Q boxes have a binary output named “Q” which can be linked further. Q boxes are used to represent memory functions, edge evaluations, and timer and counter functions (Fig. 7.22).
With Q boxes, the first binary input (and in certain cases the associated parameter) must be connected, connection of the other inputs and outputs is optional. The binary inputs of Q boxes cannot be directly connected to the left-hand power rail.
7.4.1 Arrangement of Q boxes in the ladder logic When using Q boxes as program elements, you can:
b Program one single box per network, either within the current path or as its termination
Fig. 7.21 Processing a timer function with coils
b Arrange boxes in series by connecting the Q output of one box to a binary input of the following box, and
b Position boxes following T branches and in branches which commence on the left-hand power rail
The circuits shown in Fig. 7.23 for the positioning of Q boxes use the memory box with two binary inputs as an example. This enables the possible positioning of all Q boxes to be shown.
7.4.2 Memory boxes in the ladder logic
There are two versions of the memory function as box: as SR box (reset dominant) and as RS box (set dominant). In addition to the difference in the function name, the two boxes also differ in the positioning of the set and reset inputs.
The binary tag named above the memory box is set when the set input has signal state “1” and the reset input signal state “0”. The binary tag is reset when “1” is present at the reset input and “0” at the set input. Signal state “0” at both inputs has no influence on memory functions. If signal state “1” is present simultaneously at both inputs, the two memory functions respond differently: the SR memory func- tion is reset, the RS memory function is set.
If both tags “Contact 1” and “Contact 2” in Fig. 7.24 have the same signal state “1”,
“Memory 1” is reset (box input R1 is dominant). If both “Contact 3” and “Contact 4”
have the same signal state “1”, “Memory 2” is set (box input S1 is dominant).
Fig. 7.22 Overview of Q boxes available with LAD
RS memory box SR memory box
Evaluation with falling edge Evaluation with
rising edge
Timer functions TP, TON, TOF and TONR Counter functions CTU, CTD, CTUD Represented as local instance or with own data block
Binary variable Binary variable
SR RS
S R
R1 S1
Q Q
Edge trigger flag Edge trigger flag
P_TRIG N_TRIG
CLK Q CLK Q
Function Function
#Instance
IN IN
...
...
...
...
Q Q
...
...
...
...
“Instance”
Boxes with Q outputs
Fig. 7.23 Positioning of Q boxes using example of SR memory function
Fig. 7.24 Memory boxes SR and RS
7.4.3 Edge evaluation of current flow
The edge evaluation with Q boxes registers a change in the current flow prior to the box. If the signal state changes from “0” to “1” (rising edge) at the CLK input of the P_TRIG box, signal state “1” is present at the Q output for the duration of one pro- gram cycle. If the result of the logic operation changes from “1” to “0” (falling edge) at the CLK input of the N_TRIG box, the Q output is activated for the duration of one program cycle.
In Fig. 7.25, the tag “Memory 3” is set at the moment when both tags “Contact 5”
and “Contact 6” have the signal state “1”. “Memory 3” is reset at the moment when both tags “Contact 7” and “Contact 0” have the signal state “1”.
The edge boxes may only be positioned within a current path.
7.4.4 Example of binary scaler in the ladder logic A binary scaler has one input and one
output. If the signal at the input of the binary scaler changes its state, e.g. from
“0” to “1”, the output also changes its sig- nal state. This (new) signal state is then retained until the next change, which is
positive in our example. Only then does the signal state of the output change again.
Half of the input frequency is then present at the binary scaler's output.
There are various methods for solving this task, two of which are shown below.
The first solution uses memory functions (Fig. 7.26). If the tag “Input 1” has the signal state “1”, the tag “Output 1” is then set (“Memory 1” is still reset). If the sig- nal state at “Input 1” changes to “0”, “Memory 1” is also set (“Output 1” is then “1”).
If “Input 1” is “1” the next time around, “Output 1” is reset again (“Memory 1” is now “1”). If “Input 1” is “0” again, “Memory 1” is reset (since “Output 1” is now also reset). The “basic state” has now been reached again following two input pulses and one output pulse.
Fig. 7.25 Example of Q boxes for edge evaluation
Input
Output
The second solution uses the latching principle common to circuit diagrams (Network 4). The operating principle is the same as with the first solution, but the reset condition – as usual with latching – is “zero-active”.
