# Building a PLC Program That You Can Be Proud Of – Part 1

What is the best way to program a PLC?
My answer is simple. The best way is one in which someone can look at your program and understand it. I cannot stress enough the need for good documentation of your program. The best programs are ones that I can return to after several years and understand what it is doing within a few minutes. Programs should read like a book. This will aid in troubleshooting, modifying or teaching.

How do you approach a PLC program?
You must know everything about the logic or process before starting your program. Making a flow chart is one good method of learning the logic and process. The flow chart will bring out questions like the following:
What happens after a power outage? (In each condition of the outputs)
What happens if a sensor is not made? How long do you wait?
What are the critical items to monitor? (Ex. Air Pressure, Weight, Length, etc)
What happens…
Once you have written your program and are in the troubleshooting stage you can usually go back and add to your flow chart. Usually, there is always something that needs to be added, changed or modified based upon the actual functioning of the program.
Consider each project a complete learning opportunity.

Once you know what you want to do with the PLC and have a good understanding of the logic flow, then it is time to start coding. Remember that there is no right or wrong method to program the PLC, either the program will work or it will not work.

Let’s look at an example. We will start with something that we all know how it works.
Traffic Lights

We will look at three programming examples for the lights. Two different approaches to programming will be used, but the program function is the same. The last example will modify the logic for a car being sensed.

Logic:
First Example:
Traffic Light Program
Sample program for traffic light intersection with lights facing North /South and West /East.
Green is on for 5 seconds
Yellow is on for 2 seconds
Red has an overlap of 3 seconds
This program uses discrete bits and timers to accomplish this task.
The \$FirstScan bit will reset the timers so if power is lost, the lights will start with Red / Red overlap before starting the sequence again.
The outputs are controlled by when the timers are on (Done) or off (Not Done)
North / South Traffic Lights
West / East Traffic Lights

You will notice that this program is fully documented and easy to understand.

This program is based on time events. The base rate is one second. If we create a list of what the outputs look like after each second and then send them to the physical output channel we will have the second type of approach to this logic…

Logic:
Second Example:
Traffic Light Program

Sample program for traffic light intersection with lights facing North /South and West /East.
Green is on for 5 seconds
Yellow is on for 2 seconds
Red has an overlap of 3 seconds

This program uses indirect addressing to program
Lets look at the list of outputs we want based upon the following addresses: (Notice the Bit location)
Y0 – Red_Light_NS
Y1 – Yellow_Light_NS
Y2 – Green_Light NS
Y8 – Red_Light_WE
Y9 – Yellow_Light_WE
Y10 – Green_Light_WE

We have 20 steps to do in the sequence based upon 1 second increments. (V1000 to V1019)

Here is what the hex values translated to binary look like:
(Review of numbering systems from previous blog)

The \$FirstScan bit will reset the pointers so if power is lost, the lights will start with Red / Red overlap before starting the sequence again.

Let’s look at the program:

The \$FirstScan bit will move the number 1000 into V0. V0 will act as our pointer for the list of outputs. (V1000 to V1019)
Every 1 second (\$1Second) V0 will increment by a value of 1. We will then compare the value to 1020 which indicates the end of the sequence. If the value is greater or equal to then our pointer is reset to the value of 1000. This is done by moving the number 1000 into V0.
The last step is moving our output word indirectly V0 to our output word V1. Indirectly means that the value in V0 will point to a memory location dictated by the number it contains.
ex: V0 has a value of 1000 so this means that V[V0] will move V1000 into our output word.

Set the outputs
Our physical outputs are set by casing our output word (V1) into bits. Depending on the programmable controller this can be done my moving to a word that can be addressed by bits or in our case we can cast the word into bits. [V1:#]

This program code is allot smaller than the first using discrete bits and timers. With documentation it is also easier to read.

One of the advantages of indirect addressing to program is that it makes modifications easier. Lets modify the last program…

The North will stay green until a car approaches from the West. It will remain green for 1 more second before turning yellow and completing the cycle. If the car is always there then the lights will always function.
X0 – Car West/East

Just a couple of contacts have been added to the logic on the line that increments the pointer. The setting of the outputs do not change.
If the value at V0 is equal to 1006 then stop incrementing V0. X0 (Car at intersection) comes then the pointer will increment. The cycle will complete and continue until X0 is not present. It will then stop when the pointer V0 equals 1006 again.

Watch on YouTube : Building a PLC Program That You Can Be Proud Of

In part 2 we will look at indirect addressing with a sequence that is event driven, not timed like the above.

