programming in list on a mitsubishi plc – part 1.pdf

7/29/2019 Programming in List on a Mitsubishi PLC Part 1.pdf

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Programming in List on a Mitsubishi PLC Part I- by Jim Rowell - 2003/10/19

This is a tutorial written for 2 sets of people. First its intended for those just starting out who have at leastlearned how to use the basics of their editing software in order to enter a simple program. They shouldalready understand the basic concepts such as x0 is an input, y0 is an output. Its also intended foradvanced users of Ladder who are curious about List. Hopefully its not too difficult for the former or toosimplistic for the latter. I'm not an experienced tutorial writer, so apologies in advance! ;)

You're bound to have a better grasp of PLC coding and the best ways to approach problems if youunderstand how the PLC reads and executes your code.

RELAYS VS THE PLCFirst of all, despite the fact that the PLC was designed as a direct replacement for relays, its logic is actuallyquite different. Relays are 100% parallel logic. Every single part of a relay control system operatessimultaneously. If you were to draw several rungs on a relay diagram and put one coil on each line with nocontacts on any of the lines, every relay would energize at the same time when power was applied. If youadd contacts to each line and the contacts on a line change at some time, the coil on that line is instantly

affected. This makes relay logic blindingly fast by nature (its only the relay's mechanical limitations thatmake it slow) but its often a source of trouble. There can be a race effect where different logic states occurif this relay closes faster than that relay this time but not the next time. On the other hand, the PLC issequential in operation. One thing happens at a time. In happens in the order you dictate and its the sameorder every time. In fact, the entire PLC program doesn't exist as far as the PLC itself is concerned. Onlythe single piece of code that it happens to be examining at any one time matters.

Here's how it works:First the PLC's operating system examines the state of all of the external wiring inputs. It records this datato an area of memory called the "input image". This image or record of the inputs is what will be used forinput reference while executing your code. The real inputs are not used. The PLC then looks at your codestarting at the beginning and progresses through it. The PLC only keeps track of a single logic result as itsreading your code. This result will be equal to 1 (true) or 0 (false). There are complications to this such as

when the operating system must read ahead during complex logic or when certain things require stackedresults but its true for the most part. As it reads your code, it will update any changes to the outputs bywriting to an area of memory called the "output image" rather than the actual outputs themselves. When itreaches the end of your code, it then turns the real outputs on or off by copying the output image memory tothe actual output registers. This start to finish cycle is called a "scan" and repeats endlessly over and over.In summary, the inputs are read once as a group, then your code is executed and then the outputs areupdated as a group. Without this cycle method, inputs changing midway during your program could causestrange results and you couldn't change your mind about outputs before finally turning them on or off forreal. With the scan method, we have an almost imaginary world inside the program where we can do whatwe want with no interference or consequences. Its only at the end of each scan that we have to worryabout reality.


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BOOLEAN LOGICThis is at the heart of the PLC's operation. You need to know this.There are other ways of looking at PLC logic that perhaps are more intuitive than the pure Boolean methodbut you still must understand it and have it in your arsenal of tools if you are going to have more than abasic ability with PLC's. Unfortunately, this is a scary subject to the uninitiated but stand fast! Its really notthat hard to grasp. Just remember, its the basis of all things digital. Even good old relays are digital devicesthat lend themselves well to Boolean logic.

Boolean logic uses 2 states to represent things. Something can be on (equal to 1) or it can be off (equal to0).We perform what are known as "logical" operations in Boolean. Things like "logical AND" and "logical OR"(I'll use uppercase to denote a logical operation).

logical AND means "if both things equal 1 then the answer is 1"or in other words:0 AND 0 = 00 AND 1 = 01 AND 0 = 01 AND 1 = 1

Logical OR means "if either thing equals 1 then the answer is 1"or in other words:0 OR 0 = 00 OR 1 = 11 OR 0 = 11 OR 1 = 1

Logical NOT (also called "invert") means "invert the item, make it the opposite, the current value is NOTwhat I want"or in other words:NOT 0 = 1NOT 1 = 0

You perform a logical NOT on something when you know you want the opposite result.

