Differences Between SCADAPack Modbus and Modbus

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The SCADAPack Modbus is a driver that allows connection to the Control Microsystems range of SCADAPack RTUs using the Modbus protocol. These devices support standard Modbus communications options, but also have a number of additional features that are unique to this range of RTUs that have been supported by this driver.

The SCADAPack Modbus driver is an Advanced driver, which means that it provides functionality not available in the simple drivers such as Modbus. One key addition is in the area of PSTN modem support. The PSTN support provides a way that Modbus devices can be dialled from ClearSCADA, but using the Unsolicited PLC functionality, remote devices can also dial into ClearSCADA and identify themselves.

The point polling mechanism is handled differently in the SCADAPack Modbus driver. In the Modbus driver, individual scanners are configured which determine what data gets polled and at what frequency. All points defined within that scanner will be polled at the same rate. In the SCADAPack Modbus driver, points get assigned to one of eight Retrieval Intervals. The driver then packages up requests based on the configured Retrieval Intervals, and polls the device accordingly.

The Modbus driver includes support only for Modbus RTU over serial or TCP, or Modbus TCP. The SCADAPack Modbus driver also includes support for Modbus UDP, Modbus ASCII over serial, TCP and UDP and Modbus RTU over serial, TCP or UDP.

Addressing in the Modbus driver is handled by referencing an offset within the defined point scanner. For example, in a scanner that reads 100 holding registers from address 40000, you would configure an analog point with a word offset. To read address 40050, you would configure a point with a word offset of 50 (depending on the Modbus addressing of your device). The SCADAPack Modbus driver uses direct addressing in the point objects so to reference the point at address 40050, you would enter the address 40050 directly.  See Modbus Driver Polling Differences for more information.


Examples of additional SCADAPack functionality includes support for the following:

• DLOG historic data for reading time stamped history data from the RTU.

• Remote connection objects to allow other programming tools to be used over the same communications links.

• Time synchronization

• A generic Modbus outstation type which enables use of the SCADAPack Modbus driver features

Additional functionality available for generic Modbus devices includes:

• Support for Modbus ASCII. Modbus RTU and Modbus TCP/UDP including support for Modbus ASCII over TCP and UDP and Modbus RTU over TCP and UDP.

• Restriction of the maximum packet size

• Unsolicited PLC functionality so a remote Modbus device can report into the master

• PSTN support

• Switched and shared outstation sets for multiple communications paths and redundancy

So which driver should you use?

If you have a simple configuration that does not require the additional functionality provided in the SCADAPack Modbus driver, then you can use either Modbus or SCADAPack Modbus. There is no real reason for one to be used over the other, it really comes down to personal preference. If, on the other hand, you require things such as Modbus ASCII protocol, modem support or communication to a SCADAPack Modbus device, you should use the SCADAPack Modbus driver.

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