Optimize signal clarity in an industrial network

Physical layer issues account for more than half of network installation problems—noise, ground loops, signal reflection, and cross-talk among them. Improve signal clarity with this advice.

03/05/2012


OSI Layer One (the physical layer) issues comprise more than 50% of reported problems in network installations, emphasizing that signal clarity is an important consideration when planning a network infrastructure of an industrial application. Twisted-pair cabling is a staple in industrial network infrastructure. More flexible and less expensive than coaxial cable, Cat5e and Cat6 cables are recommended for many installations; therefore, the structure of the twisted-pair cable is critical to ensure high-quality transmission. 

Differential Ethernet

How does differential Ethernet work? Cat5e and Cat6 cables transmit signals using differential mode signals, which are opposite polarity and equal amplitude from each wire in the twisted pair. At the receive end of the channel, the equal but opposite polarity signals are evaluated for the "difference" potential. If we had a 2 V signal, the difference in magnitude between the signals would be 4 V. Noise acts as a common mode signal when it is coupled into the balanced pair channel. It is "common" to both wires and is the same amplitude and phase. This interference signal does not propagate separately from the data signal but is integrated into the overall voltage signal to become a composite waveform.

The beauty of differential mode transmission is that it elegantly removes the noise from the source signal when the "difference" between the two wires is calculated. (See diagram.) With a 4 V differential signal (-2 V to +2 V) and a 1 V common mode disturber (+1 V on each wire), the voltage magnitude differential between the two wires is still 4 V (3 V to -1 V). In a perfectly balanced cabling system, the induced common mode signal would appear as two equal voltages that are simply subtracted out by the transceiver, thereby resulting in perfect noise immunity.

Differential mode transmission removes the noise from the source signal when the

In reality twisted-pair cables are not perfectly balanced, and the TIA (Telecommunications Industrial Association) has limits on cable specification intended to make sure at least a base level of twisted-pair balance is maintained. DCR unbalance, capacitance unbalance (CUB), and transverse conversion loss (TCL) are examples of specifications designed to ensure pair balance. 

Balancing twisted pairs

Balancing twisted pairs involves numerous strategies, and the better balanced the twisted-pair cabling, the more reliable the signals. Some of the most common challenges when using twisted-pair cabling are:

  • Susceptibility to noise: Twisted pairs tend to separate due to movement during installation, flexing, or handling. Each pair can be pictured as an antenna that can receive or transmit signals; thus variations in conductor-to-conductor spacing are cumulative and result in susceptibility to EMI and RFI that degrades signal transmission and network performance.
  • Signal reflections: When twisted pairs separate, they create impedance irregularities that can cause signal reflections (return loss). Impedance variations are also cumulative.
  • Pair-to-pair crosstalk: All twisted-pair Ethernet cables have crosstalk or pair-to-pair coupling, which is caused because each pair has different twists/in. (lay length). Lay length variation can increase the crosstalk that is cumulative down the length of the cable.
  • Connector issues: Twisted pairs can separate inside an improperly terminated connector, which can lead to lack of signal integrity. Tracing degradation inside connectors is a time-consuming and expensive process.
  • Lack of mechanical robustness: Like other cable types, twisted pairs are subject to degradation due to stretching and flexing during installation. When tension is applied with unequal force from one conductor to another, issues can occur.

Noise includes ground loops

If a twisted pair is not perfectly balanced, modal conversion of balanced to unbalanced signals is going to occur at RF frequencies. Differential mode signals at 20-30 MHz or higher can convert to common mode signals and vice-versa. The conversion artifacts adversely impact noise immunity from the environment as well as contribute to crosstalk between pairs and between other balanced cables. Only highly balanced twisted pairs can mitigate the modal conversion artifacts.

Signal phase in a balanced pair also is important because each signal must arrive at the end of the pair in proper phase. Both wires must be the same electrical length so the differential process acts on the "equal" but opposite signal.

Three types of noise can be coupled onto twisted-pair cabling: differential, environmental, and ground loop. Differential noise, that is, noise from nearby pairs in a cable, is called NEXT (internal to the cable), while noise from nearby cables is called ANEXT (from adjacent cables). Environmental noise is capacitive or inductively coupled to balanced pairs from external electromagnetic fields from disturbers such as electric motor noise, fluorescent light ballasts, and radio-frequency (RF) sources (in increasing order of severity). Ground loop noise is induced by a difference in potential between conductor ends or physical ground point locations in a building or between buildings. 

Shielding

Shielding can decrease the potential for modal conversion by limiting noise coupled onto the twisted pair from the environment. Shielding acts as a noise attenuator so that disturber signals are as small as possible before they impact the twisted pairs underneath the shield. A shield cannot remove noise; it can only attenuate noise. Balance twisted pairs utilizing differential signals remove noise. The more perfectly balanced they are, the more noise the balanced pairs can remove. The greater the noise source, the more critical it becomes to have good pair balances. 

Bonded-pair technology

Bonded-pair technology can alleviate many of the challenges of twisted-pair cable, exhibiting consistencies that traditionally have only been possible with coax and twin-lead designs. When conductors are bonded (that is, adjoined along their longitudinal axis), they can provide uniform conductor-to-conductor spacing, uniform twisting of insulated conductors into pairs, and a robustness that assures that the twists of the pair will not loosen up or separate during manufacturing or installation. 

Don’t cut corners

Twisted-pair cabling provides multiple benefits in an industrial networking application. However, cutting corners in cable quality can lead to downtime and costly maintenance. It is important to choose balanced-pair data cabling that can protect against noise, signal reflections, crosstalk, and other variables that can impact signal clarity.

Gareis is principal product engineer, Belden Inc. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media.

www.belden.com 

Belden offers a cable finder guide:

http://www.belden.com/resourcecenter/tools/cablefinder/index.cfm 

http://controleng.com/networks

www.tiaonline.org 



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.