PLC Tutorial | Acc Automation

People often ask “What is a PLC?” and “PLC Meaning”. A programmable logic controller (PLC) is a piece of hardware that isolates inputs from outputs. Programs are written to look at the inputs solve logic and set the outputs to perform work. Today we are going to look at the basic fundamental way we program. Every PLC company will do this…

Bits in PLC Memory

Everything in the programmable logic controller actually boils down to bits in the memory.

It is these bits that we manipulate in order to accomplish the work that we need to be done by the PLC. The instruction set is the method we use to do this. In general, there are several ways to view the bits. Discrete input and output, Numbers and Position of bits will be covered. Understanding the different ways in which we can view these bits will help in developing programs.
Bits are part of the memory of all PLC systems. The memory can be retentive or non-memory retentive. Memory retentive means that if power is lost to the PLC, the status of the bit remains the same when power is restored. If the bit is non-memory retentive, and power is lost the bit returns to the off state. Addressing refers to how the controller understands what memory location to look at. When we address memory in the PLC we can do this in two different ways:
Direct Addressing: Specify a location of the memory location
Indirect Addressing: Specify a location that contains a value to point to the memory location required.

Refer to the manual of the specific PLC that you are using for the way in which memory is addressed and if it is memory retentive or not.

Discrete On/Off Bits in the PLC

Discrete bits are the basic building blocks in the PLC. When we talk of digital I/O this is referring to the individual bits that you can wire switches, pushbuttons, proximity sensors, or any other device that is either on or off. (1 or 0) They can be usually wired to the PLC as a normally open or normally closed contact. The ladder logic is written in a way that you examine the bit as either on or off.
HOW PLC INPUTS WORK

HOW PLC OUTPUTS WORK

Bit Frequency in the PLC

We also must look at the frequency (rate of change from off to on) of the input bits or output in some cases. The maximum frequency that we can read an input to the PLC will be determined by the scan of the PLC.
Example:
A 2 ms Scan (0.002 seconds) means that we can read the inputs and solve the logic in 2 ms. In order to ensure that the input is read in both states (on / off) we have to ensure that the input is off or on for at least 2 ms. The maximum frequency (Switching / Second) that the input could switch would be 2 ms = 1/.002 times per second = 500hz

Numbers using Bits in the PLC

Numbers in the PLC are all based on binary. Analog inputs and outputs are based upon the number of bits put together in order to display the range for the input. (12 bit or 16 bit) The values from the analog 12bit input will go from 000 to FFF base 16 (Hex). Hexadecimal is used to display the binary bits in the word or register. Some of the more common numbering systems in the PLC are binary, hexadecimal, BCD (binary coded decimal) and octal (based on 8 bits)

Additional Information on understanding numbering systems in the PLC:
What Everybody Ought to Know about PLC (Programmable Logic Controller) Numbering Systems

Bit Position in the PLC

Position of the bits within the word, stack, or accumulator can be very useful. Usually, we can use this to track items. A typical example of this is to track items on a conveyor belt. The belt movement is usually a pulse input from an encoder. A sensor indicates the item on the conveyor.
PLC PROGRAMMING EXAMPLE – SHIFT REGISTER (CONVEYOR REJECT)

Bits are the basic building blocks that we use to program programmable logic controllers. The three ways to view bits (Discrete, Number, and Position) will help users to understand the different ways to program.
Here are some additional links that you may find helpful:
Five Steps to PLC Program Development
PLC Programming Example – Process Mixer
PLC Programming Example – Shift Register (Conveyor Reject)
PLC Programming Example – Paint Spraying

The Secret of Using Counters
The Secret of Using Timers

Watch on YouTube: PLC Bits Numbers and Position
If you have any questions or need further information please contact me.
Thank you,
Garry

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLCs 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.

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We will continue the series by looking at a sequencer controlling seven cylinders that can be taught. The cylinders can be operator programmed from the AdvancedHMI screen. You will be able to select what cylinders to activate at each step and program in 500 steps. Our PLC will be the Do-More from Automation Direct.

Here is a quick review of the programming series so far. If you are new to the site, we recommend reviewing the other parts in the series first.
In part 1 we looked at writing PLC programs to control a traffic light using discrete bits and then using timed sequencing using indirect addressing.
Part 2 used indirect addressing for inputs as well as output to control the sequence of pneumatic (air) cylinders in the program. Part 3 and 4 we returned to the traffic light application and expand our program significantly. We looked at the sequence of operations using Input, output, and mask tables.
In part 5 we used the AdvancedHMI software to create the game of Simon. A round in the game consists of the device lighting up one or more buttons in a random order, after which the player must reproduce that order by pressing the buttons. As the game progresses, the number of buttons to be pressed increases.

