Bill's Blog on Industrial Ethernet

Occasionally I am asked, "Which cabling is better for Industrial Ethernet: shielded twisted-pair (STP) or unshielded twisted-pair (UTP)?" Both cable types have good inherent noise rejection because of the twisted conductors which act as a balanced transmission line.

STP works like UTP, but you seldom see it in normal networking (where better noise resistance is typically unneeded), because it is less flexible and more expensive. The issue has often fueled a debate between European advocates of STP and American supporters of UTP, but advocates of STP often tout its superiority without examining both sides of the issue.

STP shielding should be grounded at just one end to avoid ground loops (in case the ground potentials at each end of a link differ). These ground potentials are often unequal. In the rare case of an extremely noisy environment, grounding both ends of a link may be required for optimal results.

Since STP encases the signal wires within a conductive shield, you might expect all outside interference to be automatically blocked; but this is untrue. Like an antenna, the shield converts received noise into current which begets equal and opposite current in the signal pairs. If these two signal currents are symmetrical, they cancel each other and no noise is passed to connected equipment. But if the symmetry fails, the current in the cable becomes a source of system noise.

A prime consideration of STP is the need for close attention to proper grounding. STP only mitigates noise as long as the entire link is properly shielded and grounded. However, even a properly grounded system will eventually suffer grounding inconsistencies as equipment is added or is serviced or simply ages. And when the inevitable grounding inconsistency occurs, troubleshooting it can be difficult.

STP should work with Industrial Ethernet equipment specified for UTP, but if you fail to ground the shield properly, the more expensive STP will not work any better and could easily perform worse since a poorly grounded shield will degrade the system by acting as a antenna for noise. STP will work with Contemporary Controls (CTRLink) Industrial Ethernet products.

Configurable and managed switches are becoming more popular in the world of Industrial Ethernet. Typically such switches are accessed through either a serial port or by a web browser. However, to optimize security, local control should be exercised over the management information -- and this normally involves a null-modem cable connection between the switch and the computer used for setting the management options. But problems may arise when laptops are used for this function.

It is becoming harder to find a laptop that has a serial port -- and under these circumstances the Industrial Ethernet installer or supervisor must employ an alternate method of connecting the laptop to the serial port on the switch. Two types of serial adapters are commonly available: a PC Card (PCMCIA) adapter or a USB adapter.

PC Card adapters are more expensive than USB adapters (sometimes hundreds of dollars more), but they are always much faster and typically more reliable. Furthermore, the low-cost choice of a USB-to-serial adapter may not even work at all. A PCMCIA-to-serial adapter offers a hardware solution whereas a USB-to-serial device requires both hardware and software for the conversion. Running the conversion through the USB adapter's software complicates the data transfer.

Depending on serial baud rates and computer programs competing for processor time, a USB adapter can easily lose sync or drop bits. Although PC Card adapters cost a lot, spending hours solving a USB adapter issue can cost more. USB adapters vary considerably in quality. Some people have tried several different types to no avail. One adapter may work for one switch but not another and vice-versa.

Our engineering department has tried some USB adapters that worked and some that did not. Surprisingly, even a $40 USB-to-serial adapter from Radio Shack did the job. But be forewarned: a USB adapter could easily take up to five times as long to transfer data as a PC Card solution.

USB-to-serial adapters that worked for us:

Radio Shack --- model 26-183
Belkin --- model F5U103

PC Card-to-serial adapters that worked for us:

Quatech --- model SSPR-100
Quatech --- model DSPR-100

Today is my last day of work before I leave for a spring vacation. Although I will be taking a vacation from Industrial Ethernet, Industrial Ethernet keeps on working! Look for my next post in a couple of weeks.

Not long ago, I received a call from a fellow seeking help in troubleshooting his new installation of Industrial Ethernet. He was trying to make sense of the Link LED behavior he was witnessing in the absence of data activity. The installation involved four Industrial Ethernet switches connected by unshielded twisted-pair (UTP) in a daisy-chain topology.

The installer observed a solid Link LED on his first unit as well as on the next one. However, the third switch in the chain exhibited a flashing Link LED and the LED on the last switch would not glow at all.

Some investigation revealed the following situation: The UTP segment between the first and second switches was somewhat less than 100 meters. The segment separating the second and third units was slightly more than 100 meters. And the installer admitted that the final segment distance was closer to 200 meters than it was to 100 meters.

The distance limit of 100 meters for Industrial Ethernet UTP cabling was being violated in the second and third segments. It is interesting to observe that the Link LED behavior was intermittent when the distance specification was violated only modestly. You might expect the LED to simply fail completely once the distance limit was exceeded, but such was not the case until the violation became extreme.

The foregoing anecdote illustrates the importance of keeping your segment lengths within spec. Furthermore, it serves as a warning that if you allow the segment distance specification to be violated "even a little bit", you are risking erratic data transfer -- even though your Link LED could be suggesting all is well.