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Programmable Logic Controllers, 5th Edition by William Bolton

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Chapter 3. Digital Systems
Digital systems work with inputs, which are essentially just off/on signals, with the two signal levels represented by 0 and 1. These are termed binary digits. The number system used for everyday calculations is the denary or decimal system. This is based on the use of the 10 digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. With a number represented by this system, the digit position in the number indicates the weight attached to each digit, the weight increasing by a factor of 10 as we proceed from right to left. Hence we have:
103102101100
ThousandsHundredsTensUnits
Denary1000100101
Thus if we have the denary number 1234, we have 1 with a place value of 103, 2 with a place value of 102, 3 with a place value of 101, and 4 with a place ...

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