Modbus Slave RTU Option

The Modbus Slave Option allows the Elite to be used as a Modbus slave device and communicated with any SCADA or HMI package that has Modbus support. The Modbus support works over a direct serial connection, radio modem or a modem.

The Modbus Slave RTU support is in addition to the normal alarm notification capabilities.

In a single industrial package the DiaLog Elite is an alarm dialer, programmable controller and RTU.

Complete SCADA systems can be created for a fraction of the typical cost and on-going maintenance is easier. Since each remote unit is a complete DiaLog Elite, they can each be programmed or interrogated over the phone. At each remote site you can view all the current conditions and perform programming using the local display and keypad.

Since every DiaLog Elite is an industrial device in a NEMA 4X enclosure, there may be no need to include any additional enclosures, heaters or fans.

Typical Applications

Most SCADA applications have one or more centrally located PCs that either periodically poll remotely located sites or have continuous radio links to the remote sites. The central PCs have a SCADA software package like Wonderware, Intellution, P-CIM or OI-2000 that communicates via Modbus RTU to remote DiaLog Elites. For example,

  • Wastewater lift station monitoring pump run times, total flow and level.
  • Water pumping stations to monitor tank level and turn on one or more pumps.
  • Oil injection wells to control pumps and valves at the well and water source.
  • Gas compressor stations to monitor flow, pressure and temperature.
  • Tank farms to monitor tank levels.

Accessible Information

All digital and analog current values, alarm states and acknowledge states can be read by the SCADA software.

Current values include:

Digital inputs open/closed
Analog inputs scaled or unscaled
Relay output state energized/de-energized
Total pulse counters counts (32-bit)
Total time counters seconds seconds (32-bit)
Alarm states in/out of alarm
Acknowledge states ack/not ack

The SCADA software can perform control by writing to the DiaLog Elite.

Values that can be written are:

Relays energized/de-energized
Pulse counter preset counts (32-bit)
Total time preset seconds (32-bit)


The Modbus Slave RTU Option does not require any additional hardware be installed into any DiaLog Elite.

If the customer has a previous version, Antx customer service downloads the latest release and turns on the Modbus Slave RTU Option.

There is no additional charge for downloading a newer version of the firmware for this option.

How the Modbus Slave Option Works

The Modbus RTU protocol defines a communication scheme between a Master and a number of remote or Slave devices. The Master initiates all communication with each Slave device by reading or writing one or more registers of information. The registers are mapped to the structure of the DiaLog Elite.

The Modbus protocol requires a response for every command, ensuring that messages are received by the Slave device (DiaLog Elite) as they were sent by the Master and visa versa.

For example, if the SCADA software wanted to read all the analog values from a remotely located Elite every 15 minutes, it would:

  • dial-up up the Elite, or communicate over a radio modem
  • request all Holding Registers,
  • wait 15 minutes and repeat step 1