15 August 2000

Magic black box links dissimilar networks

by Perry Sink Marshall

Gateways play protocol-converting role in stab at interoperability

Here’s a common situation: You’re using a particular I/O network or fieldbus on your system. You install new devices and upgrades, but one device doesn’t use that protocol. What do you do?

If you’re buying hundreds of those devices and can wait eight months, perhaps you could persuade the manufacturer to add that capability to its product. But more than likely, "That ain’t gonna happen."

Your next best choice, then, is some type of gateway or protocol converter. The following is an outline of several possible situations and how you might approach them.

Fieldbus translating to serial

Most devices such as temperature controllers, bar-code readers, drives, and transducers come with serial ports. The most common circumstance is when this serial device needs to connect to a fieldbus.

Protocol converters are an example of black boxes that accomplish this task. They have RS-232/485 on one side and Profibus, DeviceNet, Interbus or CANopen on the other.

One common question is: "Can your gateway talk RS-232? I need to connect my drive to DeviceNet. My drive speaks RS-232."

The missing piece of information there is, what’s the protocol? RS-232 defines voltage levels and physical layers; it’s the dial tone, so to speak. But are we talking English, Chinese, Swahili, or Russian? Or is it Modbus, Danfoss, or DH+?

Many devices speak protocols that are unique to a manufacturer. Uniquely defined initialization commands start and stop handshakes. Generally, connecting one of these proprietary protocol devices to a common bus requires a fair bit of custom programming.

Perhaps the most common open serial protocol is Modbus RTU/Modbus ASCII. In contrast to a device that’s proprietary, a Modbus device is easy to communicate with.

Protocol converters run a software utility on an external PC and download configurations that map Modbus registers to Profibus, DeviceNet, Interbus, or CANopen. So the black box actually has three ports: one for the Modbus device, one for the fieldbus, and one for the serial port that connects to the PC. The device connects Modbus to Profibus, to DeviceNet, and to others.

Slave serves master

The most typical application is where the programmable logic controller (PLC) is the fieldbus network master and the newly connected device is a slave.

If the serial connection is RS-232, you can connect one device. However, if it is RS-485, you can multidrop up to 31 devices and divide the cost of the gateway among more nodes.

A much less common scenario, typically found in distributed control systems (DCSs), is where the DCS (or PLC) is a Modbus master, and the user wants to connect a fieldbus-compatible device.

This is a bit more complex because the gateway must be a master, not a slave. So the mapping from one bus to another involves all of the devices on the fieldbus network, which must be individually recognized and mapped as a slave device on the supervising network. A slave connection on the upper-level network may have limited data capacity.

Two fieldbuses work one

Rather than Profibus to serial or DeviceNet to serial, you may need a solution such as Profibus to DeviceNet, DeviceNet to ControlNet, or Interbus to CANopen–a fieldbus-to-fieldbus integration.

This is more complex than the previous examples. To do this, we usually put two different cards in a PC and write an application that shares the data.

For example, if you wanted to connect DeviceNet devices to a Profibus network, you might put a Profibus slave card and a DeviceNet master card in the same PC and write a driver that exchanges data between the two cards.

If you want only to share simple I/O data, writing that driver might not be terribly difficult. If you want to access other data such as explicit messages, configuration data, and the like, then the job is quite involved.

A comparatively clean way around this formidable obstacle is to use standard interfaces such as dynamic data exchange and object linking and embedding for process control (see related story). This angle for combining protocols and making fieldbus-to-fieldbus gateways has many possibilities.

It’s even possible to have more than two cards and link three or four networks together.

Fieldbus in, Ethernet out

A growing number of manufacturers are selling various types of thin-client servers that connect Ethernet to a fieldbus.

An example is the SMS-CE4-DNM/IP, which is a black box with an Ethernet port on one side and a DeviceNet connection on the other side or, alternatively, Profibus, Interbus-S, CANopen, ControlNet, SDS, or AS-I with a Windows CE 486 engine in the middle.

This device can do simple protocol translation–by default, it maps the fieldbus I/O data to Modbus/TCP registers on Ethernet–but any Windows CE control software could also be installed for more sophisticated, distributed control architectures.WBJ

Terminology
ASCII American standard code for information exchange
AS-i actuator sensor interface
CAN controller-area network
DCS distributed control system
PLC programmable logic controller
RTU remote terminal unit