Why do we use s7onlinx communication instead of native DDE or native OPC DA protocols?

While DDE (ncdde) is a native way for HMI operator screens to communicate with CNC, it has several disadvantages when used for machine data collection. DDE is a legacy protocol that dates back to the days of Windows 3.11 and earlier. It is text-based and relies on Windows Messages for data delivery, which results in high overhead in terms of performance, system resources, and lag in timings.

While DDE may be suitable for operator screens that require relatively small pieces of data to visualize on active screens, it is not well-suited for scenarios where large amounts of data are read intensively, such as in MDC or SCADA systems. This can have an adverse effect on HMI responsiveness and may cause operator screens to freeze.

The same issues apply to Network DDE, which is essentially a network interface extension of the classical DDE protocol. Network DDE suffers from the same limitations as DDE does.

Why don’t we use OPC DA which is natively available on 840 Dpl? For the same reason. Native OPC DA server is available on 840 Dpl, but it suffers from the same limitations as DDE since it uses DDE protocol internally.

Alternatively, s7onlinx can be used to address the limitations of DDE and OPC DA protocols. By nature, s7onlinx is very similar to S7 TCP protocol, with the difference that transactions happen directly over PROFIBUS/MPI communication, rather than TCP connection. This makes it a more efficient and reliable protocol for machine data collection, particularly in scenarios where large amounts of data are collected. Even on old HMI models, s7onlinx shows good performance and low system resource consumption, even when intensive data communication occurs.