local User Interface

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

The SPLC 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 the Status 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.

The SPLC keeps a log of the last 100 events that can be viewed locally or retrieved remotely. These date and time-stamped events include:

  • Channels going into and out of alarm
  • Alarm calls
  • Alarm acknowledgements
  • Programming changes
  • Call ins and outs

For digital and analog values, the SPLC data log allows the user to specify that each day the following should be recorded and retrieved locally on the display:

  • Analog
    • Daily minimum value
    • Daily maximum value
    • Totalized value if using engineering units of GPM
  • Digital
    • Daily run-time
    • Daily number of cycles (on/off)

The data log contains the last 100 date and time-stamped recorded values.

Alarm notification

Since each channel has a unique user-recorded message with it, calls made to designated personnel easily identify what is wrong.

Calls can be made to phones, cells and pagers.