PLC/DCS I/O Module: Digital Input

I/O modules are building blocks of industrial control systems used in end equipment such as PLC (Programmable Logic Controllers), PAC (Programmable Automation Controller) and DCS (Distributed Control Systems). Digital input modules capture and measure digital input signals from a wide variety of sensors like proximity switches, limit switches and push button switches. Inputs typically are 0-24V with tolerances up to 30V.
Signal Chain
The signals from DC sensors used in PLC digital input (DI) modules are typically of a higher voltage and are usually isolated through opto-couplers before going into a shift register to be serialized. Instead, digital input serializers can be used to serialize a large number of digital inputs allowing higher channel densities in PLC I/O modules. The isolation is then limited to the SPI interface as a number of serializers can be daisy chained. They also provide the benefit of sensing higher voltages, adjustable current limits and programmable de-bounce times. Other functions that are integrated into these products are temperature sensing and voltage regulation.
Processor
Development platforms and software tools in addition to support which quickens time to market are highly desired. Connectivity options ranging from Ethernet ports to UARTs to simple SPI ports are desired. Ethernet enables integration of field devices to the corporate enterprise network. Most industrial control applications need simple low cost PLC's with lots of digital I/O and integrated functions. These include fault diagnostics, watchdog timers, low power, multiple SPI ports, UART's, integrated analog peripherals like ADCs and PWM outputs which perform similar functions to DACs. MCU's (Micro Controller Units) are a good choice for these applications. For applications which push capabilities and need higher loop rates along with advanced control algorithms to handle multiple functional domains like logic, PID control and motion, higher performance MPU's (Micro processing Units) are needed.
Industrial applications need to support a multitude of communication options. Many of TI's processors include a Real time Processing Unit (PRU) to support a truly programmable approach for these communication standards. In addition, the ARM architecture offers the performance and low power needed for mission critical functions.
Interface
There are a number of field bus options – both serial (RS-485, DeviceNet, Profibus, CAN etc.) and Ethernet based. Extended cable reach and diagnostics to debug wire breaks provides system designers significant value. Integrated digital isolation on the interface transceivers is highly desirable to reduce board space. In addition the interface between the inputs or input serializer and the controller can be digitally isolated either by using opto-couplers or TI isolators built with capacitive technology.
Microcontroller
The Hercules™ Safety MCUs offer an ARM Cortex-R4F based solution and are certified suitable for use in systems that need to achieve IEC61508 SIL-3 safety levels. These MCUs also offer integrated floating point, 12 bit ADCs, motor-control-specific PWMs and encoder inputs via its flexible HET Timer co-processor. Hercules Safety MCUs can also be used to implement scalar and vector-control techniques and support a range of performance requirements.
Power  
To protect against transients and ground loops, the field side which interfaces to sensors is electrically isolated from the control side. The design can be customized for performance and cost by using a DC-DC converter and discrete components or pre-built isolated DC-DC converter modules can be used. Other considerations include high efficiency and integration along with smaller packages. Picking a DC-DC converter with a large input range will protect again supply transients.

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