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