Relay Fundamentals

Open an HVAC control panel, and you’re likely to find at least one relay mounted inside. Sometimes you find lots of relays! Even though relay technology has been around since the 1800s, there is still a need for relays in control panels.

What components make up a relay? How are relays typically shown in wiring diagrams? What functions can a relay perform? Stay tuned for some answers…

What Components Make Up A Relay?

A relay is constructed from the following components:

  • Electromagnet (coil of wire with metal core)
  • Armature (arm that moves when attracted by the energized electromagnet)
  • Spring (keeps the armature retracted when the electromagnet is de-energized)
  • Contact(s) (one or more electrical contacts that open or close when the armature moves)
  • Electrical Terminals (so you can make connections to the electromagnet and contacts)

How Does It Work?

You might have built an electromagnet in high school science class and used it to pick up small metal objects.  An electromagnet is a coil of wire wound around a metal core.  When the coil of wire is hooked to an electrical power source and current flows through the coil, a magnetic field is produced which surrounds the coil and the core:

While the electromagnet is energized, any metal objects which come close will be physically pulled towards core.

When the switch is opened, the current flow stops and the magnetic field disappears.  When the magnetic field disappears, nearby metal objects are no longer attracted to the core:

We now place a pivoted metal arm (armature) above the electromagnet, and we connect a spring to keep the armature pulled away from the electromagnet under “normal” (electromagnet de-energized) conditions:

If we now energize the coil, the magnetic field will overcome the spring force and pull the armature down:

So… we have an armature which we can move up and down by de-energizing or energizing the electromagnet coil.  What do we do with it?

We use the moving armature to open and close electrical contacts which are separate from the coil circuit.  We can do it like this:

In this example, we’re using the metal armature itself as the “Common” or “Wiper” for a double-throw switch.  The tip of the armature makes contact with one of two possible points depending on the position of the armature.

When the relay is de-energized, the armature is up and the wiper makes connection with the “Normally Closed” (NC) contact.

When the relay is energized, the armature is down and the wiper makes connection with the “Normally Open” (NO) relay contact:

This example relay would be known as a “Single Pole, Double Throw” (SPDT) or “1 Pole, Double Throw” (1PDT) relay.  It has one set of contacts (1 Pole) with both Normally Open and Normally Closed (Double Throw) connections.

Relays can have more than one set of contacts (poles).  In the next example, the metal armature is not part of the electrical circuit but instead moves a plastic connector that activates two sets of contacts:

This would be known as a “Double Pole, Double Throw” (DPDT) or “2 Pole, Double Throw” (2PDT) relay.

The number of poles (contact sets) can be expanded (3PDT relays, 4PDT relays etc.).  Typically 4 poles is the largest relay we would see in an HVAC control panel.  If you need more than 4 poles in a relay, you would probably use two relays with the coils wired together in parallel.  Shown below are a DPDT relay and a 4PDT relay wired together to create 6PDT relay action:


Electrical Ratings of a Relay

The coil of a “classic” relay (no electronics) is typically rated for a specific voltage and either AC or DC operation but not both.

There are some modern relays (like Functional Devices “Relay In A Box”) that contain electronic circuits that allow the coil to operate over a range of voltages and either AC or DC operation.

If in doubt, the relay’s datasheet should clearly spell out the acceptable voltage range for the coil and whether it’s AC only, DC only, or AC/DC compatible.

Relay coils are generally tolerant of some variation in the coil voltage, a typical datasheet specification is 80% – 120% of rated coil voltage.  So if a 24V transformer is “running a little hot” (putting out maybe 27V) or there is a bit of voltage drop in the coil connecting wires (maybe the coil is only getting 22V) the relay will still function just fine.

The relay contacts will have ratings for the maximum voltage allowed and the maximum current allowed.  Sometimes there are multiple ratings depending on the type of load being switched (for a motor load sometimes the contacts are rated in horsepower for example).  The circuit being switched by the contacts should not put more voltage or current on the contacts than allowed by the datasheet.

Relay Symbols Used in Wiring Diagrams

The relay coil may be drawn several different ways:

The relay contacts are typically shown one of two ways:

Relays go by several possible names on an electrical drawing:

‘R’       (not a great idea since resistors can have this designation too)
‘RL’     (better)
‘RLY’   (better still)
‘K’      (Where did this come from?  We don’t know.  But it’s widely used!)

