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Chapter 5 - Mnemonic Programming Code
Chapter 5 - Mnemonic Programming Code
5-1. Objectives
Upon completion of this chapter, you will know
”why mnemonic code is used in some cases instead of graphical ladder language.
”some of the more commonly used mnemonic codes for AND OR and INVERT operations.
”how to represent ladder branches in mnemonic code.
”how to use stack operations when entering mnemonic coded programs.
5-2. Introduction
All discussions in previous sections have considered only the ladder diagram in all program example development. The next thing to be considered is how to get the ladder diagram into the programmable controller. In higher order controllers, this can be accomplished through the use of dedicated personal computer software that allows the programmer to enter the ladder diagram as drawn. The software then takes care of translating the ladder diagram into the code required by the controller. In the lower order, more basic controllers, this has to be performed by the programmer and entered by hand into the controller. It is this type of language and the procedure for translating the ladder diagram into the required code that will be discussed in this chapter. This will be accomplished by retracing the examples and ladder diagrams developed in earlier chapters and translating them into the mnemonic code required to program a general controller.
This controller will be programmed in a somewhat generic type of code. As the code is learned, comparisons will be presented with similar types of statements found in controller use. The student will have only to adapt to the statements required by the type of controller being used to develop a program for that controller.
5-3. AND Ladder Rung
Let us begin with the ladder diagram of Figure 5-1. This is the AND combination of two contacts, IN1 and IN2 controlling coil OUT1.
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Chapter 5 - Mnemonic Programming Code
Figure 5-1 - Ladder Diagram for AND Function
Ladder diagrams are made up of branches of contact logic connected together which control a coil. For instance, IN1 AND IN2 can be considered a branch. This rung has only one branch. We will see examples of multiple branches later. The code command which alerts the controller to the beginning of a branch is LD. The LD command tells the controller that the following set of contacts composes one branch of logic. The complete contact command code for these is:
LD IN1
AND IN2
The lines tell the controller to start a branch with IN1 and with this contact, AND contact IN2. LD commands are terminated with either another LD command or a coil command. In this case, a coil command would terminate because there are no more contacts contained in the ladder. The coil command is STO. The contact and coil commands for this rung of logic are:
LD IN1
AND IN2
STO OUT1
The STO command tells the controller that the previous logic is to control the coil that follows the STO command. Each line of code must be input into the controller as an individual command. The termination command for a line of code is generally ENTER.
NOTE: Two types of terminators have been described and should not be confused with each other. Commands are terminated by another command (a software item) while lines are terminated with ENTER (a hardware keyboard key).
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Chapter 5 - Mnemonic Programming Code
The complete command listing for this ladder rung including termination commands is:
LD IN1 ENTER
AND IN2 ENTER
STO OUT1 ENTER
The commands may be entered using a hand-held programmer, dedicated desktop programmer or a computer containing software that will allow it to operate as a programming device. Each controller command line contains (1) a command, (2) the object of the command and (3) a terminator (the ENTER key). In the case of the first line, LD is the command, IN1 is the object of the command and the ENTER key is the terminator.
Each line of code will typically consume one word of memory, although some of the more complicated commands will consume more than one word. Examples of commands that may consume more than one word of memory are math functions and timers, which will be discussed later.
5-4. Handling Normally Closed Contacts
Notice that the rung of Figure 5-1 has only normally open contacts and no normally closed contacts. Let us look at how the command lines would change with the inclusion of a normally closed contact in the rung. This is illustrated in Figure 5-2. Notice that normally open contact IN1 of Figure 5-2 has been replaced with normally closed contact
IN1.
Figure 5-2 - Rung With Normally Closed Contact
To indicate a normally closed contact to the PLC, the term NOT is associated with the contact number. This may take different forms in different controllers depending on the program method used by the manufacturer. Using the same form as in the previous example, the command lines for this rung would appear as follows:
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Chapter 5 - Mnemonic Programming Code
LD |
NOT IN1 |
ENTER |
AND |
IN2 |
ENTER |
STO |
OUT1 |
ENTER |
As stated above, different PLC's may use different commands to perform some functions.
For instance, the Mitsubishi PLC uses the command LDI (LD INVERSE) instead of LD NOT. This requires a single keystroke instead of two keystrokes to input the same command.