Contact me for the above programs. I will be happy to email them to you.
Thank you,
Garry

Do-more Designer Software

How to use video’s for Do-more Designer Software

One of the better PLC programming books is PLC Programming for Industrial Automation by Keven Collins. Here is the link to the free download.

http://staffweb.itsligo.ie/staff/kcollins/plc/plcprogramming.pdf

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.

Use the information to inform other people how numbering systems work. Sign up now. I have always been an old school programmer. Hardware in front of you hooked up to all of the I/O. I have recently looked at the automation direct do-more designer software solution. The Do-More Designer Software will allow you to build your ladder logic, download into a simulator (comes with the software) and run the code. The price of all of this… FREE

I will not get into a lot of details because the documentation available already will get you through the software step by step.
– Install the software
– Use the YouTube videos for help with getting around the software and making your first program.

Automation Direct – Do-more Programming Software

The simulator has a lot of great features, including PID simulation.

Do-more PLC – How to videos on youtube

Update: Here is a video from YouTube about the simulator basic instructions:

Thank you,
Garry

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.

Use the information to inform other people how numbering systems work. Sign up now. # Here’s a Quick Way to Understand PLC Inputs and Outputs

The term I/O means Input/Output. I/O can come in two different types; Discrete or Analog Most people starting out leaning about programmable logic controls (PLC) are taught all about discrete input and outputs. Data is received from devices such as push-buttons, limit-switches etc. and devices are turned on such as motor contactor, lights, etc. Discrete input and output bits are either on or off. (1 or 0) The following program will show a motor control circuit stop start. Motor off:

Motor on:

Analog inputs Common input variables for analog are temperature, flow, pressure, etc. They are converted to an electrical signal into a PLC analog input. Standard electrical signals are 0 – 20 mA, 4 – 20 mA, 0 – 10 volts DC, -10 – 10 volts DC. Note: It is recommended that a 4 – 20 mA signal is best. If voltage is required, a resistor can be added to get a voltage input. Analog outputs Common output variables for analog are speed, flow, pressure, etc. They are converted from a word in the PLC to the output of the analog. The range of signal is then outputted to the device to control the position, rate, etc. Standard electrical signals to the device are 4 – 20 mA, 0 – 10 volts DC, -10 – 10 volts DC. Both Analog Inputs and Outputs use words to determine the signal going to or from the device. Example: 4 – 20 mA current Input – 8 bit resolution 4 mA = 000000002 = 0016 20 mA = 11111111= FF16 Example: 4 – 20 mA current Output – 8 bit resolution 0016 = 000000002 = 4 mA FF16 = 111111112 =20 mA For a review of numbering systems, follow the link below: What everyone should know about PLC numbering systems

Let me know if you have any questions or need further information.
Thank you,
Garry

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.

Use the information to inform other people how numbering systems work. Sign up now. # Here’s a Quick Way to Convert Grey Code into Binary for PLC

Grey Code
Grey Code is used because only one bit of data will change at a time. The following chart shows the conversion of Grey Code to Binary.

 Number Binary Code Grey Code Number Binary Code Grey Code 0 0000 0000 8 1000 1100 1 0001 0001 9 1001 1101 2 0010 0011 10 1010 1111 3 0011 0010 11 1011 1110 4 0100 0110 12 1100 1010 5 0101 0111 13 1101 1011 6 0110 0101 14 1110 1001 7 0111 0100 15 1111 1000

It is important for absolute encoders because if the power is interrupted the encoder will know where it is within the one bit.

Example:
Power is interrupted when the encoder is between 7 and 8. If we are looking at Binary Code all of the bits would be effected and we would not be sure as to what number we are looking at for the encoder. Therefore we have lost position. In Grey Code only one bit changes so we will still be able to tell if we were on 7 or 8 if the power was interrupted.

The following sample PLC program will convert 4 bit grey code into binary code.
This code was written in an Automation Direct PLC software called Do-more Designer.

Contact me for the above program. I will be happy to email it to you.
Thank you,
Garry

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.

Use the information to inform other people how numbering systems work. Sign up now. # What Everybody Ought to Know About PLC (Programmable Logic Controller) Numbering Systems

Programmable Logic Controllers (PLC) are the same as computers. They only understand two conditions; on and off. (1 or 0 / Hi or Low/ etc.) This is known as binary. The PLC will only understand binary but we need to display, understand and use other numbering systems to make things work. Let’s look at the following common numbering systems.