If there are multiple relays, the name will be followed by a number or a letter as in RLY1, RLY2, RLY3 or RLYA, RLYB, RLYC.  There might be a hyphen (-) between the name and number or letter too.  There is no specific standard on how relays must be named.

Sometimes the relay is shown as a single assembly with coil and contacts arranged in a rectangle:

But sometimes the coil and the contacts are scattered around the drawing:

Relay Terminal Numbering

Unfortunately there does not seem to be a standard for the terminal numbering on relays.

Most of the relays we use in HVAC panels plug into a base socket.  The pins/blades of the relay have assigned numbers and then the bases they plug into have assigned numbers on the screw terminals.  If the same brand of relay and base are used together, the blade numbers usually match the terminal numbers on the base.  If, however, different brands of relay and base are used together, the blade numbers may not match the base terminal numbers even though the two parts are physically compatible (example:  IDEC SH3B-05 relay plugged into Omron PTF11A base).  It’s a good idea to use the same brand of relay and base to avoid confusion.

Some electronic relays such as Functional Devices “Relay In A Box” have color-coded flying leads rather than screw terminals.  Consult the device datasheet to determine the function of each color-coded lead.

Functions a Relay Can Perform

So what useful functions can a relay perform?

  • Use a low voltage and/or low current signal (on the coil) to switch a large voltage and/or large current circuit (on the contacts).
  • Use a DC signal (on the coil) to switch an AC circuit (on the contacts) or vice-versa.
  • Use one signal (on the coil) to switch multiple circuits (multi-pole contacts).
  • Invert the sense of a signal (using the Normally Closed contact).
  • Use a signal from one circuit (on the coil) to switch another circuit (on the contacts) where the two circuits are not allowed to have any direct electrical connection.
  • Use one signal (on the coil) to switch between two alternate loads (on the contacts).
  • Various combinations of the above.

The example below demonstrates a relay performing a combination of several of the above functions.  A low voltage, low current control signal from a controller is switching a high voltage, high current load on and off.  The relay is also allowing a DC control signal to switch an AC load.  There is also electrical isolation between the drive circuit and the load circuit so that the controller is protected from the high voltage 240V power source:

In the next scenario a smoke detector signal is being sent to three different systems simultaneously while maintaining isolation between the systems to make sure there is no unintended interaction between the systems:

Here is an application where we want to reverse the sense of an electrical signal.  We have a foil alarm loop glued to a glass door.  When the foil is intact (current flowing through foil) we want to hold off the Alarm light and horn.  When the foil is broken (current flow stops) we want to energize the Alarm light and horn.  We can get the reverse action we want by using the Normally Closed contact on the relay:

Let’s see how we can use both the N.O. and N.C. contacts to get “alternate action” between two loads.  We decide that in addition to the ALARM light, we also want a NORMAL light for our alarm circuit that is on when the foil connection is intact.  So we want the action of the NORMAL and ALARM lights to alternate, one or the other is on at any time, but never both at the same time.  We can get this alternate action by adding the NORMAL light to the N.O. relay contact:

You may begin to see how versatile relays are.  Even with all the high-tech electronics in control panels, relays will be with us for a long time to come.  The operation is easy to understand and they can be used for many different applications.

Magnetic Latching Relays

There is a special class of relays known as “magnetic latching relays.”  These relays contain one or more small permanent magnets which hold the armature in the last commanded position when the coil drive is removed.  When a coil drive signal is applied, the magnetic field from the coil is stronger than the permanent magnets and can thus move the armature to the opposite position in spite of the permanent magnet’s presence.

Magnetic latching relays must have two coil drive commands, Latch and Unlatch.  Some relays accomplish this with two separate coil windings and two command leads:

Other magnetic latching relays have just two coil wires, but the polarity of the applied DC drive determines whether the command is Latch or Unlatch:

The Latch and Unlatch drive signals are typically meant to only be momentary, you normally would not leave either signal on the coil continuously.

Magnetic latching relays are frequently used in lighting circuits.  One example is the popular General Electric RR7 lighting relay.


Some points to remember about relays:

  • Make sure that the relay coil voltage and AC/DC rating matches the coil drive signal in the application circuit.
  • Some relays are available with a “coil energized” light built in. If any troubleshooting has to be done on the panel, these lights can be very helpful.
  • Some relays are available with a manual override button built in. If any troubleshooting has to be done on the panel, these override buttons can be very helpful.
  • Make sure that the relay contact voltage and current ratings are at least as high as the volts and amps that will be applied by the application circuit. If possible, use contacts rated higher than the volts and amps that will be applied.
  • If you need more poles than are available on one relay, you can use multiple relays with the coils connected in parallel.

