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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.
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Understanding Numbering Systems



Understanding numbering systems will help you to understand various ways in which data can be monitored in the ControlLogix processor. For example, if you are reading the value of a limit switch, you would want to change the numbering system (style) to binary. If you were viewing data from an analog module, you would want to set the style for decimal. The PLC-5 modules are numbered in an Octal addressing scheme, so if you read data from a PLC-5 module, you may want to set the style to Octal. Some devices such as LED displays are wired in Hex/BCD. The two most common styles for most plants will be Binary and Decimal.


  1. Binary – Binary is a base 2 numbering system. You only have two numbers available in Binary, 0 and 1 for any position (ones, tens, hundreds, etc...). Binary is the most common style for discrete I/O such as limit switches, pushbuttons, solenoids, and motor starters.

  2. Decimal -- Decimal is a base 10 numbering system. Only 10 numbers exist in the Decimal numbering scheme (0 to 9) for any given position. The Decimal style is used most often when displaying analog data, such as a pressure or temperature.

  3. Octal – Octal is a base 8 numbering system. In Octal, 8 numbers are available (0-7) for use in any position. In older PLC's such as the PLC-2, and the PLC-5, The I/O modules were numbered in the Octal addressing scheme. The Octal style can be used when connecting to one of these older modules.

  4. Hexadecimal/Binary Coded Decimal – Hex/BCD is a base 16 numbering system. 16 numbers are available for any digital position (0 to 9 then A to F). Devices such as LED displays and thumb wheels can be BCD Devices.



On the following chart, you will write down the decimal numbers 0 to 15, and the numeric conversions for each of these numbering systems. This chart will help you understand topics covered later in the course such as masking.