Designing an efficient Programmable Logic Controller using Programmable
System On Chip
By Raja Narayanasamy, Product Apps Manager Sr, Cypress Semiconductor Corp.
A Programmable Logic Controller (PLC) is a microcontroller-based, general-purpose electronic device to control the operation
of a machine or process. Contrary to conventional microcontroller based systems, PLCs are not programmed by the device
manufacturer but by the machine builder or the end user. Some of the applications that use PLCs include:
•
•
•
•
•
•
Automatic test equipment
Pick and Place machines
Machine controls
Home automation
Elevator systems
Factory automation
PLCs are classified into following the main categories based on the number of input and outputs they can handle:
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Nano PLC: PLC that has less than 32 input/output (I/O) functions.
•
Micro PLC: PLC that has more than 32 Input/Output (I/O) points but not more than 128 input/output (I/O) functions.
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Small PLC: PLC that has more than 128 Input and Output (I/O) points but not more than 256 I/O functions. It does not
include any I/O enhancements fitted along with the basic system.
•
Medium and Large PLC: PLC that controls a large number (>256) of discrete elements using very fast input/output (I/O)
scan times.
According to a recent study conducted by the ARC advisory group, the PLC market reached nearly $9.0 billion in 2007 and is
forecasted to grow to over $12 billion in 2012.
Programming PLCs
PLCs are programmed using a Ladder diagram and are shipped with custom front-end software that allows the final users to
program them. These users are typically nonprogrammers who do not have any C or Verilog/VHDL programming knowledge.
Ladder logic programming looks similar to the wiring diagram of the electromechanical relay-based control circuits that PLCs
replace. Consider the example of a staircase light control using two switches. The staircase bulb should come ON if any one of
the switch is operated and if any one of the switch is operated again the bulb should go OFF. Figure 1 shows the control
implemented as a wiring diagram using two switches (switch 1 is on the lower floor of the staircase and switch 2 is installed at
the upper floor of the staircase) and a light bulb.
Switch 1 and Switch 2 are in the OFF position in Figure 1. If the Switch 1 is actuated, then the upper circuit will switch ON the
bulb. If switch 2 is actuated it will open the upper circuit and close the bottom circuit. However, switch 1 is already open at the
bottom circuit, so the bulb will not glow. Actuating switch 1 again will close the bottom circuit and the bulb will glow.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
Published in EE Times Design (http://www.eetimes.com/design
Page 1 of 7
January 2010
Figure 1: Wiring diagram of a staircase bulb control using two switches
Figure 2 shows the same control implemented using a PLC and ladder logic. The hardware switch contacts are replaced by -||-- (normally open contact) and --|/|-- (normally closed contact) symbols.
Figure 2: Ladder logic diagram of staircase bulb control implemented in a PLC
Figure 3 shows the same control implemented using logical gates (NOT, AND, OR) in a PLD, FPGA, or a System On Chip
(SoC) with integrated PLD functionality. Note that all the gates shown in Figure 3 can be replaced by one XOR gate.
Fig.3 Staircase bulb control logic implemented using “gates”
PLC Principle of Operation
Most PLCs on the market are microprocessor-based circuits. These PLCs read the status of all the inputs (i.e., switches) and
then execute the user-written ladder logic program to determine the final update for the outputs (i.e., bulb). Figure 4 shows the
the various components used to build a conventional PLC-based control system.
The PLC inputs are isolated and level translated using opto-isolators and connected to the ports of the microcontroller. The
PLC outputs are connected to buffers and relays to enable connecting the same to output elements as shown in the picture
below.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
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January 2010
AC mains
PC with
programming
software
RS232
24V dc
power
supply
Relay board
Micro PLC
Inputs
Outputs
From
Push buttons,
limit switches,
pr.switches
etc.
To Solenoids,
Power
contactors,
Motors, Lamps
etc.
Figure 4: Conventional PLC Block diagram
The flow chart in Figure 5 shows how the PLC operates.
Figure 5: Conventional PLC program execution flow
Limitations of Conventional PLCs
The sequential program execution used in conventional PLCs takes around 10 ms or more to complete. At this rate, they are
suitable for control applications where the input signal frequency is below 100 Hz. The scanning time is also limited by the
length of the program. For example, if you want to read a speed sensor input to measure a speed at around 1200 Rotations
Per Minute (1200/60 = 200 Hz signal frequency), a microcontroller-based PLCs cannot measure the speed correctly using this
input. Such a system may need a custom input module with decoder or counter ICs that can read the signals at high frequency
and convert them into a count value to pass to the microcontroller. Alternatively, consider controlling a flow control Solenoid
value with a PWM signal at 10 KHz frequency. Due to the above mentioned limitation, the PLC cannot output this directly and
needs a custom output module with PWM generators. Adding such high-speed counter modules and PWM generator
modules will increase the PLC cost by 2 to 3 fold.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
Published in EE Times Design (http://www.eetimes.com/design
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January 2010
PLCs that have digital programmable logic like CPLDs or FPGAs can address frequency issues. FPGAs, however, lack built-in
analog capabilities. For example if you need to measure temperature using an analog temperature sensor, you cannot
interface directly to the FPGA. Moreover, FPGAs are too expensive a component to be used in these kinds of applications.
