Today, almost every vendor of programmable logic controllers (PLCs) and their remote input/output (I/O) or block I/O units is supplying these products with Ethernet interfaces. In fact, some industry pundits, such as Harry Forbes, networks manager at ARC Advisory Group, Dedham, Mass., would even go as far as to say that Ethernet is “becoming the internal bus that glues together individual components,” even in cases in which the system isn’t truly open.
One by one, the issues that have impeded the penetration of Ethernet to the lowest tier of manufacturing are being eliminated, as years of product development and working groups begin to bear fruit. For example, a power-over-wire standard for Ethernet, IEEE802.3af, was recently ratified after three years in development. Future industrial Ethernet switches will indeed power devices, and the new standard opens up the possibility of sensors, controllers and actuators that require only Ethernet connections. However, the new standard does not address intrinsic safety because of its use of high voltage. Dick Caro, formerly with ARC and now a principal of CMC Associates, based in Andover, Mass., notes that low-energy wireless technology will be a “natural” for solving the intrinsic safety problem—combining batteries, fuel cells and other generation sources so that process control instruments can be plugged into a local source of AC power just as remote I/O is in factory automation.
Probably the most burning question facing device-level Ethernet right now is how to manage and monitor industrial Ethernet networks to address issues such as reboots on the factory floor. While several players push network management software products for enterprise-size networks, the plant floor presents unique challenges. PLCs and I/O systems are more heterogeneous than enterprise-level systems, they lack Microsoft services, their location is “God knows where,” as Forbes puts it, and if the network crashes, production stops. These are vastly different parameters than in the office, to be sure, but why reinvent the wheel when it comes to network management? Forbes writes in Ethernet at the Device Level Worldwide Outlook earlier this year: “In sharp contrast to device networks, networks employing Ethernet and TCP/IP are capable of employing the same sophisticated network management practices used by enterprise networks.”
Some vendors of enterprise networks are getting wise to this idea, such as Cisco Systems, which recently pushed into industrial devices. Meanwhile, some automation suppliers are designing special Simple Network Management Protocol (SNMP) applications for industrial devices. Note the integration between the Opto 22 Snap I/O Ethernet line and Computer Associate’s Unicenter product, which is a set of integrated components for managing the health and availability of each aspect of an enterprise computing infrastructure. Another tool is Phoenix Contact’s FL SNMP software product, which presents the network management information via an OLE for Process Control (OPC) server so that it can be integrated by human-machine interface software tools.
Not all the other concerns about device-level Ethernet may turn out to be serious setbacks. Forbes says he considers the security issue to be an “exaggerated bugaboo” now, because worries about default points in the consumer section do not translate to the industrial market. He adds that latency and synchronization problems “will be dead soldiers soon,” as much development work is being done in these areas. The IEEE 1588 standard, now in draft format, targets the issues around motion and synchronization using the dominant standard, TCP/IP.
Nevertheless, the tight synchronization of devices on the network, such as required for motion control applications, remains a big action item in the industry. To that end, an odd trend has emerged recently: the move by some vendors to bypass Internet Protocol (IP) in industrial Ethernet offerings. Bypassing IP could deprive users of a common network architecture shared across the manufacturing and enterprise domains, says Forbes.
无线以太网呢?缺乏标准ized industrial wiring technology is a hurdle to the use of Ethernet at the lower levels of industrial networks, resulting in higher costs of purchase and installation. Enterprises will likely adopt wireless local area networks (WLANs) in the next year or two, followed by a trickledown to the industrial space. As Caro puts it, “Wireless will happen simply because field wiring is too expensive to install and maintain. Objections to wireless in the field will be satisfied much as those about Ethernet are presently being addressed.”
