Programmable Logic Controllers (PLC)


Programmable logic controllers are very sophisticated industrial computers that accommodate a large number of input and output signals, used to sense and control industrial machines and environments. PLCs are available in wide ranges of sophistication and cost, depending on their control span and the speed. Resilience, power consumption, speed, and accuracy are key factors when selecting ICs for use in a PLC design.


2.75V to 4.8V Input, 10mA Output, 35μVRMS Ultra Low Noise and Ripple Boost Regulator


Compact boost with only 35µVRMS ripple and noise.

3.5V – 36V, 2A/3A, Synchronous Buck Converter with 15μA Quiescent Current and Reduced EMI


Reliable Operation (42V Input Transient Protection), Long Battery Life (15µA IQ) and EN55022 EMI Performance.

300mA, Buck Converter with 1.1μA IQ


This small, synchronous buck converter with integrated switches can deliver up to 300mA with input voltages from 3.5V to 24V, while using only 1.1μA quiescent current at no load.

Precision Uni and Bidirectional, Current-Sense Amplifiers


These ICs are single-supply, high-accuracy current-sense amplifiers with a high input common-mode range that extends from -20V to +75V.

0.4V to 5.5V Input, Synchronous Boost Regulator


Delivers 300nA IQ at 95% peak efficiency and reduces solution size by up to 50%

PLC System

Power Module Communications and Fieldbus Module Digital I/O Module CPU Module Analog Input Module Analog Output Module



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Power Module

Safety Isolated Main Power Circuit Protection Battery Backup Backplane DC-DC Temperature Fan Control CPU DC-DC LDO DC-DC LDO Signal Isolated Power Signal Acquisition Power

This diagram provides a simplified powercentric outline of PLC power requirements.

The PLC has a number of power domains: main power, backplane power, module power, and signal acquisition domain power. Maxim provides specialized voltage regulators for each domain.

Main power input can range from 24VDC to 240VAC or higher. The output power to the backplane must be regulated, cleaned, filtered, monitored, and protected from transients. Multiple backplane voltage rails must be established and battery backup circuitry must be incorporated. Total power output of a main power module ranges from hundreds of Watts for micro-PLCs to 2kW for large PLCs.

The CPU module and the various I/O modules require multiple low voltage rails. These are derived from backplane DC power. Commonly supported rails include: 24V, ±12V, 5V, 3.3V, and lower for CPU cores. Typical power usage of a CPU board is 50W to 200W while other modules typically use about 20W. Signal isolation power is typically derived from an I/O module's mid-voltage rail and is typically less than 10W.



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Communications and Fieldbus Module

Secure Authentication Voltage Monitors Backplane Interface Hot-Swap Control Watchdog-Timer Reset ICs UART Ethernet Signal Isolation DC-DC Power Profibus RS-485 CAN Fault Protection

In a complex automation system, multiple PLCs are used in a distributed control system (DCS) and they each have multiple connections. One or more PLC needs to communicate up to the monitoring level HMI (human machine interface) typically with non-time-deterministic protocols such as standard Ethernet. PLC to PLC communication must often be time-deterministic and uses industrial Ethernet. Between the PLC and smart sensors and actuators in the process, a "fieldbus" is used. It typically is NOT point-to-point allowing a single PLC fieldbus interface to communicate with multiple sensors and actuators on a process in a daisy-chain network configuration that minimizes wiring compared to a point-to-point or star configuration.

Today there are many fieldbus standards. Each standard has been designed to address a specific communication need.

Some popular fieldbus standards are: Profibus, Interbus, Modbus RTU, and ControNet are all based on RS-485; DeviceNet and CANOpen are based on CAN.



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Digital I/O Module

Secure Authentication Backplane Interface Hot-Swap Control Voltage Monitors Watchdog-Timer Reset ICs UART Digital Isolation DC-DC Power Binary Inputs IO-Link Binary Outputs

Digital input and output signals, also called binary signals, are typically high voltage, but relatively slow control signals. Digital input signals emanate from control or monitoring devices such as automated or manual switches or relays. Digital output signals drive various industrial control devices that perform a function based on the output state and include devices such as relays, motor actuators, on/off valves, solenoids, and indicators. Digital I/O signals can range in value from 0V (for off) to 60V (for on), though typically range from 0V to 24V. The voltage levels are relatively high so the intended state can be reliably determined by a receiver after the signal has traversed a long cable within an electrically noisy environment.