Binary has a base of two (2). Base means the number of symbols used. In binary the symbols are 1 or 0. Each binary symbol can be referred to as a bit. Putting multiple bits together will give you something that looks like this: 100101112. The 2 represents the number of symbols/binary notation. Locations of the bits will indicate weight of the number. The weight of the number is just the number to the power of the position. Positions always start at 0. The right hand bit is the ‘least significant bit’ and the left hand bit is the ‘most significant bit’. Let’s look back at our example to determine what the value of the binary number is:
100101112 =
We start with the least significant bit and work our way to the most significant bit.
1 x 2= 1 x 1 = 1
1 x 2= 1 x 2 = 2

1 x 2= 1 x 2 x 2 = 4
0 x 2= 0 x 2 x 2 x 2 = 0
1 x 2= 1 x 2 x 2 x 2 x 2 = 16
0 x 2= 0 x 2 x 2 x 2 x 2 x 2 = 0
0 x 2= 0 x 2 x 2 x 2 x 2 x 2 x 2 = 0
1 x 2= 1 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 128
100101112  = 1 + 2  + 4 + 16 + 128
100101112  = 151
Note that the we just converted the binary number to our decimal numbering system. The decimal numbering system is not written with a base value of 10 because this is universally understood.
To be sure we have the concept down, let’s take a look at our decimal numbering system the same way as we did the binary.
Decimal has a base of ten (10). The symbols are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
15110 =
1 x 10= 1 x 1 = 1

5 x 10= 5 x 10 = 50
1 x 10= 1 x 10 x 10 = 100
15110 = 1 + 50 + 100
151 = 151

Hexadecimal has a base of sixteen (16). The symbols are  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. Hexadecimal is used to represent binary numbers. F16 = 1111
Every for bits of binary represent one hexadecimal digit.
In our original binary number we now can convert this to hexadecimal.
100101112
The least significant four bits are:
01112 =
1 x 2= 1 x 1 = 1
1 x 2= 1 x 2 = 2

1 x 2= 1 x 2 x 2 = 4

0 x 2= 0 x 2 x 2 x 2 = 0

0111= 1 + 2 + 4 + 0 = 716
The most significant four bits are:
1001=
1 x 2= 1 x 1 = 1
0 x 2= 0 x 2 = 0
0 x 2= 0 x 2 x 2 = 0

1 x 2= 1 x 2 x 2 x 2 = 8

1001= 1 + 0 + 0 + 8 = 916
Therefore:
100101112 = 9716
We can now convert this hexadecimal number back into decimal
9716 =

7 x 16= 7 x 1 = 7
9 x 16= 9 x 16 = 144
9716 = 7 + 144 = 151

The following chart will show all of the combinations for 4 bits (nibble) of binary. Its shows the Binary, Decimal and Hexadecimal (Hex) values. It is interesting to not that Hex is used because you still have only one digit (Place Holder) to represent the nibble of information.

 Binary Decimal Hexadecimal Binary Decimal Hexadecimal 0000 00 0 1000 08 8 0001 01 1 1001 09 9 0010 02 2 1010 10 A 0011 03 3 1011 11 B 0100 04 4 1100 12 C 0101 05 5 1101 13 D 0110 06 6 1110 14 E 0111 07 7 1111 15 F
ASCII (American Standard Code for Information Interchange)
Two nibbles (8 bits of data) together form a byte. A byte is what computers (PLC) use to store and use individual information. So it will take one unique byte to represent each individual numbers, letters (upper and lower case), punctuation etc. www.AsciiTable.com
Example:
Chr ‘A’ = 4116 = 010000012
Chr ‘a’ = 6116 = 011000012
Chr ‘5’ = 3516 = 001101012
Each time you hit a key on your keyboard, the following 8 bits of data get sent.
A word is made up of two bytes, or 4 nibbles, or 16 bits of data. Words are used in the PLC for holding information. The word can also be referred to as an integer.
Long word / Double word is made up of 4 bytes, or 8 nibbles, or 32 bits of data. Long words are used for instructions in the PLC like math.
So far we have talked about unsigned words. (Positive numbers)
Signed words can hold negative numbers. Bit 15 (most significant bit) of a word is used to determine if the word is negative or not.
The following table shows you the signed vs unsigned numbers that can be represented in the PLC.
 HEX 8000 BFFF FFFE FFFF 0000 3FFF 7FFE 7FFF Signed -32768 -16385 -0002 -0001 00000 16383 32766 32767 Unsigned 32768 49151 65534 65535 00000 16383 32766 32767
Memory retentiveness:
When working with PLC’s look at the memory tables to determine what will happen if power is removed from the device. Will the bits go all off or retain their prior state?
Usually there will be areas that can be used in the PLC for both conditions.
As you can see PLC numbering systems and computers are very much related and it all boils down to individual bits turning on and off. The interpretation of these bits will determine what the value will be.

Reference:
Let me know your thoughts, or questions that you have on PLC numbering systems.

Thank you,
Garry

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.

Use the information to inform other people how numbering systems work. Sign up now. 