If the normally closed contact had been IN2 instead of IN1, the command lines would have to be modified as follows:
LD |
IN1 |
ENTER |
AND |
NOT IN2 |
ENTER |
STO |
OUT1 |
ENTER |
If using the Mitsubishi PLC, the AND NOT command would be replaced with the ANI (AND
INVERSE) command.
5-5. OR Ladder Rung
Now, let us translate the ladder of Figure 5-3 into machine code.
Figure 5-3 - Ladder Diagram for OR Function
As can be seen, the contact logic for Figure 5-3 is an OR connection controlling coil OUT1. Following the same steps as with Figure 5-1, the command lines for this rung are:
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Chapter 5 - Mnemonic Programming Code
LD |
IN1 |
ENTER |
OR |
IN2 |
ENTER |
STO |
OUT1 ENTER |
|
Notice again each line contains a command, an object and a terminator. In the case of this particular controller, the coil has a descriptive label (OUT) associated with it to tell us that this coil is an output for the controller. In some controllers, this is not the case. Some controllers designate the coil as output or internal depending upon the number assigned to it. For instance, output coils may be coils having numbers between 100 and 110 and inputs may be contacts having numbers between 000 and 010. Systems that are composed of plug in modules may set the output coil and input contact numbers by the physical location of the modules in the system.
5-6. Simple Branches
Now consider the ladder diagram of Figure 5-4, a more complicated AND-OR-AND logic containing two branches.
Figure 5-4 - Ladder Diagram AND - OR - AND Function
The previous examples have had only a single branch in each case. A branch may be defined as a single logic expression contained in the overall Boolean expression for a rung. In the case of Figure 5-4, the Boolean expression would be that of Equation 5-1. As can be seen in the Boolean equation, there are two logic expressions OR'ed together; IN1 AND IN2 and IN3 AND IN4. Each of the two expressions is called a branch of logic and must be handled carefully when being input into the controller. Incorrect entering of branches will result in improper (possibly dangerous) operation of the PLC. In cases where entering a branch incorrectly violates the controller logic internally, an error message will be generated and the entry disallowed. In cases where the entry does not violate controller logic, operation will be allowed and could cause bodily injury to personnel if the controller is in a position to operate dangerous machinery.
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Chapter 5 - Mnemonic Programming Code
OUT1 = (IN1)(IN2) + (IN3)(IN4) |
(5-1) |
This configuration of logic in Figure 5-4 utilizes four contacts, IN1, IN2, IN3 and IN4 controlling an output coil OUT1. As discussed in earlier chapters, coil OUT1 will energize when (IN1 AND IN2) OR (IN3 AND IN4) is true. The first line (IN1 AND IN2) is entered in the same manner as described in the previous examples:
(the 1. and 2. are line numbers for our reference only)
1. LD IN1 ENTER
2.AND IN2 ENTER
For the next line (IN3 AND IN4) we must start a new branch. This is accomplished through the use of another LD statement and a portion of PLC memory called the stack.
As program commands for a rung are entered into the PLC they go into what we will call an active memory area. There is another memory area set aside for temporarily storing portions of the commands being input. This area is called the Stack. Each time and LD command is input, the controller transfers all logic currently in the active area to the Stack.
When the first LD command of the rung is input, there is nothing in the active area to transfer to the stack. The next two lines of code would be as follows:
3. LD IN3 ENTER
4.AND IN4 ENTER
The LD command for IN3 causes the previous two lines of code (lines 1 and 2) to be transferred to the Stack. After lines 3 and 4 have been input, lines 1 and 2 will be in the stack and lines 3 and 4 will be in the active memory area.
The two areas, active and the stack, may now be OR'ed with each other using the command OR LD. This tells the controller to retrieve the commands from the stack and OR that code with what is in the active area. The resulting expression is left in the active area.
This line of code would be input as shown below:
5. OR LD ENTER
Up to now, most controllers would have recognized the same type of commands
(AND, OR). The LD and OR LD commands will, however appear differently in various controllers depending upon how the manufacturer wishes to design the controller. For instance, the Allen Bradley SLC-100 handles branches using branch open and branch
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