Sequence of Operation – Building a PLC Program 6

We will be using AdvancedHMI to communicate Modbus TCP to the Automation Direct Do-More Designer Software Simulator. The following is the sequence of operation:
Watch on YouTube: Running the Cylinder Sequence (PLC / HMI)
Note: All of the programs used are provided free of charge and are an excellent way to learn PLC / HMI programming.

The following table is the Modbus TCP memory map to the Do-More PLC:

Coil/Register Numbers	Data Addresses	Type	Do-More PLC	Table Name
00001-09999	0000 to 270E	Read-Write	MC1 to MC1023	Discrete Output Coils
10001-19999	0000 to 270E	Read-Only	MI1 to MI1023	Discrete Input Contacts
30001-39999	0000 to 270E	Read-Only	MIR1 to MIR2047	Analog Input Registers
40001-49999	0000 to 270E	Read-Write	MHR1 to MHR2047	Analog Output Holding Registers

Here are the inputs and outputs we will be using for our program:

Device	Data Addresses	Type	Do-More PLC	Description
Start Pushbutton	00011	Input	MC11
Stop Pushbutton	00012	Input	MC12
Jog Pushbutton	00013	Input	MC13
Reset Pushbutton	00014	Input	MC14
Run/ Jog Selector	00008 / 00015	Input/ Output	MC8 / MC15	MC8 is the value and MC15 is the click
Light Stack	00010 / 00009 / 00008	Output	MC10 / MC9 / MC8	Red / Green / Amber
Set Pushbutton	00071	Input	MC71
Jog / Teach Selector	00070	Input	MC70	MC70 on is teach mode
Sequence Step (Panel Meter)	40001	Output	MHR1	Current step in the sequence
Inputs Actual	40002	Input	MHR2	Show the actual inputs in binary format
Output Sequence	40003	Output	MHR3	Show the actual outputs in a binary format
Input Sequence	40004	Input	MHR4	Show the input sequence bits in a binary format
Cylinder 1 to 7 – value	00001 to 00007	Output	MC1 to MC7	Determine if cylinder is on/off
Cylinder 1 to 7 – set (click)	00041 to 00047	Input	MC41 to MC47	Set the cylinder button
Cylinder 1 to 7 – retract indicators	00021 to 00027	Input	MC21 to MC27	Indicate cylinder has retracted
Cylinder 1 to 7 – extend indicators	00031 to 00037	Input	MC31 to MC37	Indicate cylinder has extended
Cylinder 1 to 7 – extend / retract error indicators	00050 to 00063	Output	MC50 to MC63	Indicate cylinder input error when jogging

HMI Design – Building a PLC Program 6

The first thing we will do is design the HMI. We have three main areas on the screen. Basic Controls, Cylinder Visualization, and the Sequence Step/Teach area. Please refer to the above reference chart for the inputs and outputs programmed on the screen.

Basic Controls:
This area will allow us to see what mode we are in via the stack light. Red – Stop
Yellow – Jog / Teach Mode – Troubleshooting
Green – Run

Cylinder Visualization:
Each cylinder will have indication lights to determine the status of the cylinder. (Extended / Retracted)
The cylinder will also have red indication lights to reflect the differences between the current sequence and the next sequence step. This is visible when we are in jog mode.

Sequence Step/Teach – Building a PLC Program 6

When in a jog or teach mode the sequence step is visible, which indicates the current step that we are on. The inputs and outputs are displayed as a binary value which represents the actual inputs and outputs. This is valuable when troubleshooting and finding errors in the system. The set button is visible when in teach mode. When pushed the outputs and inputs are set for that step and the sequence will then increment.

The following is the code for each of the words that the DataScribers are reading. This includes the code to change the word into a 16-bit binary value.

