Simplify Your PLC Design with a Digital I/O IC

Aug 4, 2020

Timothy Leung  By: Timothy Leung
 Business Management Director; Maxim Integrated 
Josh Fankhauser  and Josh Fankhauser
 Business Manager, Industrial Communications; Maxim Integrated  

Whether your industrial application is a robot, factory automation equipment, or part of a utility distribution control system, you'll need to meet the same common requirements: deliver more functionality in increasingly smaller spaces, with the flexibility to adapt to ever-changing requirements.

Consider the automated factory. As intelligence moves to the edge, smart sensors need to be ubiquitous in order to enable machines to engage in real-time decision making. With the factory humming along, networked programmable logic controllers (PLCs) continuously monitor inputs from hundreds or even thousands of these sensors, sending signals to output devices. (Digital inputs take high-voltage inputs from binary sensors, digitize the signals, and then relay the information to the controller. Digital outputs are used to drive various high-voltage (24V and up) loads on the factory floor, such as relays, lamps, valves, and other actuators.) As such, these sensors as well as the PLCs or motor control systems have to be tiny to fit into existing enclosures, dissipate little heat, and be power efficient.

Automated Factories Rely on Small, Thermally Efficient ComponentsAutomated factories such as this vehicle manufacturing line benefit from components that are small and thermally efficient.

Electronic marshalling makes it easier to connect this expansive set of field I/O devices back to the controller. Replacing the more traditional wired marshalling approach, electronic marshaling presents a new signal routing methodology in process automation. In wired marshalling, technical staff on the factory floor route multi-core cables from the field to the right side of the terminal blocks in a marshalling cabinet. The same happens in electronic marshalling; however, in the I/O room, each terminal block is connected to the appropriate controller I/O channel electronically instead of manually. Physical wiring changes are no longer needed to connect an I/O device to a specific controller.

A PLC typically has either a digital input channel or a digital output channel. To change from one type to another, the physical card for the channel must be changed. The total number of digital input and digital output channels is limited by how many of each type of card exists in the rack that houses the cards in an electronic marshalling system. Digital input/digital output (DIO) devices, however, provide the flexibility to be configured, on a per-channel basis, as a high-side switch, a push-pull driver, or a Type 1 and 3 or Type 2 digital input. Using such a device eliminates the need for discrete chips, whether it's a thermistor for temperature control, or power- and current-hungry optocouplers, resistors, or bulky MOSFETs for the digital input.

Another consideration is safe demagnetization of the actuator's load. Many actuators have high inductance. When they're turned off, you have to wait for the energy to spin off the coil in order for it to be used again; otherwise, the peripheral or the chip itself can sustain damage. One way to address this is to limit the maximum load inductance value. Alternatively, a DIO that provides safe demagnetization prevents such damage during turn-off or discharge of the load.

To recap, some of the key challenges you'll need to address when designing a PLC or motor control system include:

  • Supply tolerance: 65V supply tolerance is required to provide supply headroom during surge testing of a 24V supply input
  • Flexibility: The high cost and space consumed by mechanical connectors presents a burden on the design
  • Power dissipation: Reducing the power dissipation in the module will allow you to shrink the size of the enclosure

New DIO with SafeDemag Technology

Maxim Integrated has launched its newest DIO product, the MAX14906, which addresses the key PLC and motor control system design challenges. A quad-channel device featuring SafeDemag technology for fast turn-off of unlimited inductance, the MAX14906 operates with a supply voltage of up to 40V and is tolerant to 65V. It can be configured on a per-channel basis as a high-side switch, push-pull driver, or a Type 1 and 3 or Type 2 digital input. In a PLC, the 7mm x 7mm device can be used in the DIO modules, replacing both the digital input card and the digital output card. Low RDSON and ICC reduce power dissipation in the module. The device complies with IEC 61131-2.

The SafeDemag feature in the MAX14906 means that any value inductive load can be safely discharged. By contrast, most digital inputs on the market have a fast demagnetization feature, but in these cases, there can be thermal runaway where too much power is dissipated too quickly, burning leads or the chip itself. The device also has integrated features such as wire-break detection and other diagnostics. Wire-break detection provides an alert when there's a short on an output line and notification where there are opens. In addition, the digital output current limit is selectable between 130mA and 1.2A, which eliminates wasted energy and enables you to stay within your power budget.

You can evaluate the MAX14906 for your next PLC, motor control, or distributed control system by buying the MAX14906EVKIT evaluation kit. The kit is available for $59, is fully assembled and tested, and is RoHS compliant.