“Currently” Playing: Connecting A 4-20 mA Constant-Current Signal To Multiple Loads

One of the most popular and long-lived methods for transmitting analog control signals in the HVAC and industrial control worlds is the “constant-current signal loop.” In this scheme, the value of the current flowing in the circuit is the control variable, rather than any voltages that may appear at different points in the circuit. The constant-current loop is very robust and noise-tolerant and can run for long distances. Even in this high-tech digital age, it remains a very popular analog signaling method.

This article will deal with how to connect multiple loads to a constant-current signal loop. Connecting multiple loads to a constant-current signal loop is totally different than connecting multiple loads to a voltage signal source (which is covered in a previously-published article for those interested).

In the HVAC world, the most common range of current used for constant-current signaling is 4-20 mA (1 mA = 1 milliampere = 0.001 amperes of current). 4 mA represents 0% of the variable’s value and 20 mA represents 100% of the variable’s value. Other current ranges can be used, but today we’ll work exclusively with 4-20 mA current loops.

Current loop load “resistance” or “impedance”

Every load to be connected in the current loop will have an electrical characteristic called “resistance” or “impedance.” The load’s datasheet might use either term. Don’t worry about whether the datasheet says “resistance” or “impedance” we’ll take them to be the same thing. In this article we’ll use the term “impedance.”

Impedance is measured in units called “ohms.” If you don’t know what ohms are, don’t worry. All you have to do is use the numbers in some simple calculations later on.

Basic constant current loop configuration

Before we discuss connecting multiple loads to a constant current loop, let’s get familiar with the basic constant current loop containing a single load:

currently figure 1

On the left we have a constant-current source that is regulating the current to some desired value (the desired value is determined by a calculation in a controller, measured physical value in a sensor, etc.). The constant current leaves the source, flows through the load, and returns to the source.

Simple, right? What could go wrong? As usual, “the devil is in the details.”

Two different styles of current sources

There are two different styles of current sources you should be aware of. We’ll call the first style the “locally powered” current source. It has its own source of power for the 4-20 mA output and the only other item needed to complete the loop is the load:

currently figure 2

You will usually find this type of current source on the output of a powered controller.

We’ll call the second style the “loop powered” or “2-wire” current source. It does not have its own power source built-in, but rather depends on a separate power source (usually 24VDC) wired into the loop:

currently figure 3

You will find this type of current source in many 4-20 mA loop-powered temperature, pressure, and humidity transmitters.

Current sources have a “maximum load impedance” they can drive

Current sources like small values of load impedance, in fact they can drive their current straight into a short circuit (zero ohms load impedance)!

Current sources run into trouble when the load impedance gets too large. When the load impedance gets too large, the current source will not be able to make the higher values of current (near the 20 mA end) flow. The current value will plateau at some value less than the desired value.

How do I know the “maximum load impedance” my current source will support?

For a locally-powered current source, you will need to find the maximum load impedance value on the product’s datasheet.

For a loop-powered current source, you may find a parameter called “maximum load impedance (with 24V supply)” on the datasheet. If you do not, you will have to perform the following steps:

1. Find a parameter called “minimum operating voltage” (or some very similar wording) on the current source datasheet.

2. Calculate maximum load impedance as follows:

Maximum load impedance = (Loop power supply – minimum operating voltage) / 0.020

For example, suppose we are using a 24V loop power supply and we see on the datasheet that the 2-wire current source needs a minimum of 11V to operate. Then we would calculate:

Maximum load impedance = (24V – 11V) / 0.020 = 650 ohms

Now that we know something about how a single-load constant-current loop works, let’s see what kind of complications we get into when trying to drive multiple loads!

Connecting multiple loads to the current loop

Rule #1: All loads must be connected in series around the loop, never in parallel.

