Ethernet note
InTech Magazine-01 June 2003
By Nicholas Sheble - Fieldbus News.
Write him at nsheble@isa.org.

Networking engineers continue to have questions regarding the efficacy of Ethernet and where it comes down regarding determinism. Is it or isn't it deterministic?

Networks that can reliably deliver messages on time are deterministic. However, the industry has defined that networks in which the time to deliver messages is unpredictable under the worst-case conditions are nondeterministic.

The property of the original 10Base2 Ethernet that makes it nondeterministic is the protocol for recovery from collision detection. The original ethernet used statistical setback which means that once the system detects a collision, both transmitting nodes stop their transmissions and wait a random interval before trying again.

This random number interval will be different in each node, so that one will retry before the other. It is the random number and statistical nature of resolving Ethernet's collisions that make it nondeterministic and unsuitable for most automation applications.

However, all modern 10/100/1000BaseT Ethernet-based automation networks totally avoid collisions and are therefore entirely deterministic and quite suitable for any automation purpose.

The technology that replaces collisions uses segmented networks based on active switching. With an Ethernet switch, each Ethernet segment consists of only one device and its own port on the switch.

The switch buffers all traffic and resolves all potential collisions while serving many nodes at the same time. Furthermore, most modern switches are full duplex, allowing nodes to receive at the same time as they send.

With full duplex-switched Ethernet, there are no collisions or delays in transit. These networks exhibit the property that the worst-case transport delay for a message between any two nodes is predictable. That makes Ethernet fully deterministic.

Dataforth's isoLynx(tm) Data Acquisition System uses modern ethernet 10baseT to reliably transmit your analog and digital I/O to and from your host processor. The isoLynx(tm) system provides software examples in Visual C/C++, Visual Basic, and LabVIEW that demonstrate the ethernet communications port for data acquisition applications.

isoLynx(tm) accepts single-channel digital or analog I/O modules, and all I/O is channel-to-channel isolated. The company’s highly successful SCM5B signal conditioning module family is used for analog I/O, and its SCMD miniature digital modules provide digital I/O.

One I/O Controller module can operate up to 60 channels of differential analog I/O and 128 channels of digital I/O. You can mix and match any I/O module type on a per-channel basis thus reducing wasted I/O channels and saving costs.

The A/D and D/A system is built around 16-bit converters and provides a system throughput rate of 2000 channels-per-second. By selecting from over 650 single-channel SCM5B analog I/O modules, virtually any industrial signal can be interfaced to the isoLynx(tm). These modules offer superior specifications such as ±0.03% accuracy, ±0.005% linearity, six poles of filtering, 1500Vrms continuous input-to-output isolation, low output noise, and much more.

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at techinfo@dataforth.com.