So now if we said given that X=1 and Y=1, you'd know that X AND Y = 1 and also that X OR Y = 1or given that X=0 and Y=1, you'd know that X AND Y = 0 but X OR Y = 1.Or, to get tricky, given that X=0 and Y=1, you'd know that NOT X AND Y = 1 but X OR NOT Y = 0You should read and re-read this until you understand it. Its important.

When something is equal to 1 we say that the result is "true". When something is equal to 0 we say that theresult is "false".

In proper Boolean, there is a defined order for performing operations just like in math where multiplication isperformed before addition. In Boolean, AND operations are performed before any OR operations. In fact,the short form method of writing Boolean uses the times symbols "x" or "*" to represent AND while the plussign "+" is used for OR. (eg. 1+0=1 is the same as writing 1 OR 0 =1). The "times" symbol expresses the

fact that AND has precedence over OR. You don't have to worry about this in the PLC. It simply performsthe operations in the order they are presented. Just be aware that its a deviation from what goes on outsideof the PLC arena.


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PROGRAMMINGTo program the PLC we have several options in the way of languages. The two common ones are Ladderand Instruction List. There are many tutorials that discuss Ladder so we'll stick with the one most near anddear to my heart: "Instruction List". List is a "mnemonic" or text-based language. Its exactly like the

Assembler languages used to program microprocessors in their native language. A native language is onethat the processor directly understands. You type a "mnemonic" which is simply an abbreviation consistingof a few letters to represent a command, and the editing software on the PC sends the corresponding codenumber to the PLC. There are no interpretations or changes made other than the fact that you type in lettersand symbols and the PLC reads numbers. When working with a PLC, you are not really programming theprocessor. There is a high-level operating system on-board that is programmed to be a PLC but the netaffect to you as a programmer is identical to working directly with a processor. In other words, you are aboutto become an Assembler programmer! Pretty exciting, right? ;)

Mitsubishi uses mostly obvious mnemonics such as LD to mean "load the accumulator", AND to mean"AND", OR to mean "OR" and OUT to mean "send the result to the output image". These and the followingmay seem like gibberish to you at first but plough through it. There is a bit of a learning curve and then thesun breaks through the clouds!

LDI (Load Inverted; more commonly thought of as Load Not) means "perform a NOT operation on thefollowing value, then load the accumulator with the result".

AND means "perform an AND operation".ANI (And Not) means "perform a NOT operation on the following value, then do an AND"

eg. given that X1=1 and Y1=0, X1 ANI Y1 =1 because although Y1 contains a zero, it is first inverted toa 1. The AND operation is performed as 1 AND 1 which = 1.ORI (Or Not) means "perform a NOT operation on the following value, then do an OReg. given that X1=0 and Y1=0, X1 ORI Y1 =1 because although both contain zeros, Y1 has first been

inverted to a 1. The OR operation is performed as 0 OR 1 which = 1.


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WHAT THE PLC SEESPeople generally think of the PLC as being more complex in its method of reading lines than it really is.They picture it reading an entire Ladder rung like they would and then making a decision at the end as towhether or not to turn on the output. It actually never makes a decision when performing basic logic such as

AND, ANI, OR, ORI. It just keeps track of the current state of affairs in a reserved memory location knownas the Accumulator or just "A" for short. This is an old, tried and true processor method that the PLC'soperating system emulates.

Example:

If we were to write some standard English control logic as such:

if x1 is on and x2 is on or y1 is on and y2 is off then turn on y10 else turn y10 off.

and then rewrote the same thing for the PLC it would look like the code below. The comments show howthe PLC executes it.

LD x1 ;the PLC "loads" the Accumulator (A) with the value of x1 (ie. the value of x1 is moved into A).AND x2 ;the PLC performs A AND x2. The result is placed in A.OR y1 ;the PLC performs A OR y1. The result is placed in A.

ANI y2 ;the PLC performs A AND NOT y2. Result is placed in A. OUT y10 ;the PLC "outputs" the value of A into the output image memory location reserved for y10.

