PLC Sample Assignment Structure FOR REFERENCE ONLY

Introduction

Introduction

This report is a study to find out what the following PLC requirements mean in

laymen’s terms. The document will contain information that will be broken down in

the form of explanations and drawings of each of the following.

Capacity

Program Memory (words) 6.5 k

Ladder Memory (words) 3.5 k

CMOS RAM

UVPROM

EEPROM

Total I/O 1152

Local / Local Expansion 640

Remote 512

Interrupt Input 8pt

Remote I/O channels 2

I/O module point density 8 / 16 / 32

I/O module slots per base 4 / 6 / 8

Performance

Scan time / K (Boolean) 3.0 ms

Functionality

Run Time edit

RLL

Built-in Communications 1x 15 Pin RS232C, 9600 baud Port

1x 25 Pin Networking Port, RS232C or RS422, baud

Rate selectable via CPU dipswitch.

Instructions 113

Control Relays 440

Timers 128

Counters 128

Real-Time Clock / Calendar

Speciality modules

Telephone / radio modem

Modbus slave

Modbus Master

High Speed PID

Thermocouple

12 bit Analogue Input/ Output

Magnetic Pulse Input

100kHz high-speed Counter

Report

And

Analysis

Example of a rack or rail mounted PLC.

Capacity

Program Memory (Words) 6.5k

The program memory is all the information that is stored in the PLC unit; this may

take the form of Ladder Memory, CMOS RAM (CMOS Random Access Memory,

UVPROM, EEPROM (UVP, EEP Read Only Memory). The PLC unit uses the Read

Only Memory information for start-up before programming. After programming in

day-to-day running both Read Only Memory and Random Access Memory are

utilised within the system. There are several different types of memory in a PLC unit

and the amount of memory is measured in words. Smaller memory is measured in

bits, nibbles and bytes, 1 bit, 4 bits = nibble, 8 bits = byte, 16 bits = 1 word, 32 bits

(double word). There are 16 bits of information to one word therefore 6.5k x 1024 =

6656 words 6656 x 16 = 106496 bits of information.

Ladder Memory (Words) 3.5k

The Ladder Memory contains a list of instructions called upon in order to write ladder

programs and is where the ladder programs that have been written for special

purposes are stored and saved. The PLC unit then utilises the Ladder program to run

the system that is connected to it. Again this will be 3.5k x 1024 = 3584 words 3584

x16 = 57344 bits of information.

Below is an example of a typical ladder program utilised for programming PLC’s.

CMOS RAM

CMOS RAM stands for Complimentary Metal Oxide Semiconductor random access

memory. This is a battery powered form of memory that will only span the life of the

battery, then the information will be lost until the battery is replaced. This memory is

very small and contains information that changes from day-to-day such as real time

clocks, dates and months. The CMOS RAM also contains system start-up information

therefore the system will not run without this memory. Changes can be made to the

parameters of this memory (date and time), but the initial base program remains the

same and cannot be erased in day-to-day use.

UVPROM

UVPROM stands for Ultraviolet Programmable Read Only Memory. The ultraviolet

form of memory programming is an outdated form of reusable chip for the Read Only

Memory. This memory is permanent and can only be erased by UV, so when

programming is done and a mistake is made in the program the only way to erase the

mistake is to place the processor in an ultraviolet light compartment whereby the

complete memory will be erased, only then can programming be started from the

beginning again.

EEPROM

EEPROM stands for Electrically Erasable Programmable Read Only Memory. In

layman’s terms this means the memory of the PLC unit can be programmed or the

program erased by using electrical voltage (5 volts to program approx 6 volts to erase)

instead of a UV light source, this makes the unit reusable and without having to

remove the chip makes this method cost effective. Although the EEPROM memory

does need a special programming device in order to program the Read Only Memory

this will have been done by the manufacturer prior to purchasing.

