Introduction to Industrial Automation

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  Introduction to Industrial Automation

7.4.4   Complementary  Instructions

Load (L) and transfer (T) instructions. With the L and T instructions, we can move various mem-

ory data of the PLC massively from one location to another. The data may be related to inputs, 

outputs, auxiliary bits, or other accessible memory locations. The density of the data movement 

can be a byte, a word, or a double word. The L and T instructions do not depend on the RLO, 

and thus they are executed continuously in each scan cycle. The instruction L can also refer to 

a constant numeric value while it transfers its defined content always to accumulator 1. From 

the other side, the T instruction always transfers the contents of accumulator 1 to the specified 

variable-memory location. The usefulness and utilization of these instructions will be illustrated 

in corresponding examples.

Counting instructions. The CU and CD counting instructions only concern the operation of 

the counters. When executing the CU and CD instructions, the content of the counter is increased 

or decreased by one, only if the RLO pulse rises. Increasing or decreasing the content of a counter 

is associated with the RLO pulse rise for a very simple reason, which will be considered in the 

following example. Let us assume that a conveyor belt transports bottles in a production process, 

while a photocell is placed on the conveyor belt for counting the bottles. The photocell is a digital 

input to the PLC, while there is a running program whose CU instruction refers to the input of 

the photocell. Whenever a bottle is passed in front of the photocell, the content of the counter is 

increased by one. Suppose now that for some reason the conveyor belt stops temporarily, and there 

is a bottle in front of the photocell that keeps it activated. For as long as the bottle remains in front 

of the photocell, the program will be executed many times. If there was no pulse raise condition 

in the CU instruction, then the PLC would list a lot of virtual bottles from the encountered stop 

situation, while in reality, there is only one (stopped) bottle in front of the photocell. Instead, with 

the RLO pulse raise condition, the stopped bottle should continue its movement for some time, 

turn off the photocell, and then be switched on again by the next bottle, to count another passing 

bottle.

In Table 7.1, the counting instruction symbols CU or CD, in the case of FBD and LAD 

programming languages, are simplified. Figure 7.11 presents the actual graphical form of the 

CU symbol or block in the LAD language (Step7—Siemens), where it accepts six logical con-

nections or definitions. The input to the CU point of the block is the counting input. This input 

can be either connected or denoted to the counter C

xx

 where xx is the number of the counter. 

In the inputs of S and R, proper input signals will be declared or connected for performing the 

counter’s set and reset instructions. However, as mentioned above with the set instruction, the 

counter is placed at a specific numerical value, and the latter should be declared somewhere in 

the program or in a memory position. This operation can be achieved with the proper connec-

tion to the PV input. To the CV output, the current value of the counter is provided by the 

CU

S

PV

C

xx

R

CU

Q

CV

CU

S

PV

R

CU

Q

CV

CV = current value of counter

Q = counter output (0 or 1)

CU = counting input

S = setting counter to an arithmetic value

PV = arithmetic value used by set instruction

R = resetting counter to zero value

xx = number of counter 

CU

S

PV

R

CU

Q

CV