Introduction to Industrial Automation

Figure A.9  Conversion of a hexadecimal number to a decimal.

---

Appendix A 

◾

 

417

When receiving or reading the transmitted data word, the total number of units (1) are checked 

if they are even or odd, and thus it is determined whether there was an error in data transmission. 

When establishing a communication connection between two PLCs, the user is usually invited to 

select an even or odd parity in the corresponding software environment.

A.7   ASCII  Code

A digital device, such as a computer or a PLC, should have the ability to process anything, except 

for numbers and letters, since this functionality is needed for enabling the programming of the 

PLC with text commands (e.g., an instruction AND). As in the case of numbers being represented 

based on an arithmetic system in a binary form that can be processed by a PLC, letters should 

also be represented in binary format. For this purpose, the coding of the letters of the alphabet 

and other symbols used to write text has been provided by the introduction of the ASCII code 

(American Standard Code for Information Language). Based on the ASCII code, 7 bits were ini-

tially used to represent the various letters and symbols. Since the number of letters and symbols 

surpassed the maximum 128 (2

7

), which could cover the 7 bits, an eighth bit was added. A table 

with ASCII codes is not listed here, because it is a well-known issue and they appear in many 

books for someone who would like more details.