(see DDC for an explanation of Analog and Binary signals). The Analog input logic block with receiving an Analog signal from a device wired to the DDC controller.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. An example of a Binary input logic block would be a Binary input logic block that gets its input from a smoke alarm. 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.
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. DDC Programming Logic Symbols - Description of Inputs and Outputs Binary Input – Analog Input – Binary Output – Analog Output Understanding ladder diagrams and basic electrical circuitry is necessary to follow these logic gates and symbols. We’ll start with simple blocks or symbols and work our way up to the more complex blocks. 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. To someone skilled with this, they can use the “logic” page in the DDC software to troubleshoot problems. 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. 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. These are the basic logic blocks that, when combined, make up the DDC algorithm control program. Some of these logic blocks are rather complex but others are very simple. 
Some software and hardware manufacturers utilize symbols to build their logic programs for HVAC DDC control. DDC Programming Logic Symbols 1 - DDC programming in HVAC Control can be rather complex especially to those who don’t understand software or programming.
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