7.4.5 Controlling IEC timer functions in the ladder logic with Q boxes You can use the timer functions to implement timing processes in the program such as waiting and monitoring times, measurement of a time interval, or the generation of pulses. An IEC timer function can be started with two different program elements: a coil or a Q box (see Chapter 7.3.6 “Starting IEC timer functions in the ladder logic with coils” on page 225). Both variants are equally useful. A de- tailed description of the timer functions is provided in Chapter 10.4 “Time func- tions” on page 344.
A timer function can be started with one of the four behavior patterns TP, TON, TOF, and TONR. A timer function requires internal data for each application. You can specify where this data is to be saved when programming: For the Single instance entry in its own data block with the data type IEC_TIMER and for the Multi-instance entry in the instance data block of the calling function block with a data type that depends on the behavior of the timer function (TP_TIME, TON_TIME, TOF_TIME, Fig. 7.26 Examples of binary scalers in ladder logic
TONR_TIME). You address a timer function with the name of the instance data – data block or local data.
The top timer function “Timer 1” in Fig. 7.27 saves its data as a local instance with the name #“Timer 1” in the calling function block. This function is started with
“Contact 1” and “Duration 1”. The tag “Coil 1” has the signal state “1” for as long as defined by the tag “Duration 1”.
The bottom timer function “Timer 2” saves its data in a separate data block
“Timer 2”. This function is started with “Contact 2” and “Duration 2”. After expiry of the duration, the tag “Coil 2” has signal state “1”.
The name of the local instance (#“Timer 1”) and the name of the data block (“Timer 2”) address the respective timer functions. Component Q of the data structure supplies the status of the timer function.
7.4.6 Controlling IEC counter functions in the ladder logic with Q boxes You can use the IEC counter functions to execute counting tasks directly using the control processor. The counter functions can count up and down; the numerical range depends on the data type of the preset value. The data types USINT, UINT, UDINT, SINT, INT and DINT are available.
The counting frequency of the counter functions depends on the execution time of the user program. In order to count, the CPU must recognize a change in the signal state of the input pulse, i.e. the input pulse and the pause must be present for at least one program cycle. The longer the program execution time, the lower the counting frequency. A detailed description of the counter functions is provided in Chapter 10.5 “Counter functions” on page 349.
Fig. 7.27 Examples of timer functions
A counter function can be controlled with one of the three behavior patterns CTU, CTD, and CTUD. A counter function requires internal data for each application. You can specify where this data is to be saved when programming: by specifying Single instance for storage in a separate data block, and by specifying Multi-instance for storage in the instance data block of the calling function block.
The data type of a counter function is based on the data type of the count value. If, for example, an up-counter (CTU) with a DINT count value is programmed as a sin- gle instance, the data type IEC_DCOUNTER is taken as a basis for the data block (see Chapter 4.8.2 “IEC_COUNTER system data type” on page 112); as a local instance, the counter function has the data type CTU_DINT (see Chapter 4.6.2 “Parameter types for IEC counter functions” on page 108). You address the counter function with the name of the instance data – data block or local data.
The top counter function “Counter 1” in Fig. 7.28 saves its data as a local instance in the calling function block. The current count value “Count value 1” is set by
“Contact 4” to zero. “Contact 3” increments the current count value by one unit with each positive edge. If the count value reaches the default value “Preset value 1” and then exceeds it, the tag “Coil 3” at output Q is set.
The second counter in the example is an up/down counter. “Contact 7” sets the cur- rent count value to zero, “Contact 8” loads the default value “Preset value 2” as the current count value. “Contact 5” increments the count value by one unit with each positive signal change, “Contact 6” decrements the count value by one unit with each positive signal change.
Fig. 7.28 Examples of counter functions
The QU output has the signal state “1” if the actual count value at the CV output is equal to or greater than the default value at the PV input. The QD output has the sig- nal state “1” if the actual count value is zero or less than zero.
The upper QU output can be further connected directly. In the example, it is used to control the tag “Coil 4”. The QD output cannot be supplied, but can be scanned indi- rectly via the corresponding component QD of the counter structure. (For the QU output, this would be the component QU.)
The name of the local instance (#“Counter 1”) and the name of the data block (“Counter 2”) address the respective counter function. In the example, “Counter 2”
has its own data block, and the QD output is scanned as usual in LAD with a contact named “Counter 2”.QD. The result of the scan can be connected further, e.g. assigned directly to a coil.