Below are sketches of the right way (series) and the wrong way (parallel) to connect loads in a current loop:

currently figure 4

currently figure 5

At first glance this requirement seems simple enough. That’s because we’ve drawn all the loads as 2-wire loads that don’t require power supplies. A not-so-obvious problem may occur if the loads do require power supplies. The current will always take the “path of least resistance” back to the current source whatever that path might be. If one of the loads before the last load has a power connection back to the current source, this connection will be the “path of least resistance” and the 4-20 mA will bypass the rest of the loads in the loop:

currently figure 6

If the powered load is a standalone module, you may be able to use a dedicated “floating” transformer or small DC power supply to power the load and resolve the issue:

currently figure 7


If multiple loads in the series connection are powered, each load would need its own power supply that is floating with respect to all the other supplies. For example, if Load 1 and Load 2 were both powered, Load 1 would need a power supply that is floating with respect to Load 2 and the current source, and Load 2 would need a power supply that is floating with respect to Load 1 and the current source. It can get messy quickly if there are several powered loads in the loop!

Consider also that If the powered load is a controller input and the controller is serving other inputs and outputs, it wouldn’t be practical to “float” the entire controller subsystem’s power connections.

In cases where a floating power supply for each load is not practical, it’s time to employ a signal isolator module such as the DT-13E from Kele:

currently figure 8


The DT-13E signal isolator has its inputs isolated from its outputs, and both inputs and outputs are isolated from the power terminals. So with the DT-13E installed, it just doesn’t matter where Load 2’s power source comes from, it will not corrupt the 4-20 mA current in the loop.

Rule #2: The sum of all the load impedances must be less than the maximum load Impedance the current source can support.

When the mA loads are wired in series (as they must be) the total load impedance seen by the current source is the sum of the individual load impedances. So if each of the three loads in the drawing below is 250 ohms, the total load impedance seen by the current source would be 250 + 250 + 250 = 750 ohms:

currently figure 9


Suppose your current source datasheet shows that it can only drive 650 ohms maximum impedance? Houston, we have a problem…

What can you do if your total load impedance adds up to more ohms than the current source can handle? This is another way that the DT-13E signal isolator can sometimes help. Many HVAC 4-20 mA loads have impedances of 250 to 500 ohms, but the DT-13E mA input impedance is only 125 ohms. So by isolating one or more loads using DT-13Es, not only do you remove grounding concerns but you may also lower your total loop impedance to a value the current source can handle:

currently figure 10


Now the current source only sees 250+125+250 = 625 ohms which is lower than its maximum load impedance, and Rule #2 is satisfied.


Multiple loads inserted in a 4-20 mA current loop must be connected in series. If the series-connected loads are powered devices, “sneak paths” between the load power supply connections and the current source may misdirect the 4-20 mA so that not all the loads receive the signal. Isolating the series-connected load with a DT-13E or similar signal isolator will solve this problem.

The sum of the series-connected load impedances may be too high for the current source to handle. The DT-13E signal isolator’s impedance of 125 ohms is less than that of many common HVAC loads. Isolating a load with the DT-13E may lower the total loop impedance to an acceptable value.

Connecting A Voltage-Output Signal Source To Multiple Loads

The Tech Support crew here at Kele frequently gets quizzed by customers who have a voltage-output signal source which must drive multiple voltage-input loads. Requests for “voltage signal replicators” come in frequently. Sometimes extra hardware is needed, but sometimes it isn’t! So we thought it might be a good idea to write an article addressing this topic. Here we go…

*** A note about the terms “resistance” and “impedance”

When looking at datasheets for HVAC control modules, you may see the terms “resistance” or “impedance” used in describing signal input/output loading characteristics.  Strictly speaking, “resistance” and “impedance” are different entities (impedance = resistance + reactance) but for a slow-moving control signal which is what we usually have in the HVAC world, the two terms can be used interchangeably.  So don’t worry about whether the datasheet says “resistance” or “impedance” we’ll take them to be the same thing.  In this article we’ll use the term “impedance” because it’s one letter shorter than “resistance” and we’ll save keystrokes! J

Impedance is measured in units called “ohms.”  If you don’t know what ohms are, don’t worry.  All you have to do is poke the numbers into some simple calculations later on.  You may see a suffix “k” or “K” appended after the ohms value on a datasheet.  This is important to note!  The “k” or “K” means “times 1000” so an impedance of “100K” means 100,000 ohms and that’s the number you would put into your calculations.

On with the show…

The first thing we must do is answer a few questions about the signal source and loads:

  1. Do the Signal Commons on the different loads have to be isolated from each other?
  2. Does the Signal Common on the voltage source have to be isolated from all loads?
  3. What is the “minimum load impedance” the signal source can drive (datasheet item)? (Alternately, this could be specified as “maximum output current”).
  4. What is the “input impedance” of each of the voltage-input loads (datasheet item)?

