Introduction to Industrial Automation

to have the same result (b).

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  Introduction to Industrial Automation

programming error. The solution to this problem is to look at what specifically the circuit o

the conventional circuit. This modification of the logic on the functionality is presented i

Number of LAD instructions per rung. Every manufacturer of a PLC introduces a limit to the 

number of LAD instructions that may exist in a program rung. Additionally, limits are being 

introduced on the total number of parallel sub-branches that can be present in the same program 

rung, as shown i

per program rung, as adopted by Westinghouse and Square D of the Schneider Electric group, 

9 instructions and 8 sub-branches of General Electric, and 11 instructions and 7 sub-branches 

of Allen-Bradley. In each rung of a LAD program, some manufacturers allow only one digital 

output, such as the above companies, while others allow more than one output, such as Modicon, 

which allows up to 7 digital outputs per program rung. Restrictions are also present in the number 

of embedded loops in a program rung, a problem that can be solved by doubling the number of 

contacts’ instructions. In the Siemens Step7© software initial version, there were 7 instructions 

per rung for the LAD and FBD languages, 2000 for the Boolean language, as well as 999 as a 

maximum number of branches and an almost unlimited number of sub-branches. In today’s TIA 

portal software environment, there is no such limitation for real applications. In reality, the limi-

tation on the number of branches is set by the size of the memory of the PLC, something that is 

valid for all PLC manufacturers.

The order of rungs. At this point, the issue of rung order in a LAD program will be examined, 

e.g., whether the order in which the rungs are inserted influences the final result of the program 

execution. It is obvious that in a conventional automation circuit, there is no such issue of the 

branches’ order, since all branches are under the same electrical voltage and operate at the same 

time. Subsequently the LAD program o
input I2.0 and the auxiliary bit M0.0 are initially deactivated and remain in this state without a 

change. The execution of the program will result in the permanent activation of the Q1.0 output 

and the simple activation of the auxiliary bit M0.0. The program o
as i
becomes the second (first) rung in the program o
from the execution of the program o
Number of instructions per branch

Parallel sub-branches

Program rung or network