Coming Soon—New Kele.com & 2014-15 Kele Catalog

At Kele—we’re committed to providing you the best products, services and solutions in the building automation and HVAC/R industries. And we’re starting 2014 off with two new solutions that will help you get the job done right! In the first quarter we will be launching a new Kele.com and will be releasing our 2014-15 Kele catalog.

New Kele.com:
Fresh, sleek and easy-to-use are just some of the things people are saying to describe the new Kele.com. But we didn’t just update the look and feel of the site—once the site launches you’ll also find:
• New product landing pages and search functions to help you find the products you’re looking for faster,
• New product recommendations based on what you’re shopping for,
• A new video library to help stay ahead of the game,
• An enhanced cart and checkout process,
• And new line card and link shortcuts for quicker site navigation.

And don’t forget—online orders of $750 or more qualify for FREE SHIPPING!* So be on the lookout for the new Kele.com—you’ll be glad you did!

New 2014-15 Kele Catalog:
We may be biased, but the new 2014-15 Kele catalog is our best yet. When it comes to catalogs, Kele sets the industry standard. With this new catalog you’ll get:
• Information on 965 product families including 206 new product families,
• Easy-to-find wiring diagrams, dimensions and everything you need to spec a product,
• Valuable information on Kele’s services: in-house custom panel fabrication, custom valve and actuator assembly, custom calibrations, kitting and tagging,
• As well as valuable information on how Kele is your one-stop source for all of your building automation and HVAC/R needs.

The catalog should be ready to ship sometime in March 2014. Click here to request your new 2014-15 catalog today.

 

*Eligible in the 48 contiguous U.S. state only. Qualified customers only.

When the Controls Just Won’t Work

I have an old engineer friend I’ll call Bucky (not his real name). Bucky was burned years ago when he designed an HVAC system that turned out to have insufficient capacity to keep the building comfortable in winter. In fact, the perimeter offices were in the low 50s (°F) (low teens °C) when the first cold snap hit. When I say he was burned, I mean it figuratively – but the occupants of the building were thinking about burning him literally.

Well, old Bucky was not going to be burned again. We joke that the architect has to specify stronger door hardware when Bucky is doing the mechanical design, so that the doors don’t blow off the hinges from his absurd supply air quantities.

This leads to a control problem on Bucky-designed jobs. I had to install and program a building automation system for a Bucky job, and it wasn’t a good experience. How can one tune an office temperature control loop when the reheat box can warm the room faster than the temperature sensor can respond? The occupants would essentially be subjected to supply air temperature, which could reach 130°F (54°C) in heating mode.

I first went with my tried-and-true PID tuning method that I learned from a DuPont instrument engineer in the early 1980s. This method had never failed me until I tried it on Bucky’s HVAC system. I worked for a couple of hours on a single office but I could not get anything near stable control. I tried adding a feedforward loop to give the PID loop advance notice that the oversized hot water valve was about to open. That took programming time and it didn’t help at all.

So I went back to the office and batted the problem around with a group of my peers. We discussed, we calculated, we got out our controls books. We came to no good conclusion.

The next day, I programmed all of the interior spaces with no problem. There was way too much air but my tuning method resulted in stable control on the first pass. Then I went to ponder the perimeter offices again. As the building was approaching occupancy time, the painters were gone and the carpets were being installed. There happened to be a carpet layer in the office I went to first. I thought out loud for a minute, then I vented to him about what a pain the air system was for me. He sat up on his heels and listened, then said, “Seems to me there ought to be a way to reduce the air and water flows.”

I turned mighty red with embarrassment at that time. I thanked the carpet layer and went to call the test and balance fellows. They agreed to cut back on the water and air to the perimeter spaces if I could convince Bucky that I needed it. Well, Bucky came to the job site and it was pretty easy to convince him by getting him to stand in a perimeter office for a while. Problem solved, but not by me. They say that if your only tool is a hammer, then every problem looks like a nail. That was my problem in a nutshell. I was a controls guy, so I focused only on the controls.

The lessons I learned were: 1) Engineers can be wrong (yes, really!); 2) When a system can’t be tuned, the system might need fixing; and, 3) When all else fails, ask the carpet guy.

THE (RS-485 Network) TERMINATOR Or The Dance of the Data Pulses

If you’re involved with building automation systems you know (unless you’ve been living under a rock like the guy in that insurance commercial) that the modern trend is to connect all your building controls together on networks. Networks make it easy to add or move control nodes as your building control needs change since the nodes all connect to the network in a consistent, simple manner.

Obviously the various monitoring and control nodes on a building automation network must be able to talk to each other over some sort of medium. Both wired and wireless networks (or a hybrid combination of the two) are possible. Almost all wired networks deployed for building automation use twisted-pair communications cables. There are three popular types of twisted-pair communication schemes in use:

RS-485 (BACnet MSTP, Modbus RTU, Metasys N2 protocols)
FT-10 Free Topology  (Lontalk protocol)
Ethernet (BACnet IP, Modbus TCP protocols)

 

Today we are going to discuss the RS-485 twisted pair communications scheme and the significance of a little component called the “network termination resistor.”