Total I/O 1152

The I/O is an abbreviation and in layman’s terms stands for Input / Output. The Inputs

and Outputs are the points where cables of devices on a system run by PLC are

connected to the CPU (Central Processing Unit) and the unit’s slaves (Remotes). The

Inputs and Outputs are normally referenced to on a ladder program with the letter P

and a number following (e.g. P1, P55, P134); this is called addressing the devices. An

Input (or information provider) may take the form of a thermostat, external clock or

even a pulse sensor that pulses every time an object passes in front of it. This then

would be linked to the Input terminal and programmed to a counter within the CPU to

be shown on the ladder program as a counter. An Output (or controllable device) is

used in the same way as an Input, but on an Output the CPU controls the devices

actions (on / off, move etc). The 1152 is the number of Inputs / Outputs on the

complete system. The Inputs are normally the lower numbers and the connection

points are found at the top of the PLC and visa versa for the Outputs.

Local / Local Expansion 640

Local / Local Expansion refers to the combined number of Input and Output

connections on the Master CPU unit 640 in total, the CPU unit may have an even

number of Inputs and Outputs (320 Inputs, 320 Outputs) or odd number of Inputs and

Outputs (440 Inputs, 200 Outputs) as the split of these connections is not known the

information will have to be obtained before the PLC is obtained.

Remote 512

The remotes or slaves (2 off) on the PLC system have a combined number of Inputs /

Outputs 512 in total. Once again the split of the Inputs / Outputs has not been

documented on the specification sheet and therefore more information must be

obtained.

Interrupt Input 8pt

The Interrupt Input module is able to interrupt a CPU’s scan at any point and stop the

program in hand to operate a high speed counter or to move to another part within the

same program, or move to another program altogether which might be stored in the

memory but with different commands relating to specific events that may have

happened or are in the process of happening within the system, (i.e. ”interrupts the

sequential execution of the ladder program” A. J Crispin). The specification states that

this is an 8-point Interrupt Input therefore the program can be interrupted in 8

different places. The Interrupt Input comes as a separate module that connects to the

base unit.

Remote I/O channels 2

The Remote I/O channels as is understood are the slave units by the Master CPU. As

the specification states there are 2 Remote (slave) units on this system. The Remote

(slave) can be installed with a the master unit to increase the number of Inputs /

Outputs on the system, or if one PLC is utilised in different areas some distance from

the master unit the Remote (slave) can be placed closer to the points were the unit is

needed therefore cutting down on the number of cables that the system will be

utilising, therefore making this method more economical or cost effective.

I/O module point density 8/16/32

The modules on a rail type PLC unit contain Input / Output connection points.

Therefore the point density is the amount or number of Input / Output connection

points on a single module that fits into the base unit. As seen on the specification the

point density of the modules are 8 connections per module, 16 connections per

module, 32 connections per module.

I/O module slots per base 4/6/8

As is now known rack or rail type PLC’s come in a module form, module slots per

base means how many modules can clip into a base as unit to form the master PLC

unit. The PLC unit does not need to have all the slots filled with modules in order for

the unit to work, blank fillers may be obtained to fill the spaces in the base, therefore

a larger unit may be obtained if later updates are planed for the system. The

specification states that three types can be obtained; a 4 slot per base, a 6 slot per

base, and an 8 slot per base.

Performance

Scan time / K (Boolean) 3.0ms

The scan time on a PLC system is the amount of time the PLC takes to loop scan the

complete program 3.5k rung by rung, and therefore making periodic checks on the

states of the Inputs within the program, and in so doing the CPU will be able to make

changes to the states of the Outputs in the program and therefore execute the changes

on the running equipment in the field. On this particular system the specification

states the scan time to be 3ms. Boolean is a form of algebra that is useful when

analysing switching circuits like ladder diagrams.

Functionality

Run Time edit

Run Time edit means that the PLC ladder program can be modified or updated while

the program is in use. This allows the programmer to find the faults that may be in the

program and make modifications without having to shut the system down.

RLL

Relay Ladder Logic is the form of language or form of writing that is utilised when

designing a ladder program. Ladder logic initiated in the electrical industry when

relays were used to control equipment; therefore the relays in Relay Ladder Logic do

much the same thing within the PLC. The PLC’s internal relays are not physical

relays but simulated relays on a ladder rung in the ladder program. These internal

relays can be used to activate or de-activate devices within a system it is operating.

