Ethernet IP Real Time

Myth: Some control system vendors and standards organizations equate real time with modifications to the Ethernet standard associated with scheduling and time slots. Because EtherNet/IP does not modify Ethernet or TCP/UDP/IP it cannot comply with these definitions.

MOSTLY FALSE.

It is true that EtherNet/IP does not modify Ethernet, but false to assume that these modifications are required for real time control.

The Collins English Dictionary definition of ‘real time’ would make it appear that all systems using EtherNet/IP are real time. The reasoning is that EtherNet/IP provides the ability to update data at the same rate as receiving data. The true measure of being ‘real time’ is not measured in microseconds, but in the overall response function being sufficiently fast enough to adjust and control the process in question. For process loops, loop update times are on the order of seconds with requested packet intervals on order of 100-250ms.  More time critical loops like I/O are typically 30-50ms with a requested packet interval of 20 ms. Motion is the area that needs the most scrutiny as this is where we do need to consider microseconds and even smaller unit – nanoseconds!

Ethernet Requirements

With CIP Motion and its IEEE1588 time stamp, clock synchronization of better than 200ns can be readily achieved, meeting the needs of the most demanding motion control applications. Because the clocks in the end devices are tightly synchronized, a small amount of jitter in receipt time of the message is unimportant since information in the message is time stamped. It removes the requirement for strict determinism from the network infrastructure and entrusts the end devices with the timing information necessary to handle the real time control needs of the application. EtherNet/IP with CIP Motion allows 100 axes to be coordinated with a 1ms network update to all axes.

In an interview with IEB, Katharine Voss, executive director of ODVA answers the question of what the definition of 'realtime' in the context of Industrial Ethernet is. “In this context, 'real time' communications means in-time for the application. This clearly reflects the applicable IEC standard, IEC 61784- 2, for the definition of 'realtime' in the context of Industrial Ethernet which states that [realtime is] "the ability of a system to provide a required result in a bounded time." In fact, system determinism is the most critical factor in the user's ability to achieve command and control performance that meets the requirements of the application.”

When asked what cycle times/latency does your protocol achieve, and under what conditions, she explained “For example, in an 100-axis motion control application, it is possible to achieve an update rate of <1ms per axis in an EtherNet/IP system running at a gigabit data rate. More important, the axis update rate can be optimised for maximum system performance by individually configuring axis update rates to meet the performance needs of each axis, rather than all axes needing to operate at the same rate as the most demanding axis in the system.

EtherNet/IP, per se, does not segment the network into a protected real time and unprotected "non-real time" part. In fact, EtherNet/IP allows a single converged network architecture utilizing standard switching technology. Overall system determinism can be realized in EtherNet/IP networks using industry standards for Quality of Service in IP-based networks in accordance with The EtherNet/IP Specification and IETF standards (e.g., RFC2474 "DiffServ").