Private Sub DataSubscriber1_DataChanged(sender As Object, e As Drivers.Common.PlcComEventArgs) Handles DataSubscriber1.DataChanged
'Label1.Text = Hex(DataSubscriber1.Value)Dim i As Integer = DataSubscriber1.Value
Label1.Text = Convert.ToString(i, 2).PadLeft(16, "0") '16 bits
End Sub

Private Sub DataSubscriber2_DataChanged(sender As Object, e As Drivers.Common.PlcComEventArgs) Handles DataSubscriber2.DataChanged
'Label2.Text = Hex(DataSubscriber2.Value)
Dim i As Integer = DataSubscriber2.Value
Label2.Text = Convert.ToString(i, 2).PadLeft(16, "0") '16 bits
End Sub

Private Sub DataSubscriber3_DataChanged(sender As Object, e As Drivers.Common.PlcComEventArgs) Handles DataSubscriber3.DataChanged
Dim i As Integer = DataSubscriber3.Value
Label3.Text = Convert.ToString(i, 2).PadLeft(16, "0") '16 bits
End Sub

Private Sub DataSubscriber4_DataChanged(sender As Object, e As Drivers.Common.PlcComEventArgs) Handles DataSubscriber4.DataChanged
If DataSubscriber4.Value = True Then
     Label1.Visible = True
     Label2.Visible = True
     Label3.Visible = True
     Label4.Visible = True
     Label5.Visible = True
Else
     Label1.Visible = False
     Label2.Visible = False
     Label3.Visible = False
     Label4.Visible = False
     Label5.Visible = False
End If
End Sub

We will now look at the PLC ladder program. The program is broken down into several parts as follows:

ACC Automation Sample PLC Program – Building a PLC Program 6

This is the main start / stop circuit of the program.
If we are in run mode the green light will be on. (MC9)
If we are not in jog mode (MC8) this circuit will be functional.

If we are not run mode (MC9) or in jog mode (MC8) then the stop mode is active.
This will turn on the red light. (MC10)

Run / Jog – Toggle Circuit
Flip Flop circuit to set the jog function

Move the outputs to the physical outputs when we go to run mode.

Indirect Addresses for the Program
V0 – Input pointer – 100 – 499
V1 – Output pointer – 500 – 999
V2 – Input pointer last step in the sequence
V3 – Output pointer last step in the sequence
V10 – Inputs to the sequencer
V20 – Outputs from the sequencer

Jog Mode – Jog Pushbutton

Reset the sequencer pointers. This will happen automatically in run mode or by hitting the reset button in jog or stop mode.

Teach Function – Building a PLC Program 6

This first rung will activate the values so we can manually turn them off/on with the HMI screen.

This will reset the pointers when going into teach mode.

This will set the teach point and increment to the next step.

Show the current step of the sequence.
Note: 0 is the first step

Set the inputs for cylinders.
The actual physical input points would be inserted here.
HMI inputs from the cylinders have a 500ms delay to simulate the movement of the actual cylinder.

Set the actual inputs/sequencer inputs/sequencer outputs so we can monitor this on the HMI.

Set the outputs for cylinders.
This will set the physical output points Y1 to Y7.
This will also set the HMI cylinders MC1 to MC7 (00001 to 00007)

Diagnostic Bits for indicating the difference for the inputs to the PLC. This will show up as a red indicator light on the cylinder represented on the HMI.

This section of PLC logic will mimic the inputs from the cylinders.
Cylinder 1 – Retract MC21 (00021) – Extend MC31 (00031)
Cylinder 2 – Retract MC22 (00022) – Extend MC32 (00032)
Cylinder 3 – Retract MC23 (00023) – Extend MC33 (00033)
Cylinder 4 – Retract MC24 (00024) – Extend MC34 (00034)
Cylinder 5 – Retract MC25 (00025) – Extend MC35 (00035)
Cylinder 6 – Retract MC26 (00026) – Extend MC36 (00036)
Cylinder 7 – Retract MC27 (00027) – Extend MC37 (00037)

Between each function of the cylinder, there is a time delay for the input to turn on and off of 500msec.

Cylinder 2 – Retract MC22 (00022) – Extend MC32 (00032)

Cylinder 3 – Retract MC23 (00023) – Extend MC33 (00033)

Cylinder 4 – Retract MC24 (00024) – Extend MC34 (00034)

Cylinder 5 – Retract MC25 (00025) – Extend MC35 (00035)

Cylinder 6 – Retract MC26 (00026) – Extend MC36 (00036)

Cylinder 7 – Retract MC27 (00027) – Extend MC37 (00037)

This is the end of the PLC program. You can see that the program is not very complicated once you break down the individual steps.

Download the PLC program and the Bin directory for the AdvancedHMI screen.

Watch on YouTube: Building a PLC Program that You can be Proud Of – Part 6b – Explaining the Program
If you have any questions or need further information please contact me.
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.