The input relays or contacts and the output relays or coils on the ladder rung are

physical relays as they send a signal to the devices on the system that is being

controlled by the PLC, and therefore the devices relay contacts are made and / or

broken on external devices

Built-in Communications 1x 15-pin RS232C, 9600-baud port

1x 25-pin Networking port, RS232 or RS422,

The 1x 15-pin RS232C, baud port and the 1x 25-pin Networking port, RS232 or

RS422 are connection points that allows interface between two systems or pieces of

equipment (male & female plug and socket). The male is normally on the cable end

and the female on the equipment side, but not always as all systems do differ. Not all

the pins are used in every application as this all depends on the systems requirements

in the action being performed. These connectors or plugs and sockets are also known

as D connectors. The D connector is not only found on PLC’s, they can also be found

on personal computers (PC) and most equipment that is used with a PC (printers,

scanners etc). Therefore connectors can be associated as part of the network highway.

The RS232C is limited to shorter distances (approx 15 meters) as noise begins to limit

the numbers of bits per second transferred through the cable, the RS422 is utilised for

longer distances as this method utilises a balanced form of transmitting with two lines,

the noise affects both lines equally thus having no affect on the transmitted signal.

The make up of these cables are governed by standards. This standard says that each

pin on the port should have a special purpose and this purpose will be the same on all

PLC’S, PC’S and equipment utilised by them.

Hewlett Packard developed the Ethernet or IEEE-488 standard; this was done to link

computers and instruments and was known as the General Purpose Instrument Bus.

The list of pin numbers is as follows.

Pins 1-4 Data lines.

Pin 5 End of message or identify

Device.

Pin 6 Data Valid (device identified).

Pin 7 Not ready for Data. (Devices

ready to accept Data).

Pin 8 Not Data Accepted. (Device

informs Data being accepted).

Pin 9 Interface Clear.

Pin 10 Service Request.

Pin 11 Attention.

Pin 12 SHIELD.

Pin 13-16 Data.

Pin 17 Remote Enabled.

Pin 18-24 Ground / Common.

As can be seen by the above, the standard set-up should remain the same on all the

GPI Bus’s that are produced within the IEEE (Institute of Electronic and Electrical

Engineers).

Baud Rate selectable via CPU dipswitch

Baud rate stands for the number of bits per second exchanged in data transfer or the

speed in which data is transmitted, so 1 baud = 1 bit per second. The specifications

state the PLC does 9600 baud; therefore 9600 bits per second are transmitted. The

DIP switch allows the baud rate to be adjusted to suit the devices and the PLC as the

PLC will only work on a digital signal (sequence of pulses at normally 0-5 Volts) and

not Analogue (signal size related to the amount of info being sent) and not Discrete

switching (just on / off signal) therefore the DIP switch is essential to the running of

the PLC.

Common Baud Rates used with RS232C

50

75

110

150

300

600

1200

2400

4800

25 Pin Networking Port

25 Pin Network Port

Telephone connection

Baud rates continued

9600

19200

38400

76800

Instructions 113

The instruction is found in the ladder program and this is normally done in order to

tell the machine or device what to do next (e.g. move / compare / end). Due to there

not being a standard for abbreviations all PLC’s do not utilise the same instruction

abbreviations, therefore IMO will not be the same as Allan Bradley. The instruction is

always found on a ladder program after an open or closed contact on the right hand

side of the ladder or rung in a small box. In this PLC unit 113 different instructions

are found (e.g. move / compare / end / timer on / timer off). As is known different

PLC companies utilise different instruction abbreviations, therefore a list of

instructions within the PLC manuals would generally accompany the unit.

Control Relays 480

The Control Relays do have a few different names that companies utilise e.g. flag,

auxiliary relay, coil or as it is addressed within a ladder rung M but not all, once again

companies use different addresses. The relay or coil is able to switch devices on and

off, themselves being switched on and off within a running program. A device on an

output can be addressed to the internal relay, which is symbolised in the ladder rung

with ( ) and can addressed to for example M111. As can be seen in the specifications

this particular PLC contains 480 relays and in theory this would mean 480 devices can

be addressed to a coil / relay I / O’s permitting.

Timers 128

The timers within the PLC come standard and can be either delay on timers, which is

the most commonly used timer or delay off timers. The timers in a PLC can vary from

1millisecond to 1 second and the use of these timers depends on the operation being

performed within a system. In the specifications it is stated that this particular type of

PLC contains 128 timers, therefore 128 different devices can contain timers, or a

device can be timed out 128 times.

