PLC/DCS I/O Module: Analog Output

Analog Output for PLC/PAC/DCS I/O Modules – System Block Diagram

Block Diagram

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Design Considerations

PLC/DCS I/O Module: Analog Output

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). Analog output modules are used to drive and control actuators and other machines and form part of an overall control system. Current-loop outputs typically range from 4-20mA while voltage outputs range from 0-5V, 0-10V, +/-5V and +/-10V.

Signal Chain

The linearity, settling time, response and accuracy is determined by the DAC used. Resistor string based DAC's are small, inexpensive and inherently monotonic. For higher resolution and for applications requiring higher linearity like open loop valve control, R-2R DAC's are a good choice. Finally for achieving the fastest settling times and speeds, R-2R multiplying DAC's should be used. Selecting the right reference and pairing it with the right buffer can impact system performance. Considerations for references include initial accuracy, temperature drift and source/sink current capability.

The output amplifier must be picked based on speed, output voltage level and power. TI's XTR family of industrial drivers is a good choice to implement 4-20 mA current loops. Some products allow the designer to make the output programmable to be either a voltage or current. Newer products integrated the DAC and XTR together to minimize board space and optimize power.

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 DAC 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. This is done on a per channel basis or by isolating groups of channels from each other and 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. Finally, using LDO's with good PSRR (Power Supply rejection Ratio) to supply the precision analog circuitry will reduce the power supply ripple and preserve system accuracy and resolution.

Application Notes (11)

Title Abstract Type Size (KB) Date Views
HTM 9 KB 28 Feb 2013 5936
HTM 8 KB 26 Oct 2012 3687
HTM 8 KB 26 Oct 2012 3322
HTM 8 KB 26 May 2010 2297
HTM 8 KB 28 Oct 2009 3383
PDF 190 KB 12 Oct 2007 3996
PDF 177 KB 10 Oct 2005 1554
HTM 8 KB 14 Feb 2005 1750
HTM 8 KB 02 Oct 2000 1813
HTM 8 KB 02 Oct 2000 4968
HTM 8 KB 27 Sep 2000 1878
    

Reference Designs

Description Part Number Company Tool Type
AM1810 Profibus Evaluation Module TMDXEVM1810 Texas Instruments Reference Designs

Selection and Solution Guides

Selection Guides (1)

Title Abstract Type Size (KB) Date Views
PDF 4.16 MB 23 Jul 2009 8738

Solution Guides (5)

Title Abstract Type Size (KB) Date Views
PDF 4.07 MB 12 Nov 2013 5065
PDF 896 KB 12 Nov 2013 7484
PDF 1.08 MB 31 Oct 2013 7661
PDF 599 KB 30 Oct 2013 3737
PDF 2.13 MB 30 Oct 2013 3385

Tools and Software

Name Part # Company Software/Tool Type
ADS1248 Performance Demonstration Kit ADS1248EVM-PDK Texas Instruments Evaluation Modules & Boards
Code Composer Studio (CCStudio) Integrated Development Environment (IDE) v5 CCSTUDIO Texas Instruments SW Development Tools, IDEs, Compilers
PROFIBUS on AM1810 ARM® MPUs and OMAP-L138 DSP+ARM Processors PROFIBUS Texas Instruments SW Development Tools, IDEs, Compilers

Product Bulletin & White Papers

White Papers (2)

Title Abstract Type Size (MB) Date Views
PDF 522 KB 06 Feb 2013 778
PDF 2.62 MB 17 Nov 2011 2950

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