Synchronizing industrial Ethernet networks
Use of time by field devices
Having network-wide synchronized clocks is nice, but they are ultimately useless unless users can implement relevant features and benefits with them. Getting the slave device to behave in relation to the time is the ultimate goal of a synchronized system. Here, the differences between IEEE 1588 and DCs are quite apparent.
IEEE 1588 devices are typically somewhat complex because a microcontroller is required regardless of whether the core function of the device is a simple one (such as a digital I/O) or not. However, the devices will typically be implemented within the slave with the microcontroller running some kind of software, which reads directly from time registers to facilitate any and all functions, from simple digital signals to complex motion profiles.
On the other hand, the internal DC unit inside the EtherCAT slave controller is tied directly to the internal frame transceivers, the logical processing unit, an interrupt request to an optional microprocessor, and to a set of input and output pins. These input (latch) and output (sync) pins can be used directly as I/O signals for simple digital devices, or can be used with a microcontroller to achieve interrupts that can enable more advanced functionality.
At the basic level of device implementation, the sync pins can be used as outputs for digital signals. Associated with each sync pin is a time value register. When a value is set to this register, the sync pin will fire automatically with nanosecond resolution after the local time reaches this preset. In this way, a very inexpensive digital output module can be constructed, because there is no accompanying cost of a microcontroller, RAM, or software stack. These devices will have very low jitter, and can be synchronized to other network-wide DC devices in the network.
Again, the corollary to the sync pins are the input latch pins. These pins can be used as input pins in a simple I/O device, because a time value will latch into their associated registers when the configured rising or falling edge is detected. Both the sync and latch pins give functionality that is not tied to the scanning of a traditional PLC, but allows input capture or output command to occur at any point in time with nanosecond resolution.
When precise input time can be paired with precise output response, the user can measure the exact time that an event occurred and command an exact time to react to this event. This can include, for example, reacting to an alarm in a manufacturing line and calculating the future time required to avoid the rejection of product that is not at risk, but reliably eliminating the product that is at risk.
Oversampling can be accomplished through the use of subordinated interrupts, which will allow a device to take many samples of a signal at a rate that can be a multiple of the controller scan rate. This permits the capturing of an event (or even multiple events) that would otherwise be invisible to a traditional PLC, because their duration is short enough to fit between PLC scans. Similarly, oversampling digital output devices can generate pulse trains that would be impossible to detect with simple I/O, which at the maximum would only be able to create square waves of half the scan frequency (see Figure 4).
Using oversampling with analog capture ability enables analog modules to capture or create waveforms, which can add great functionality to a high-speed process. Continuously running oversampling modules can be used for adding condition monitoring and preventive maintenance functionality to an automation process, where fast Fourier transform (FFT) algorithms can self-diagnose motors, bearings, or gears for wear and tear. All this is possible while providing the control of the process and equipment itself.
The preceding paragraphs are not meant to imply that DCs and IEEE 1588 are necessarily rival synchronization schemes, nor that they are incompatible. The key point is that the internal synchronization methodology inside EtherCAT is based on DCs because of the aforementioned benefits of simplicity, cost efficiency, and flexibility in design. Also, DCs are already built into the EtherCAT slave controllers, so there are no additional hardware requirements. As a matter of fact, there are several sources of EtherCAT-to-IEEE 1588 boundary clocks that allow the bridging of time values from one to another. These are used for synchronizing an EtherCAT network to an exterior IEEE 1588 timing source, such as from a grandmaster clock, or another fieldbus system that uses IEEE 1588.
Both IEEE 1588 and DCs offer the automation engineer the ability to implement a network of highly synchronized devices spread across a large area and long network distances. Whereas IEEE 1588 offers a viable solution for Internet- and switch-based protocols, EtherCAT uses the more streamlined, bandwidth-efficient DC solution, which also ensures very low jitter. Whereas IEEE 1588 requires special and complex microcontroller-based hardware even for the most simple of digital I/O devices, EtherCAT DC devices can be implemented without microcontroller support, resulting in lower device and system costs. These two synchronization schemes can still be used together, bridged via boundary devices. This allows the network time to be shared between an EtherCAT network and an IEEE 1588-based system. Between these two synchronization schemes, automation engineers can develop bottom-up or top-down architectures to meet the demands of their application or their customers.
About the author
Joey Stubbs is the North American representative of the EtherCAT Technology Group. He is a registered professional engineer in control systems and holds a BS in electrical engineering from the University of South Carolina, as well as several other technical degrees. He has nearly 30 years of industrial experience in implementing industrial Ethernet technologies, automation projects, motion control, nuclear power, and power distribution.