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Short on I/O? Try This Out!

Tricks of the trade, time-tested and industry-accepted

Steven R. Calabrese

Steven R. Calabrese
Automated Logic Chicago

Read Steve's previous AutomatedBuildings columns:
December - Wiring For Combustion Air
November - Terms & Terminologies – Avoiding Confusion 
October - Electric Duct Heaters – Application & Control 
September - Pressure Transmitters – Selection & Placement
August - Rooftop Unit Economizer – Operation & Control 
July - RTU Operation Via Conventional & Digital Controls
June - Interlocking of AHU Safety Devices

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Tricks of the trade, time-tested and industry-accepted

Like most of us in our chosen professions, we in the building automation industry, particularly those of us who design, install, and commission DDC systems, sometimes find ourselves in mini-dilemmas. How we go about solving these dilemmas is part of what makes our jobs challenging, to say the least. It always helps to develop a good toolbox of design, installation, and programming techniques that we can go to in a pinch, and pull out a solution to a problem. One of these mini-crises that we encounter in our day-to-day is a shortage of input/output points on a controller, where we’re maxed out from the get-go, and we’re trying to add one more sensor or control one more stage of heating that hadn’t originally been accounted for. Next time you find yourself in one of these situations, consider the following tricks to help you resolve your issue.

Bear in mind the following abbreviations, used gratuitously throughout the rest of the article. DI stands for Digital Input (discreet, two-state value), and AI stands for Analog Input (continuous value). DO stands for Digital Output (discreet, two-state output), and AO stands for Analog Output (continuous signal output). We’ll look at some applications on the input side of things first, and then we’ll tackle some output tricks.

Multiplex a single AI to get multiple DIs – Some third party manufacturers of building automation system products and peripherals have come up with packaged electronic interface devices that can turn an analog input into multiple digital inputs. Sounds pretty fancy, but the reality is that these devices are nothing more than resistor banks on a printed circuit board. A typical multiplexer, as they are often called, may accept up to four (or more) digital inputs. On the output side of the device is a resistance whose value is dependent upon which and how many DIs are closed. So for each and every possible combination of closed DIs is a unique value that presents itself at the output of the device, which is then wired to an AI. The programming must account for each unique value, so that the controller can accurately process the digital input information, a one-time task that can be tedious at best.

For you do-it-yourself electronics buffs, you can build a multiplexer on the fly using standard ¼ watt resistors. A two-input resistor-based multiplexer requires three uniquely valued resistors. Sketch out the resistor/contact closure network on a piece of paper (cardboard box, napkin, what have you…). Design it so that, with neither input closed, the circuit still reports a resistance value to the AI. A two-input multiplexer has four possible output states. Each output state must have a unique resistance value, for the controller to be able to discern between the input combinations. Check your circuit with an ohmmeter prior to connecting it to the AI. Record the output resistance value for each of the four input combinations. Program the controller to accommodate your setup, and viola, you have magically converted an AI to two DIs!

Tip of the Month:
While most of these tools described here are helpful for when you’re in a pinch and need to squeeze one more point out of an existing or pre-designed control panel, prudent design practices usually dictate that controllers be furnished with more than enough inputs and outputs for the initial application, so that there are spares available when the inevitable added point comes up down the road. Still, the above tricks and tidbits can prove invaluable when you find yourself outta spares, outta room, outta time, and outta money!

Feed two temperature sensors into one AI – You can do this by using a DO to toggle between the two sensors. Have the DO control an outboard relay. Wire one lead of each sensor together, and into one side of the analog input. Wire the other lead of each sensor to the normally open and normally closed contact of the relay, respectively. And wire the common of the relay to the other side of the analog input. When the relay is de-energized, the AI reads the value from the sensor connected to the NC contact. And when the relay is energized (via the DO), the AI reads the value from the sensor connected to the NO contact.

