Appendix; Of Bits, Bytes And Words - Siemens SIMATIC S7-200 Manual

A.1

The smallest unit of information in a digital system is known as a "bit". A bit can only have

the states "0" (false or not true) or "1" (true).

A light switch, for example, only has the states "light on" or "light off". The value of the light

switch in answer to the question "Is the light on?" is either true (the light is switched on) or

false (the light is switched off). The light switch thus has an information width of one bit. The

state "light switch on but bulb defective" is ignored in this example.

In a PLC, bits are organized into groups. A group of 8 bits is called a byte. Each bit in the

group is defined exactly by a separate position with its own address. Each bit has a byte

address and bit addresses 0 to 7. A group of 2 bytes is called a word. A group of 4 bytes is

called a double word

The numbers in this logical system are binary numbers and they are counted with the base 2

number system.

A word in a PLC can represent an integer number from -32768 to +32767. The bit with the

value 2

number is negative).

Getting Started - Beginners

Training Documents, 07/2007, A5E01031470B

is used to denote a negative number (when position 2

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A

has the value "1" the

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