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Crystals and oscillators for industrial Ethernet

Created by Jochen Neller, Technical Support Inductors & Timing Devices at Rutronik Elektronische Bauelemente GmbH |   Knowledge

Industrial Ethernet enables continuous communication between actuators and sensors at field level and management systems. It also combines real-time capability with the robustness and security of fieldbus systems – which is why it is gradually replacing it. For Industrial Ethernet to also satisfy tough real-time requirements, crystals and oscillators of high signal precision and reliability are needed.

There are currently many application protocols available, among them Ethernet/IP, EtherCAT, SERCOS, Profinet and Powerlink. According to HMS, EtherNet/IP, Profinet and EtherCAT currently have the largest market share.

 

EtherCAT – Fast and flexible

The by far fastest technology is EtherCAT, which also offers exceptionally fast synchronization measurable in nanoseconds. With their short reaction times, they speed up all applications with transitions. Because the load placed on the CPU by EtherCAT is around a third less than other bus systems with a comparable cycle duration, it provides a basis for higher performance and precision at lower cost – and therefore also serves as a basis of control concepts that cannot be implemented using conventional bus systems. The protocol is processed entirely in hardware here.

 

Data communication follows the master/slave model. EtherCAT offers absolute flexibility in terms of topology: It supports linear, tree and star topologies as well as any combination of these. There is practically no limit to the number of nodes. EtherCAT therefore also makes the structures known from bus systems available for Ethernet, making EtherCAT ideal for use in time-critical motion control applications.

 

Hard and Soft Real-Time

Soft real-time is adequate for communication in management systems. At control and field level, however, hard real-time is increasingly demanded so that the components in a multi-layered system can work cleanly in sync. Where different processes distributed across multiple locations require simultaneous actions (e.g. if multiple servo drives perform coordinated motions simultaneously), minor deviations and synchronized processes in the network nodes are also a critical aspect. Distributed clocks provide a high-precision, network-wide reference time for this purpose.

 

The basic Ethernet standard is not sufficient for these requirements and functions. They can be implemented in a variety of hardware solutions such as FPGAs, ASICs or fully integrated controllers. Crystals, oscillators or real-time clocks (RTCs) with high signal quality and reliability are required for signal clocking.

 

Precise and reliable crystals and oscillators

For oscillators, there is a choice between MEMS and crystal-based models:

  • MEMS oscillators are considered to be mechanically more resilient.
  • Crystal oscillators benefit in terms of precision, phase jitters and temperature stability.

 

Crystals and oscillators that offer hard real-time support, short cycle times and low jitter are available from EPSON, for example. The manufacturer covers all needs relating to current protocol using high-quality frequency-defining components.

 

For the standard frequencies of industrial protocols with a requirement of +/-50ppm over a range of -40°C to +85°C, the 2.5x2.0 SG-210STF series crystal oscillators are recommended – that are also available in 7x5 (SG7050) and 5x3.2 (SG5032). The expanded temperature range up to 105°C or 125°C is covered by the SG-210S*B series.

 

For non-standard frequencies, smaller quantities and short delivery times, the programmable crystal oscillators of the SG-8018 and SG-8101 series are recommended. They include an internal PLL (Phase-Locked Loop) that generates the corresponding frequencies.

 

The SG-8101 series can be used in the expanded temperature range up to 105°C, including under harsh environmental conditions. It is also distinguished by a frequency tolerance that is around 66% tighter (+/-50ppm to +/-15ppm) and power consumption that is half of comparable products. As such, the SG-8101 crystal oscillators make a significant contribution to the system’s properties with hard real-time, low power consumption and fast development cycles. They are also suitable for smaller production volumes.

 

The SG-8018 series encompasses the cheapest crystal oscillators with PLL from Epson, which already have specifications of +/-50ppm at a temperature range of -40°C to 105°C as standard.

 

Real-Time Clocks for maximum precision

If even greater precision is required, real-time clocks (RTCs) are the technology of choice. RTCs offer a simpler design thanks to their integrated quartz crystal, a high level of reliability and lower power consumption, and can for instance be used as backups. For maximum precision, the RTCs of the RX8900CE series from Epson offer integrated temperature compensation, which allows them for instance to contextualize the timing of various events in relation to one another. The RTCs have a clock precision of up to ±3.4 x 10-6 at temperatures of -40°C to +85°C.


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