Industrial Network Standards Need Good PHY SIlicon Implementations

March 27, 2008
Industrial networks are moving from analog to digital operation to realize higher functionality while reducing design effort and bill-of-materials costs. Although proprietary solutions have emerged, there is great momentum behind the use of

Industrial networks are moving from analog to digital operation to realize higher functionality while reducing design effort and bill-of-materials costs.

Although proprietary solutions have emerged, there is great momentum behind the use of open standards to speed design, reduce costs, and ensure interoperability.

Several networking standards suit industrial applications, including Fieldbus-based solutions such as Profibus and the emerging Foundation Fieldbus. Also, some networking solutions first adopted by the automotive sector are showing good industrial usage potential, including the Controller Area Network (CAN) and Local Interconnect Network (LIN).

INDUSTRIAL NETWORK STANDARDS The original Fieldbus enabled a 4- to 20-mA current loop. This created an analog bus, with the advantages of a standardized physical interface to the wire, bus-powered devices on a single wire, and intrinsic safety options for a wide range of control applications.

Demands for enhanced process data and more extensive control capabilities led to the emergence of hybrid analog-digital solutions such as the HART Fieldbus, which superimposed digital information on the analog current loop.

The Foundation Fieldbus supports all-digital, serial, twoway Fieldbus communications. It retains the advantages of the analog system but also allows multiple variables from each device to be brought into a control system.

High-voltage mixed-signal technology supports the implementation of a highly integrated Fieldbus physical-layer (PHY) solution. This enables a complete physical interface between network wiring and actual measurement devices, meeting specifications of IEC physical layer standards, including Foundation Fieldbus H1 and Profibus PA protocols.

A Media Attachment Unit (MAU) allows common industrial functions, including closed-loop continuous control, batch sequencing, high-speed process automation, information integration, recipe management, and data gathering, to be easily performed using Fieldbus protocols.

Already proven in the automotive arena, CAN is steadily growing in popularity in industrial applications for the interconnection of servos, sensors, controllers, and a host of other devices used in machine control and automation. The CAN protocol is now ratified as the international standard ISO 11898, and it includes provision for 1-Mbit/s communications, as well as the 500-kbit/s rate that’s favored by the automotive sector.

However, industrial CAN implementations must be able to drive considerably longer cable lengths than those found in automotive applications. These longer cable lengths also place an extra burden on designers to protect circuit elements against electromagnetic interference.

LIN controls individual sensors and actuators, including motors, directly across a network. The automotive industry first adopted LIN to reduce the weight and complexity of wiring harnesses, a trend that began with the introduction of bus-type infrastructures such as CAN.

The CAN infrastructure is too expensive to use all the way to individual sensors and actuators. LIN makes these connections viable, but it also can connect into a CAN environment via a LIN master controller. Hence, a hierarchy of networks is emerging in modern vehicles, and the same situation is occurring in industrial applications.

PHYS, MOTOR DRIVERS, AND MORE Building a network requires PHY implementation, with particular attention to electromagnetic immunity, protection against short-circuited bus lines, and precautions to prevent blocking of network communications. Designers must often make their own provisions for such features. Actuators like motor drivers are traditionally implemented as discrete components, demanding additional design and development effort.

In the past, such network PHY implementations were created as discrete circuits using standard parts. Now, mixed-signal semiconductor technologies offer an alternative approach.

In particular, high-voltage mixed-signal semiconductor technology can be used to implement a single chip with all the functional blocks necessary to build a complete PHY, along with high-voltage drivers for motors and actuators, analog interfaces for sensors (one of the most common requirements for industrial applications), embedded digital processing, and high levels of system protection.

For example, designers can implement CAN transceiver and control functionality directly into a sensor interface, actuator, or motor or add additional new functions and enhance performance without sacrificing existing board space.

The use of high-voltage mixed-signal design in implementing industrial standard-compliant PHYs opens the door to higher levels of integration, reducing overall circuit size, weight, and power consumption.

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