High Performance HVAC DDC Programming Logic and Symbols

HVAC Control DDC Logic Quickpage Jump links:

DDC Programming Logic in HVAC Control

Binary and Analog Inputs and Outputs

The And Gate

The Or Gate

The Not Gate

The "If" Logic

Low Limit Analog Logic

High Limit Analog Logic

Delay on Make Logic

Delay on Break Logic

PID Logic Block

HVAC DDC Programming Logic and Symbols Page

Basic HVAC Control DDC Program Logic 101

DDC programming in HVAC Control can be rather complex especially to those who don't understand software or programming. Some DDC software and hardware manufacturers utilize symbols to build their logic programs for HVAC DDC control. Some of these logic blocks are rather complex but others are very simple. These are the basic logic blocks which, when combined, make up the DDC algorithm DDC control program. Everything from And gates to Or gates to Timers blocks to complex mathematical blocks allow a DDC Control programmer or DDC Control engineer to assemble programs in little time simple by arranging a set of logic blocks in a software program and then compiling this set of logic blocks into a basic machine line code and necessary tables which the casual observer and even the engineers never see. When this program is downloaded into the controller one can use the logic page in the DDC software to observe real time information from inputs to outputs to alarms and schedules. To someone skilled with this they can use the logic page in the DDC software to troubleshoot problems. It is simply a tool along with trend charts and graphics to watch the system, collect data, and make changes as necessary or to use to find a problem in the program or in the HVAC or Electrical equipment which the program is controlling. We'll start with simple blocks or symbols and work our way up to the more complex blocks. Understanding of ladder diagrams and basic electrical circuitry is necessary to follow these logic gates and symbols.

Binary Input - Analog Input - Binary Output - Analog Output

• The Binary input logic block will get an input signal of either on or off (0 or 1 in Binary speak) from a device wired to the DDC controller. It will send an appropriate response out through other logic blocks for program processing through other logic blocks and eventually end at an output block. An example of a Binary input logic block would be a Binary input logic block which gets its input from a smoke alarm. If the contacts on the smoke alarm close the Binary input logic block becomes true and sends this through the program for the appropriate response.

• The Analog input logic block with receive an Analog signal from a device wired to the DDC controller. (see DDC Page 2 for an explanation of Analog and Binary signals). An example of an Analog input logic block would be a logic block which gets its input from a pressure transducer. The pressure transducer will send an analog signal to the Analog input logic block and the Analog input logic block will send this information through the program logic.
• The Binary output logic block will get its instructions from other logic blocks in the DDC program and give a binary output response based on the input from the program. An example of a Binary output logic block would be a Binary out logic block assigned to a motor start/stop. When the program called for the motor to energize the Binary output logic block would energize the output and a really would energize the motor starter.
• The Analog output logic block will do the same as the Binary output logic block except the signal coming from the Analog output logic block will be an analog signal. An example of an Analog output logic block would be an Analog output logic block assigned to control a variable frequency drive which works off of an analog signal like 0 to 10 volts or 4 to 20 milliamps.

And Gate

The And gate is simply a series switch. In the DDC graphic to the left is an And gate and it is the same as the series switch to the right of the And gate. If a single input to the And gate is open the And gate will not allow an output. The input side of the And gate can have as many gates as necessary for proper functioning whatever is being controlled.

Or Gate

The Or gate is simply a circuit that says on or the other or both. If one or the other have an input signal to the Or gate the Or gate will allow an output. For example, you could have a three gate Or gate which would be linked to three specific alarms like the smoke detectors, the freeze stat, and the high static pressure switch. If one or the other tripped then the Or gate would send an output signal to an alarm logic block and an emergency shutdown logic block.

The Not Gate

The Not gate is a simple gate which will change a 0 input to a 1 output or a 1 input to a 0 output. It used for various logic programming to change a binary value from a 0 to a 1 or from a 1 to a 0.

If Less Than and If Greater Than Logic Blocks

This is basic line code programming that goes back to the basics of if, else, then. An example of the usage of these blocks would be if the outside air temperature was less than 65° then it is okay to use the economizer. Of course the program would have other conditions which may be linked through an And Gate logic block but the If Less Than logic block would be one of the conditions to enable the economizer. There are other If logic blocks such as If Equals to and others which can give an analog or a binary output to the program.

Low Limit Analog Logic Block

The Low Limit analog signal block will place a low limit an analog signal coming from the program going to the analog output. An example of this would be a minimum speed for a VFD or a minimum damper position for a modulating damper.

High Limit Analog Logic Block

The High Limit analog signal block will place a high limit an analog signal coming from the program going to the analog output. An example of this would a high analog limit to the analog output of a damper that you only want to open to maximum of 80%.

Delay on Make Logic Block

Every control or HVAC control technician knows the value of a Delay on Make logic block. You can delay fans and compressors using this control logic block and prevent short cycling.

Delay on Break Logic Block

Again, every control or HVAC control technician knows the value of a Delay on Break circuit in HVAC Controls. You can delay a fan shut down or damper closing using this block.

PID Logic Block

Proportional (P), Integral (I), and Derivative (PID) control responses are very complex algorithms. This is the logic block which will help the DDC control program tune the device being controlled using PID tuning calculations and parameters. This logic block allows user input to tune different devices with PID control to prevent overshoot and undershoot. It prevents devices from under responding or over responding to program inputs and allows for smoother operation based on the PID control algorithm.

There are many more logic blocks to be covered. We will update the site with more logic blocks and an example of how some of these blocks would work together in a future update.

DDC Programming Logic Page Two

DDC Related Links

DDC HVAC Control Home 1 | DDC Technical Aspects 2 | DDC HVAC Control VAV's | DDC HVAC Control Economizers | DDC Controls Programming Example | DDC Controls Technician Descriptions | Variable Speed Drives | Energy Recovery Wheels | DDC HVAC Control Logic Page Top

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written by Oleg, April 01, 2010

Hi I appreciate affords of writer of this article.
Wary simple and popular explained basics of automatization and control in HVACR system, on level of hi-tech computerize equipment, which one rite now in everywhere.
I think need to mention about of falls in DDC, cost of it, troubleshooting and etc. (An example Toyota)
I like electronics toys on stage of sensors and monitoring but not a Direct Control.
It’s just my opinion.
Now the question: Why we using mechanical direct expansion valve not an integrated computerize logical gate.

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