Counters 128

The PLC comes standard with Counters installed, and as with timers this system has

128 Counters. The specifications do not specify how many up counters, down

counters and up/down counters the PLC contains; these counters can also be standard

or high-speed counters. They can be programmed to work off light pulses to count up

and down. The specification also does not state how many of these counters are

volatile and how many are not, this means when the system is powered down some of

the counters will reset and some will not, (these are non-volatile counters) this may

cause problems if a counter resets when it is not meant to, so the correct counter must

be selected by the programmer.

Real Time Clock / Calendar

The specification states that this particular PLC unit contains a real time clock and

calendar. The “real” in this statement means that the clock and calendar within the

PLC is working on the same day-to-day time as a normal clock and calendar (time

and date are always changing). The clock and calendar are very useful if the system is

to shut down at certain times and start up again at certain times. The real time clock

and calendar as mentioned earlier in this report are located in the CMOS RAM and

are kept working (even if the system is shut down) with a battery based micro chip,

therefore the clock and calendar will remain accurate for as long as the battery lasts,

then the battery can be replaced and the clock and calendar reset.

Speciality Modules

Telephone / radio modem

The telephone and radio modem may be the best way of running a system, as a

programmer will be able to monitor and work on a system without having to actually

physically be by the system, all modifications to the running of a system may be done

via telephone therefore costs will be cut time wise. Below is found a simple diagram

of how a radio and telephone system can make the running of a plant a lot simpler. In

this diagram it is seen that a master modem has been used to drive slave or remote

units via radio and telephone. The radio modem is ideal for remote areas where there

are no telephone cables and no mobile telephone towers to receive a signal. Below is

found a prime example of radio / telephone communications.

Modbus Master, Modbus slave

The Modbus is a type of standard or Protocol messaging structure of a PLC that is

made for Master-Slave communication “communication between intelligent devices”.

The Modbus protocol states that when the Master sends a signal to the Slave the Slave

will contain an address within the master therefore the signal will be sent to this

address. As is known companies who develop PLC’s all have different PLC standards

or protocol’s to which they follow. Therefore the equipment or devices that link or

work off the PLC Master and Slave must follow the same standard in order for the

equipment or devices to work. If the standard is not followed and the equipment or

devices are not compatible problems may arise within the system or the system may

not work at all. Therefore the Master and Slave must be of the same standard or

protocol. Below is a list of a few different standards from other companies.

Alan Bradley Micrologix

Mitsubishi Medoc

Siemens Profibus

Telemacanique Interbus

Toshiba Toshibaline

Telephone

Telephone Link up

Radio link up

High Speed PID

PID stands for Proportional Integral Derivative. PID is a combination of controls and

is used in various fields, to control airflow, water flow, pressures, temperatures and

many other fields. PID controls the output frequency of an inverter according to a PID

calculation. PID works by reducing the error and bringing a motor as close to a set

value as possible without actually going passed that set value and not to far below the

set value. This is done by adjusting the output signal to the correct deviation. Below is

diagram that will help in the explanation of PID.

Thermocouple

The thermocouple is a temperature sensor that consists of two dissimilar wires

forming a junction. When this junction is heated the thermocouple converts the

temperature into voltage thus registering at the PLC as a voltage. The thermocouple is

not a very accurate system as this device is only reliable within 1 degree Celsius

though it is the most widely used temperature-sensing device as the output voltages

are very predictable and they can be utilised at most temperatures. But not all

thermocouples register the same voltage at the same temperature so each

thermocouple will have a list of calibrations of voltage to temperatures.

Fused joint on thermocouple

Hot junction

Cold junctions

Cold junctions x 2

Hot junction

12 bit Analogue Input / Output

A typical analogue Input / Output will accept either voltage or current signals. The

voltage works on 0 – 5, and the system is configured to accept 4 – 20 milliamps.

When the analogue signal reaches the PLC input it is converted into a digital signal

(binary) by an analogue-to-digital converter, therefore an analogue-digital converter

card can be fitted to the PLC module allowing this conversion to take place on the

inputs and on the outputs.