The first two questions are sometimes difficult to answer, and are application-dependent.  If you cannot find concrete answers, you can always assume that every device must have isolated signals.  This approach will always work, but it will also cost more money as signal isolator hardware is required.

With regards to question #3, the signal source datasheet may specify a “maximum output current” value instead of “minimum load impedance.”  The “maximum output current” will be expressed in units of mA (milliamps).  No worries, we can calculate what we need by this equation:

Minimum load impedance = Largest output voltage required/maximum output current

As an example, suppose the signal range of interest is 0-10V and the maximum output current available from the signal source is 2 mA (0.002 amps):

Minimum load impedance = 10V / 0.002 amps = 5000 ohms

Calculating “equivalent load impedance”

When connecting voltage loads in parallel, you’ll need to calculate the “equivalent load impedance” of all the loads connected together.  There are a couple of ways to do this:

If all load impedances are the same—

Equivalent load impedance = impedance of one load / number of loads

If the load impedances are different –

Equivalent load impedance = 1 / (1/Load1 impedance + 1/Load2 impedance + …)

The second calculation is most easily done using the 1/X key on your calculator.  Just do a 1/X calculation for each load impedance and add the terms together.  Then do a final 1/X calculation on the first result.  Here’s an example—

Load 1 = 10,000 ohms (10K)

Load 2 = 50,000 ohms (50K)

Load 3 = 100,000 ohms (100K)

Equivalent load impedance = 1 / (1/10000 + 1/50000 + 1/100000)  =  7,692 ohms

The two rules that must be satisfied

There are basically two rules that must be satisfied when driving multiple loads from a voltage source:

  1. If electrical isolation is required, you must add a hardware signal isolator module to each device to be isolated.
  2. At each voltage output-to-input interface, the equivalent load impedance of the loads connected together must be higher than the minimum load impedance of the signal source

Case 1:  It’s OK for all devices to have their Signal Commons tied together 

This case can be done using direct wiring (NO extra hardware) if the equivalent load impedance (calculated above) is greater than the signal source’s minimum load impedance:

direct connect method

That’s great UNLESS… your equivalent load impedance is smaller than the voltage source’s minimum load impedance.  Then what are you going to do?

When the equivalent load impedance is too small for the signal source to drive directly, you will need to insert a “signal booster” device of some sort between the original signal source and the loads.  The signal booster doesn’t need to boost the voltage (you typically want the same voltage out that’s coming in).  It needs to boost the available current so there is enough to drive all the loads connected together.

The Kele UAT is a good device to use as a signal booster in this application.  A look at the datasheet shows that its voltage output can drive up to 20 mA of current.  Let’s assume that the signal range of interest is 0-10V so the maximum voltage we ever need to output is 10V.  Then the minimum load resistance supported by the UAT would be:

UAT minimum load resistance = 10V / 0.020 amps = 500 ohms.

This is plenty of drive power for most parallel-connected voltage loads.

Now when we insert a signal booster device, we must insure that the input impedance of the booster itself is not too low for the original signal source.  We see that the UAT input impedance on the 0-10.9V range is 156K ohms (156,000 ohms).  If your UAT datasheet shows 156 ohms input impedance, don’t panic!  It’s really 156K, some datasheets have a typo.  Our apologies.  So the UAT input impedance of 156K is far higher than the signal source’s minimum load impedance of 5K, and it’s all good:

signal booster method

Now you should understand that the UAT does not provide signal isolation as it has one Common terminal for both signal input and output.  If you require isolation between devices, read on…

Case 2: Load Signal Commons can be tied together, but voltage source needs isolation

In this case you will need a signal isolator device such as the Kele DT-13E.  On the DT-13E, the input signal terminals are completely isolated from the signal output terminals and both of those terminal sets are completely isolated from the power terminals.

We look at the DT-13E datasheet and see that the input impedance on the 10V input range is 13.3K.  This is higher than the signal source’s minimum load impedance of 5K, so there is no problem driving the DT-13E input from the original signal source.

Looking at the DT-13E voltage output, we see that the maximum current available is 6 mA.  For a 0-10V signal, the max voltage we need to drive out is 10V so the DT-13E’s minimum load impedance will be:

DT-13E minimum load impedance = 10V / 0.006 amps = 1667 ohms.

