As Ethernet technology has evolved to meet the needs of the industrial automation market, its capabilities provide significant advantages compared to the older proprietary networks.
Some of these advantages include:
- High speed communications, 10 Mbit, 100 Mbit, and 1 Gbit options
- Large data packet size coupled with higher speeds improve communications to large I/O drops and intelligent field devices
- More predictable communications with the introduction of Ethernet I/O scanning in automation systems
- No need to specially train personnel on proprietary networks as Ethernet is taught in universities and understood by personnel in other industries
- Simplified configuration and troubleshooting allows management of the entire network from one central location and access to a wide range of existing Ethernet diagnostic tools.
Yet despite such advantages, some organisations continue to share concerns regarding the use of Ethernet at plant level. This has chiefly focused on nervousness regarding its real-time capabilities and robustness to operate in the harsh environment of a plant floor.
While some of these concerns may have been valid before now, developments in Ethernet technology mean they should no longer be an issue. Today, Ethernet solutions are available that address head-on previous limitations expressed by end users.
In the 1990s, vendors recognised that the inherent advantages of Ethernet would make it an attractive fieldbus network, so they began to build open application protocols based on Ethernet. The protocols used layers 1 & 2 of the Ethernet stack and a new application layer optimised for automation applications.
The resulting protocols needed to have the flexibility to meet a range of industrial requirements while being easy to use for non-IT personnel within the plant.
An additional requirement was for these open protocols to use standard Ethernet hardware technology so users could utilise common off the shelf network components. Large device catalogues allow customers to pick best-in-class devices for their system, and be assured that these devices will work together.
Ethernet ring architecture.
Today, the most widely accepted Ethernet protocols used for distributed I/O communication and other industrial automation applications are Modbus TCP/IP and EtherNet/IP.
The system response time is defined as the time taken from the receipt of an input signal by the end device until receipt of response from the controller by the end device ie the time a system takes to react to a given input.
For a distributed I/O network that uses Ethernet communications, the response times for most devices will be equivalent or better than systems using other fieldbuses.
With an Ethernet distributed I/O network that has multiple I/O drops, an Ethernet switch is needed, either as a standalone device or built into an I/O drop adapter.
The typical switch adds about a 50 microsecond delay to the response time, however provides capabilities such as broadcast limits, priority services, and unicast/multicast traffic types. Thus enabling the fast response times necessary for industrial applications.
Ethernet infrastructure devices are now available that meet the same environmental requirements as the I/O systems for operating temperature, shock, vibration, and noise immunity.
Ethernet components also support the same agency approvals as traditional fieldbus solutions. Ethernet I/O products, infrastructure devices, and cables are now available with an IP67 watertight rating with operation temperature ranges from -20 to + 70°C.
In addition, the new generation of Ethernet Interfaces for distributed I/O systems includes built-in Ethernet switches that allow daisy chain connection of I/O drops without the need for an external switch, as shown in the figure below.
Ethernet daisy chain architecture.
This further reduces the installed cost of the I/O system and improves the ease of maintenance by reducing the required hardware.
Ethernet networks include both single and dual ring configurations (refer to Figure B) to increase the distributed I/O network availability for applications that require continuous operation.
In the event of a cable break or interface failure, the network will automatically reconfigure to re-route the data traffic, bypassing the failure to continue communication with the I/O system.
Legacy concerns regarding Ethernet are no longer valid given Ethernet has evolved into an Industrial Communication Protocol that is capable of being used at all level of an Industrial Automation application.
The performance offered by Ethernet distributed I/O networks has surpassed many of the older vendor specific networks.
While the easiest specification to quote is always the communication speed, automation controllers have the ability to communicate 4,000 I/O messages per second, with each message containing up to 125 words of data, with the capability to scan an Ethernet device in under 0.25 msec.
With this type of performance, concerns about Ethernet’s ability to be used in automation applications where system response is a critical requirement, has virtually been eliminated.
[See part 2: The power of Ethernet: Understanding design principles]
[Shaun Loesch is Product Manager Programmable Automation Controllers (PACs) at Schneider Electric.]