As you can see, no decisions were made. The PLC didn't even turn the output on or off. It just moved theresult to the output image when it was told to! Later, at the end of the scan, the entire output image will becopied to the real output registers. If there ends up being a 1 in a register, the associated output willmagically turn on. If it contains a zero, it will turn off. The turning on, turning off part is done by thehardware. The operating system has nothing to do with it.The PLC doesn't even perform the operations in the proper order according to the rules of Boolean logic. Itsimply AND's and OR's things as presented.This is how the PLC with a simple and comparatively slow processor can scream through your code at suchspeed. Its efficient and only concerned with simplicity.

When we start a new "series" or collection of statements in List or begin a new rung in Ladder, what we arereally doing is telling the PLC that we want to start over and it should not use any previous results. In otherwords, we want it to "Load the accumulator" with the first value rather than perform an operation with the oldvalue from the previous line. Loading the accumulator with the new value overwrites whatever was alreadythere.

STARTING SIMPLEOkay, so now lets back up and start simple:

You want a motor to come on but only if you turn on a selector switch. So you connect a switch to x0 on thePLC and you connect the motor's contactor coil to y0.Now what? First we name the inputs and outputs. Never, ever, ever, use the raw I/O names in a program.Its fine for describing things to your friends in snippets of code about how to do something... but don't use

x0, y0, etc in a real program. Its hard to read, its hard to debug and its hard to change. Okay, so we'll callx0 "switch" and y0 "motor". Right?

Here's how your program will look:

LD SWITCHOUT MOTOR

That was pretty easy, no? To make it easy to read, instead of thinking "LD", think "IF" and instead of OUT,think "then" or "turn on".


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In your head it becomes:

if switchthen motor

What if you want the motor to shut off if it gets too hot? Oops, that's right, we have overload contacts wehave to monitor. So lets put them into x1 and call them "overload". Typically, we want the motor to only turnon if the overload contacts are closed. Same as the switch. So now we have:

LD SWITCHAND OVERLOADOUT MOTOR

Now someone adds a special sensor on the machine and tells you the motor must turn off if the sensorturns on.

LD SWITCHAND OVERLOADANI SENSOROUT MOTOR

So now in English, this would read...If switch (was that supposed to be open or closed?) and we have an overload... no, wait! If we don't havean overload... and the sensor is not on (not open or not closed?)... hmmm.... this is getting confusing!!

We've only got 4 lines of code and already its unclear exactly what we are trying to do. Time for some moreRULES!

NAMING CONVENTIONSName your inputs and outputs according to what they do when they are "active". That's an important word.Forget "open" and "closed" for a minute. Think "active" and "inactive". Every item has a purpose or youwouldn't be connecting it in the first place. You must always name an item according to its purpose. It willbe active when its fufilling its intended purpose. The purpose of the switch we used was to turn on the

motor or make it run. We should have called it something like "RUN". Even better would be to call it"RUN'SEL" (for Run Selector). Notice I used 2 words separated by an apostrophe. We are only allowed asingle word but often two or three make the purpose clearer. The apostrophe is a good way of cheatingyour 1 word into several. The more traditional underscore also works but its easy for your eye to miss it anda tired and worn printer will often drop it. Now you know the following about RUN'SEL:

If the real switch in the field is active then our input called RUN'SEL is active (x0=on),and itsname tells you what the intended result should be (RUN).

It doesn't matter how that was achieved in the field ie. normally-open or normally-closed contacts. An inputis only active if you have a live electrical signal coming to it.Now, how about our overload contacts. Their purpose in life is to detect overloads. They become "active"when an overload occurs. So they can properly be called "Overload" contacts. Since we have decided to

use normally-closed contacts from the overloads, we have a problem. The signal into the PLC will be OFF(inactive) when the overloads are active. In other words, the input should be named "NO'OVERLOAD"because when the input is active, the overload contacts are inactive and there is no overload.

A better method is to use a slash in front of the name which is read as "NOT". This is just my personal idea,stolen from Boolean, but it works really well. The slash tells you that the input is inverted (inactive when theinput is on). So "/OVERLOAD is read as "not overload". The field device is called "Overload" but the inputon the PLC is called /OVERLOAD. When you see the slash, it tells you the field device is using normally-closed contacts. Otherwise, everything is from normally-open contacts. Beautiful, no?Let's see where we are now (the comments show how YOU would read it):


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LD RUN'SEL ;if runselAND /OVERLOAD ;and not overloadOUT MOTOR ;out motor

That's better. We need a name for the sensor. Let's say that when its active, its telling us that the machineis busy and the motor must remain off. Let's call it BUSY.

