Ethernet flourishes in the shop

Modern Machine Shop, March, 1996 by Golden E. Herrin

Users looking to advance to the next level of communications capability, above a serial line for communicating to CNC machines, have been more concerned about price than about having the ultimate in communications technology. Ethernet LANs have met that requirement with its baseband technology. Even though an Ethernet baseband system supports only one transmission at a time, its widespread use has held the cost down. Combined with its relative ease of maintenance, and its acceptable speed, it has become an attractive next step up from a serial line. MAP as created for the manufacturing environment, on the other hand, was a broadband network that supported simultaneous voice, video, and data transmissions over the same cable. But, with this additional capability and higher speed came a higher price tag that the manufacturing community was not willing to pay. So the demand for Ethernet communications interfaces on CNCs has steadily increased while the demand for MAP has diminished.

Ethernet was developed in 1976 by Xerox Corporation as a prototype network for internal use at their Palo Alto Research Laboratories. The objective was to link together a set of minicomputers that were scattered throughout their research center. This link was to provide their researchers with the capability to exchange programs and data, in addition to providing access to specialized peripherals. Before Xerox completed the development, Digital Equipment Corporation (DEC) and Intel Corporation joined with Xerox as backers of this new LAN. This joint effort by three industry powerhouses gave Ethernet a tremendous boost. Even though the majority of Ethernet installations are in office environments there is no element of its design that prohibits use at the shop floor level.

Ethernet uses "Carrier Sensed Multiple Access with Collision Detection" as covered under OSI (Open System Interconnection) sanctioned standard IEEE 802.3. With this arrangement, all devices share a common data line. Any device wishing to transmit, listens to the line first to see if another device is transmitting. If there is existing transmission, the device waits until the line is clear. There is a chance that two devices can start sending information at the same time and thus create a collision. When collisions are detected, the data sent is ignored and each device stops transmitting for a random interval of time. The random interval assures that the same two devices do not create another collision on retry.

Ethernet provides a data transfer rate of 10 megabytes per second and considerable transmission flexibility in that it supports four transmission media: thick wire coax, thin wire coax, twisted pair and fiber optics. The distance without repeaters ranges from 230 feet on twisted pair to 5,000 feet on fiber optics.

To illustrate Ethernet's speed, tests were made to compare the speed of an Ethernet link to a serial data link. The first test consisted of transferring a very lengthy part program from a Silicon Graphics workstation to a VAX host computer. Using an RS-232 serial data line with Kermit protocol running at 9600 baud, the transfer time was six hours. The same part program transferred between an IBM PC and a Silicon Graphics workstation over an Ethernet link running at 10 megabytes per second took only 3.5 minutes. This savings in data transfer time can be a valuable asset to any shop where very long part programs are frequently transferred.

Ethernet interfaces are available options on many CNCs today. For CNCs that presently do not offer them, an Ethernet connection is available through stand-alone PCs connected to the CNCs and used as network interface units providing protocol conversion. It appears that Ethernet, the most common office network access method, is now firmly established in the manufacturing environment.