Introduction to Industrial Automation

6.8.3   Addressing I/Os and Other Internal Elements

Every digital input or output of a PLC is characterized by a unique name that will be utilized dur-

ing the automation logic programming. This name is nothing else but the address of the digital 

I/O that expresses the corresponding position of the I/O between others. Thus, every digital I/O 

has an address that defines its position in the specific I/O module, as well as its position in the I/O 

mapping memory. The type of the addresses that are provided to the I/Os of a PLC varies with 

each manufacturer, since until now there has been no standardized way in addressing them. The 

same unstandardized addressing situation is found in the other internal components of a PLC, 

such as the auxiliary bits, timers, and counters.

Every PLC manufacturer uses its own addressing system, which may be based on the decimal 

or octal system, use numerical or alphanumeric data, etc., while subsequently, reference will be 

provided to the most common addressing methods. The decimal I/O addressing system uses num-

bers 1–8 for the first group, 9–16 for the second, 17–24 for the third, and so on. The octal system 

uses only eight digits from 0–7, that is, numbers 0–7 for the first group, 10–17 for the second, 

20–27 for third, and so on. The distinction between inputs and outputs is done by adding a letter, 

such as X, I, or E for inputs, with I as the predominant one; and Y, O, or Q for outputs, with Q 

as the predominant one. Other addressing systems do not use letters but only five- or four-digit 

numbers. For example, each interconnection point with a code between 0000 and 0999 may be 

an output, while with a code between 1000 and 1999 may be an input.

The digital I/Os of a PLC, with a byte memory structure, are also grouped by octets. In this 

case, a digital I/O is uniquely characterized by the octal bit corresponding to it, as shown in 

Figure 6.33. For an octal system of addressing, the first digit represents the number of the byte and 

the second digit is the number of the bit into the byte. Thus, digital input I 6.3 corresponds to the 

bit 3 of the byte 6, with respect to the input mapping memory and, at the same time, corresponds to 

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Basic Operating Principles of PLCs 

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the fourth input of the sixth input group, with respect to the input module. In addressing systems 

with a five-digit number (e.g., 110 00), the first digit determines whether the connection point 

is an input 1 or an output 0, the second digit expresses the number of the base with various I/O 

modules, and the third digit is the number of the I/O module. Very small PLCs, with 10–20 I/Os, 

follow the decimal addressing system. In some large PLCs, the addressing system may be “flex-

ible”, i.e., the I/O addresses are set by the user via DIP switches or by programing the addressing 

data into an EEPROM memory. In the case of non-flexible addressing systems, each I/O module 

and every corresponding I/O has a fixed address. The procedure of determining the specific I/Os 

for the PLC where the corresponding I/O devices will be connected, known as I/O address assign-

ment, is an important task that may or may not simplify the automation programming task, the 

diagnosis of errors, and dealing with general logical programming problems.

The other internal elements of a PLC are addressed in an analogous or even simpler way. The 

auxiliary bits or logic coils and the corresponding memory locations are encoded with the letter 

M or F (from the English terms “memory bit” or “flag”) followed by a two- or three-digit number 

with decimal or octal numbering. Also, with “Txxx” the memory locations for the timers are 

defined, with “Cxxx” the memory locations for the counters, and with “Sxxx” usually the gen-

eral content memory locations, where “xxx” is an integer number, the maximum of which varies 

according to the size of the PLC.