SoC-based PLC
To address the needs of low-cost PLCs, developers require a device that can handle high-speed digital input, high frequency
output, and also to directly handle analog signals. Today, there are many System On Chip (SoC) devices that combine a
microcontroller with integrated programmable logic and configurable analog blocks to achieve this in a cost-effective manner.
For example, these SoCs can implemented an HDL-based quadrature decoder that uses two 90 degree phase-shifted sensor
signals to detect the speed and direction of rotation of a motor by reading speed/position sensor signals at high speed.
Likewise, a PWM waveform generator can be implemented using on-chip programmable digital resources. Any timer and
counter modules required for normal machine control operations can also be implemented in the same way. These
components (quadrature decoder, PWM generator, timers, counters, and so on) can be coded once by a programmer and
made available as configurable library components. This allows end users with no HDL experience to program the PLC using
these components with no low-level HDL/C programming or without having to learn the low-level architecture of the device.
Apart from this, other control functions can be implemented using AND, OR, and NOT gates that are available as standard
components in a SoC.
An Example System
Figure 6 shows the components of a SoC suited for PLCs, including CPU, configurable analog blocks (comparators, op-amps,
ADCs, DACs), and programmable digital blocks (PLD-based). With these components integrated into a single device,
developers can build a single-chip, low-cost PLC that can overcome the speed limitations of standard PLCs and still provide
ease-of-use programming for end users.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
Published in EE Times Design (http://www.eetimes.com/design
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January 2010
Figure 6: SoCs for PLC applications, such as the Cypress PSoC, combine a microcontroller with programmable
digital and analog blocks.
PLD-based Universal Digital Blocks (UDBs) are for used to implement gate-based logic while configurable analog components
like ADCs, DACs, Op-amps, and comparators handle the analog signals. An integrated CAN controller block can be used to
interface more than one PLC on a network to support more input and outputs. A built-in USB controller can be used as a
programming and debugging interface for the PLC.
With a programmable SoC architecture such as the Cypress PSoC, any internal analog or digital signal can be routed to any
GPIO (General Purpose Input/Output) pin. This flexibility enables PLC users to implement various machine control functions
using a single PLC across designs. Consider a product line with Machine “A” (10 digital inputs, 2 analog inputs, 7 digital
outputs, and 1 analog output) and Machine ”B” (12 digital inputs and 8 digital outputs). A PLC with 12 inputs and 8 outputs can
be configured by firmware to control either of these machines. This is the major advantage of using a mixed-signal
programmable device as a controller for PLC applications.
PC with
PSoC
Creator
For
expansion of
IOs
0-10V
analog
out
CAN tranceiver
Opamp
SWD
24V dc
power
supply
24V
to
12V
PSoC3
Opto isolaters
12V
to
5V
Relay driver
Relays
Inputs
From
Push buttons,
limit switches,
pr.switches.
To Solenoids,
Power
contactors,
Motors, Lamps.
Figure 7: Block diagram of a PSOC-based PLC with integrated relay board and programmer
For example, if a PID speed control loop needs to be built for controlling a DC motor using a quadrature velocity encoder for
speed feedback, it is possible to use the same SoC-based PLC subsystem. The PLD-based Universal Digital Blocks (UDB)
can be configured as a quadrature decoder using a pre-built component to read the velocity signal at very high speeds (> 100
KHz) and one more UDB can be configured as a PWM generator to generate a pulse width modulated signal at the required
frequency (i.e., 32 KHz) to control the MOSFET H bridge to control the DC motor speed and direction. Together, these can be
developed as a single component so that the end user who has to program the PLC can just drag and drop these components,
configure the parameters for each, and begin using the PLC to control a motor.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
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January 2010
Networked PLC
To control a machine that has many input and outputs requires the ability to network many PLCs to function as a single, large
PLC. This can be achieved by supporting a CAN bus interface between PLC modules to support more inputs and outputs (see
Figure 8).
Figure 8: PSoC-based modular and scalable PLC network
Such an architecture allows developers to build a networked PLC that can control a bigger system with many inputs and
outputs. In addition, many industrial control applications utilize touchscreens for control panels for machine tools. Integrated
touchscreen and configurable HMI (Human Machine Interface) capabilities can further consolidate cost by eliminating separate
and expensive custom touch screen-based HMI panels.
Ease-of-Use Programming
Developers may also be able to utilize a silicon vendor’s development tools for end user PLC programming instead of having
to create a custom ladder logic programming application. For example, Cypress’ “PSoC Creator” programming software
allows users to visually program PSoC-based PLCs from a schematic-level view using the AND, OR, and NOT gate
components. As a result, users do not need C or VHDL/Verilog programming knowledge to program the PLC to its full
capabilities. Figure 9 shows the staircase switch logic implemented using the PSoC Creator. The gate-level implementation
can be an alternate to ladder-based implementations, allowing users to take advantage of all of a system’s features.
Designing an efficient Programmable Logic Controller using Programmable System On Chip
Published in EE Times Design (http://www.eetimes.com/design
Page 6 of 7
January 2010
Figure 9: Staircase switch implementation using PSoC Creator.
Cypress Semiconductor
198 Champion Court
San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
© Cypress Semiconductor Corporation, 2007. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the
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Designing an efficient Programmable Logic Controller using Programmable System On Chip
Published in EE Times Design (http://www.eetimes.com/design
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January 2010