A twisted-pair communications cable, as the name implies, has two insulated signal conductors twisted around and around each other at a consistent (N turns per inch) twist rate. Twisting the insulated conductors around each other reduces noise radiating outward and also improves immunity to external noise pickup. Twisted pairs are especially beneficial when used with a certain type of transmitter and receiver hardware known as “differential” signaling hardware which is used in RS-485 communications.

Twisted-pair communications cables have an electrical property called “characteristic impedance.” A cable’s characteristic impedance could be simply described as “how the cable looks to a high speed data pulse traveling down the cable” without getting into a lot of electromagnetic theory.

A cable’s characteristic impedance is expressed in units called “ohms.” You don’t need to worry about what an ohm is for purposes of this article.

Those of you who have some electrical experience are thinking that maybe you can measure the characteristic impedance of a cable by attaching your DC ohmmeter to the conductors and taking a reading. Sorry, it won’t work! You’ll just measure infinite resistance or pretty close to it. A cable “looks different” to a high speed data pulse than it does to a steady state DC voltage applied to it.

Sometimes a data cable will have its characteristic impedance stamped on the cable jacket, sometimes not. Most twisted-pair data cables will have an impedance somewhere between 100 and 150 ohms. A data cable specifically marked for RS-485 applications will have a characteristic impedance fairly close to 120 ohms.

Now as a data pulse travels down a twisted-pair data cable, you might say it “gets used to” the cable’s characteristic impedance. As long as the cable’s impedance doesn’t change unexpectedly the data pulses happily propagate along:

*** RS-485 WIRING TIP #1:

RS-485 will sometimes work with only the twisted pair connected between nodes, but you have a much better chance of making it work reliably if you also run the RS-485 Signal Common wire between the nodes. This topic really deserves its own tech article and we aren’t going to delve into it any deeper today! Just remember to provide the signal common hookup whenever possible.

Now RS-485 architecture allows many nodes to co-exist on a communications cable. So the transmitted data pulses will be read by all attached nodes. To keep from loading the transmitter too heavily, each RS-485 receiver has a high-impedance (12000-96000 ohm) input.

At each intermediate node (nodes not connected at the ends of the cable), the data pulses arrive on a 120 ohm twisted pair and leave on a 120 ohm twisted pair. The high impedance receiver inside the node does not load down the line, and so the data pulses happily travel on to the next node on the line:

*** RS-485 WIRING TIP #2:

For intermediate nodes on an RS-485 line, DO NOT make “stubs” that “tee” into the main twisted-pair trunk line! Run the incoming pair and the outgoing pair directly to the screws on the intermediate node as shown above.

So our data pulses are happily traveling down the twisted-pair communications cable being read by each intermediate node on the line until they come “to the end of the line” (cue ominous background music!).

At the end of the line, the data pulses traveling on the 120 ohm twisted pair suddenly encounter the high-impedance input of the last receiver on the line. This is known in transmission-line theory as “impedance mismatch” and it isn’t good!

When the data pulses hit the impedance mismatch at the end of the twisted pair, some of the energy in the pulses is literally reflected backwards up the line where it collides with the other data pulses. If the energy reflections are bad enough, the RS-485 receiver may not be able to interpret the data pulses correctly:

Obviously we’re going to have to do something about the impedance mismatch at the end of the line!  Fortunately, there is an inexpensive fix for this.  A small electrical component (a 120 ohm resistor) can be purchased and wired across the ends of the twisted pair.  Then, when the data pulses get to the end of the line they continue to see an impedance of 120 ohms due to the presence of the resistor.  Instead of reflecting, the energy travels into the 120 ohm resistor where it is converted into miniscule amounts of heat, and the data pulses fade away gracefully:

The 120 ohm resistors are inexpensive and easily obtained from distributors.

*** RS-485 WIRING TIP #3:

Only place 120 ohm termination resistors at the ENDS of the RS-485 twisted-pair cable.  Do not install termination resistors at any of the intermediate RS-485 nodes:

 

Conclusions

120 ohm network termination resistors placed at the ends of an RS-485 twisted-pair communications line help to eliminate data pulse signal reflections that can corrupt the data on the line.

We have heard anecdotal stories about how adding termination resistors did not help, and in some cases made matters worse!  That’s always possible, real-world network installations don’t always follow the assumptions made for a “typical” installation.  But on the whole the termination resistors will help network performance more often than they will hurt it.

Remember, network termination resistors are yet another tool in your network installation/troubleshooting toolkit.  They are not a cure-all for all network problems.  Keep a bag handy, and use them when it helps!