If we have the same three 10K loads paralleled as in the previous example (3333 ohms equivalent load) then the DT-13E has enough drive and this setup should work just fine:

isolated signal source

Case 3: Load Signal Commons must be isolated from each other

In this situation, each load will need its own signal isolator module like the DT-13E.  The most obvious setup to use is shown in the next figure.  When we do the load calculations, however, we find that three DT-13E inputs in parallel will present an equivalent load impedance of 13.3K / 3 = 4433 ohms.  Oops, that is lower than the minimum load impedance of 5000 ohms specified for the signal source:

individually isolated loads

How do we fix this problem?  Here are some possibilities:

  1. Use a different signal source with more drive capability.
  2. Use different signal isolators with higher input impedance.
  3. Try rearranging the wiring using the devices that we have.

Let’s see what we can do with option #3.  Realizing that inputs and outputs are isolated on the DT-13E modules, it’s perfectly legal to take one of the DT-13E inputs and drive it from another DT-13E’s output.  Let’s check the math to see if this would work.

With only two DT-13E inputs paralleled on the signal source, the equivalent output impedance would be 13.3K / 2 = 6650 ohms.  This is higher than the signal source’s minimum load requirement of 5000 ohms, so that works.

With the new arrangement, one DT-13E output will drive a parallel combination of the original 10K load and one DT-13E input (13.3K).  The equivalent load impedance will be:

Equivalent load impedance = 1 / (1/10000 + 1/13300) = 5708 ohms.

This is higher than the DT-13E output’s minimum load impedance of 1667 ohms so that works too!  And this is the final configuration:

idividually isolated 2


By following the two interface rules discussed above, multiple voltage-input loads may be successfully driven by a common voltage-output signal source.  The math is simple algebra and can be done on any calculator.  Happy interfacing!

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:



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!


Lights Out?

As many of you may be aware there was a very highly important football contest televised this past Sunday.  I will not name the contest because I do not wish to draw the attention of the sanctioning body or it’s league offices.  During this unnamed football contest there was a strange occurrence, an anomaly, a freaky thing: the power went out.  There were no lights on one side of the stadium.  It brought the game to a halt and many, myself included, stayed tuned to the broadcast.  I read the following day that (however they estimate these things) not a single viewer was lost.  Hmm….

After thinking about it for a while my guess is that many like me stayed glued to the broadcast because in America today power outages are a bit of a novelty.  The power was restored 34 minutes after it went out and the game went on.  It was a good one too.  But I still had to wonder about the power outage and what happened.  I’m sure that everything was checked and double checked – but it still happened.  Chances are that if you are reading this you have most likely seen something like in your professional career.  Have you been part of a project that was checked, double checked, and something still went wrong?  Please share: either reply here or send to  I might even pick a winner or two and send out some prizes. 



What is next, endicator?

In my last blog entry I raved about the endicator power monitor and how it will make my life easier and that is all true.  What the designers have done, unbeknownst to them, is they have made life easier for future DR too.  I’ll try to explain.

Chances are if you are reading this you are either a building automation professional or my Mom (love you Pumpkin) so I can be a little snarky with this part.  There is a commercial for a home security company that shows a home being blanketed in a warm blue blanket of 0s and 1s.  What could be more comforting than knowing your worldly possessions are protected by technology that is older than my father (no offense Pop)?  Don’t get me wrong, digital switches are a very effective and highly efficient way of detecting abnormalities and initiating alarms, but it is hardly innovative.

Innovation is used to advertise almost anything that exists.  I don’t know much about marketing but I’m sure that the wheel V2.0 was depicted on a cave drawing as being so advanced that if you were still using wheel V1.9 or less you were behind the times.  To get the benefits of wheel V2.0 you had to upgrade.  The axle was not available as an upgrade for older versions of the wheel.

This is where the endicator power meter shines and what makes it, in my humble opinion, the most exciting product Kele has ever introduced.  The endicator is ready for the next big thing you need to do with a power monitor.  The endicator can be ordered with or without communications cards.  Units can come from the factory with communication cards for BACnet, LON, Modbus, and N2 OR they can be added/changed in the field. Firmware updates can also be done in the field.   This means that as your customer’s needs change you can upgrade them – and it doesn’t stop there.  I don’t know what the next big thing will be.  It might be adding internet capability or (gasp) cellular.  Being able to upgrade in the future is truly innovative.

I guess to sum it up; the endicator design team has given me a gift with this product.  There is not much in life that is easier than being able to say “yes” to a customer and I’ll be able to say that a lot with the endicator.  This is where you come in.  Where do you see power monitoring going?  I’d like to know.