LD RUN'SEL ;if runselAND /OVERLOAD ;and not overloadANI BUSY ;and not busyOUT MOTOR ;out motor

In English: If the run selector switch is on and there is no overload and the machine is not busy then turn onthe motor.

You can see how important the names are when reading List. They are everything. Another benefit of the"slash" is when you have a line like "LDI /OVERLOAD", you can cancel out the 2 "NOTS" in your head andit makes more sense. Instead of saying "If-not not-overload", its just "If overload". It means the same thing!


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ORDER OF OPERATIONSI suspect this is the area where most people give up. They don't understand how combinations of OR and

AND are seen by the PLC. And it can soon become very confusing to read. Complex Boolean is actuallymore clearly displayed in Ladder format than it is in List. But List can manage it, and Ladder is not veryuseful for displaying non-Boolean operations such as math and the higher functions. So the challenge is tomake your List readable when doing Boolean so that you can take advantage of the rest.

First thing to remember is that the PLC completes each statement as it reads them. It doesn't read theentire collection of lines at one go. For those familiar with Ladder, its like saying that the PLC reads onecontact and adds it to its sum in the Accumulator, forgets that the contact exists and goes on to the nextcontact. It doesn't read the entire rung at once. If it encounters an AND, its the same as if you have acontact in series with everything that came before. If it encounters an OR, its like a contact in parallel witheverything that came before. On a Ladder diagram, the AND is always in series with the ENTIRE rung. TheOR is always in parallel with the ENTIRE rung (in other words, its a new line that starts back at the left rail).That being the case, how the heck can we do anything?!

There are a couple of methods for telling the PLC to hang on to a result for later use. Just like on yourcalculator where you have the Memory feature. On your calculator you might add some figures together,place the result into the Memory, do some multiplying with new figures and then finally add the result towhat you previously put in the Memory. We can do the same thing on the PLC in several different ways.

Write out a series of statements such as follows:LD RUN'SEL

AND /OVERLOADANI BUSY

Don't complete the series by using an OUT statement. Now start a new series:

LD HIGH'SEL ;high-speed selector switch

The PLC knows you are not done with the first series because you havent "used" the result yet in an outputtype of statement. When it encounters the new LD statement, it puts the old result into a temporary storagearea called a stack. It knows you will need it later.

Now carry on:LD HIGH'SEL ;high-speed selector switchOR LOW'TEMP ;low temperature sensor

ANB


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That last command was "AND BLOCK". It means pull the old value from storage and then AND it with thecurrent value in the Accumulator. In other words, "I want the old series (the old block of code) to be true

AND I want the new series (new block of code) to be true before performing any outputs. You could alsohave used ORB. It means "If the old series is true OR if the new series is true then...". Now, you follow withwhatever output code you wish. Let's put it all together:

LD RUN'SELAND /OVERLOADANI BUSY

LD HIGH'SELOR LOW'TEMP

ANBOUT MOTOR

In English: If the run selector switch is active and we have no overload and the machine is not busy... if thehigh selector switch is active or the low temp sensor is active... if both of those series of things are true thenturn on the motor.

Make sure you use a blank line to separate the two series. It keeps things readable.

When you have seriously twisted logic to express, and even the handy ANB, ORB won't manage it, there isan all-powerful stand-by at your disposal. Its called MPS for "Memory Point Store". Its exactly like the oneon your calculator. No different. You should avoid it like the plague in most cases because it can lead tocode thats difficult to read. There are times, however, when it s exactly what the doctor ordered. Usually,its better if you repeat sections of code or store your own result with its own descriptive name rather thanusing MPS.

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That's all for Part 1. We'll try out some more examples and look at some more advanced topics in Part 2.

Happy coding!

Jim RowellRowell Electric

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programming in list on a mitsubishi plc – part 1.pdf

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