The programming must be set up such that the state of the DO is linked with the status of the AI. Care should be exercised when trying to apply this with sensors whose values are used as a part of a sequence of operation. The ideal fit for this trick is when the sensors are required for monitoring purposes only, and simply show up as values on a graphic. That way the relay need only be cycled on a periodic basis, and will be less prone to premature failure. Also, the value of each sensor displayed on the graphic can be set up to lock on to the last known good value, so as to display a meaningful number when it’s the other sensor’s turn to report to the AI (make sense?).

Average multiple temperature sensors into one AI When you have a requirement for taking the average of multiple sensor values, you don’t necessarily need to dedicate an AI to each sensor. To average four thermistor sensors into a single input, first split the four sensors into two pairs. Wire the two sensors of each pair in series. Now wire the two sensor pairs in parallel, and into the input. What you have is a series-parallel averaging network of variable temperature-controlled resistors. The value read at the input is the average of the sensors. You don’t even need to perform any special programming to derive the average. It’s already done for you!

This can be done with sensor quantities other than four, however it becomes a bit more complicated. The ideal application for this technique is temperature averaging of several areas within the same zone, say for instance a large open office area. The office area is a single zone, served by a single air handler, yet is large enough that there may be differences in temperature throughout the space. Remote sensors can be mounted in “strategic locations” throughout the space, all wired back to the same input at the air handler’s main controller. You wouldn’t be able to ascertain the value of each individual sensor, but you would know the average, and that just may be good enough!

Use a DO for proportional control – A relatively obscure method of proportional control, Pulse Width Modulation (PWM) uses a time-based pulse from a digital output to modulate a PWM-enabled end device. Manufacturers have taken this a step further by creating a module that can accept a PWM type signal and output an analog (0-10 volts DC) control signal that is proportional to the PWM signal. A typical time base for PWM to analog conversion is 10 seconds. The digital output is programmed to go to a “high state” for a certain duration of every 10-second time period. The output of the module converts the pulse within the time period to an analog signal that is proportional to the duration of the pulse. To control a proportional damper actuator, send a pulse with a duration of zero to the module, to result in a 0-volt DC signal sent to the actuator. Send a pulse with a duration of 10 seconds to the module, to result in a 10-volt DC signal sent to the actuator. For any continuous voltage value to the actuator, simply send the corresponding pulse to the module, and let the module do it’s thing. Quite the alternative method of proportionally positioning a modulating end device!

CatNet Systems Use an AO as a DO – To get a contact closure output from an analog output, simply use what is called (for lack of a better term) an analog relay. An analog output is typically a voltage type output, with a voltage range of 0-5 or 0-10 volts DC. Several manufacturers offer what in reality is nothing more than a DC relay with a small coil “pull-in” voltage and small amp draw. An analog output off a controller, when forced to its maximum output value, can typically drive one of these DC relays, provided that the current draw of the relay coil does not exceed the maximum DC output current of the AO. The programming can accommodate the required sequence of operation with simple if-then conditional logic. For example, if condition is true, then set output value to 0 volts (for relay to be de-energized). And if condition is false, then set output value to max volts (to energize the relay).

Use an AO to sequence multiple DOs – Building upon the last trick, you can get multiple sequential contact closure outputs from a single analog output. Individual DC relays, each with a different coil “pull-in” voltage, can be all be fed from the same AO. The contacts of these relays can then be used to stage an electric duct heater, for example. In practice you’ll be more inclined to purchase a ready-made sequencing module to perform this task. The module may have up to four (or more) independent output contacts. Each contact can be adjusted to close at a certain input voltage. Feeding a four-stage sequencer with a 0-10 volt signal from the AO, the contacts may be adjusted so that the first one closes at 2.5 volts, the second at 5 volts, the third at 7.5 volts, and the fourth at 10 volts. Sequencer modules are commonplace these days, and are often a standard feature on equipment comprised with multiple staging.

About the Author

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS).  You can visit his website at www.pcs-engineering.com.

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.


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