At the PLC, processing of digital input signals involves level translation and signal isolation. Digital output signals are also sent through isolation, and then fed to an output buffer/latch that performs level translation and circuit protection functions. Important factors to consider when designing a digital input or digital output PLC module include power dissipation and design size.   

With the advent of digital signal isolator circuits, today’s most efficient digital input and output board designs serialize multiple digital I/O signals and send the serialized stream through one or more digital signal isolator chips. Maxim provides both digital isolator ICs and function-specific digital input and output ICs for this application. Click the binary input and binary output blocks to view these products.  

I/O-Link is a more sophisticated protocol-based digital I/O control link and can communicate information beyond on and off. An I/O-Link device can communicate an actual value such as a temperature value.  Maxim manufactures an I/O-Link approved transceiver for this application.

Also see the fieldbus module above for more information about high-speed digital communications interface within a PLC.



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CPU Module

DC-DC Power Circuit Protection Clock Watchdog-Timer Reset ICs Backplane Interface Console Port Thermal Management Secure Authentication USB Indicator Drivers Switch Debounce Display Backlighting

The computing module within a PLC requires very high performance processing capabilities along with strong I/O capabilities. The module has to manage multiple real-time communication streams multiplexed between numerous I/O modules, while also computing real-time control-loop equations to adequately manage complex industrial processes.

Maxim provides numerous best-in-class ICs that increase the performance, robustness, and security within the compute module.



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Analog Input Module

Sensors RTD or Thermocouple to Digital Converters MUX Hart Modem Current-Loop Protection Scaling Op Amp Instrumentation Amp Active Filtering Amps Digipot Precision Resistors Switched C Filters Multichannel A/D Voltage Reference Single-Channel A/D Signal Isolation Secure Authentication UART Backplane I/F Watchdog-Timer Reset ICs Hot-Swap Control DC-DC Power

PLC analog input modules accept voltage and current-mode input signals and specialized modules can accept, for example, RTDs or thermocouples directly. Standard voltage inputs ranges include 0 to 5V, 0 to 10V, ±10V, though there are different ranges depending on the transducer type and the amount of amplification, if any, provided inside the sensor. Thermocouple input modules will see voltages in the tens of mV range, both plus and minus.

RTD modules must accept a range of resistance from near zero to thousands of ohms.

Likewise, 4–20mA current loops are an industry standard input interface. Current loop communication offers high EMI/RFI immunity and loss-free analog signal transmission over long distances (up to potentially kilometers) and so is a workhorse in industrial environments.

The choice of the analog input circuit components depends on many factors: The number and type of inputs to be handled, whether they need to be individually isolated or group isolated, sensor stimulation and/or biasing needs, multiple sensor sampling simultaneity, signal frequency range of interest, anti-alias filtering needs, AD converter resolution, and sampling rate required.



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Analog Output Module

Secure Authentication Backplane I/F UART Hot-Swap Control Watchdog-Timer Reset ICs DC-DC Power Signal Isolation Voltage D/A Voltage Reference Current D/A Digipot Precision Resistors Output Conditioning Hart Modem Current-Loop Protection

PLC analog output modules generally drive high-voltage analog actuators, like proportional control valves, as well as current loop driven actuators and gauges. Each board generally supports multiple channels depending on the type of outputs. These output channels can be individually isolated or group isolated.

In this block diagram we are showing primarily a serial architecture where the control information from the on-board processor passes serially through an isolator using an I2C or SPI bus that sends the signal to the assigned channel DAC for conversion to analog. For individually isolated outputs, each channel must have its own output DAC. For group isolated outputs, DACs followed by a de-mux with hold capability can be used to maintain the output until the next DAC update to that channel. After the DAC there is often additional calibration and output buffer circuitry to fine tune the signal and provide the necessary drive at the required voltage or current levels.

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