Magnetic Pulse Input

The magnetic pulse input is a module that directly receives magnetic pulse pickups

from equipment like turbine meters and tachometer signal generators. This

information can then be configured within the module to read rate of flow or volume

of flow. The module can also be configured to provide a direct indication of speeds

like the RPM (Revolutions Per Minute) of the turbine blades, the information can then

be stored in a non-volatile memory in the PLC for easy access at any given time for

updates ect.

100kHz high-speed counter

The high-speed counter is normally fitted to a section of machinery where there are

fast moving parts e.g. a spinning shaft on a motor or fan blades. This counter normally

works off digital pulses through a cable. These counters can be linked to an interrupt

device on the PLC allowing the count to take place in the ladder program by

interrupting the scan, although most PLC’s do not need an interrupt device to allow

the count to take place and the count will not be slowed down while the program is

running, due to the counter circuit being separate, provided the buffer memory is

accessible to the ladder program so that it can be read at a point when the CPU needs

the count value in order to perform the next operation.

High-speed counter module

Installation diagram of a typical PLC module

Inputs & OutputsInputs &

outputs

Below are 3 types of rack or rail type PLC bases

Summary

Summary

This report contains information about a PLC unit specification. This document will

be helpful in ascertaining the different abbreviations and meanings on the PLC

specification form that was received from the PLC supplier. All aspects of the

specification document have been broken down into easily readable layman’s terms

and have therefore been made understandable. The document will be helpful for use

in explaining the terms and conditions that go with the rack mounted modular PLC

unit. There are drawings included in this report to help with the understanding of a

PLC unit as the drawings show the inputs / outputs, the type of ladder program

utilised in programming, also shown is a similar type of module used in rack mounted

PLC’s and how to install the module.

Discussion

Discussion

In this document we have discussed the rack or rail mounted modular PLC. The pros

of the rack or rail mounted PLC will be the main topic as not enough information was

provided in order to look at the cons, (e.g. the PLC make and / or supplier) therefore

no other PLC of a better make, design or quality could be taken into account. The rack

mounted PLC contains modules that can be added and removed depending on the

needs of the system that the PLC will be installed on. The modules that can be added

or removed from the PLC come with a variety of functions, for example high-speed

counters (counting fast moving objects), magnetic pulse input modules (rates RPM),

telephone / radio modems (for linking up modems and PLC’s over long distances),

thermocouples (calibrating voltage to temperature) etc. The rack or rail mounted PLC

can also operate a number of remote or slave units in this scenario the master only

operates two slaves, therefore adding to the amount of Inputs / Outputs on the PLC

system. The slave units can as we know also be placed at a distance from the Master

control unit therefore reducing the number of cables that need to be laid and therefore

reducing the cost of a particular system.

Conclusion

Conclusion

In conclusion it is known that the PLC can be utilised for numerous functions in the

day-to-day running of a plant, as the PLC can be programmed to perform repetitive

and / or detailed work with limited or no supervision. The PLC can also do the work

with little or no mistakes and has an ability to respond almost instantaneously and

without thought or hesitation, therefore removing the human error factor arising from

boredom, distractions and or thought behind the actions. The PLC is also able to

perform a number of functions at the same time, therefore removing the need for

manual labour and over staffing of a plant or factory, in turn cutting the costs of

running said factory / plant. The rack or rail mounted PLC is prime example as this

PLC can be programmed to fulfil the needs of a particular job description and the

work will be accurately repeated.

References

References

References Programmable logic controllers W. Bolton

An introduction

Second edition

Printed 2001

Programmable logic controllers Alan J Crispin

And their

Engineering applications

Second edition

Printed 1997

HTI News Article Internet

Programmable logic controllers

Application programs (Oct 1997)

By Eugene Kowch

PID consultant

PID Controls Internet

PID Explained

Automation Direct Internet

Technical support

PLC Direct Internet

Quick designer

Modbus technical overview Internet

Triconex Internet

References continued

Y.K Malaysia Internet

Programmable Logic Controllers

Explained

RACO Remote alarms and controls Internet

Telephone/radio modems

Explained on water treatment

Plants

Transtronics Internet

EEPROM, EPROM,

UVPROM Explained

Telebyte Internet

ROM / RAM

CMOS RAM Explained

Contents

Contents Page No

Report and Analysis 1 – 14

Summary 15 – 16

Introduction 17 – 19

Conclusion 20 – 21

Discussion 22 – 23

References 24 – 26