Modern industrial systems have a multitude of sensors, switches, buttons and other low-frequency inputs that need continual monitoring. The network of input devices that an industrial system (such as a programmable logic controller [PLC] or a building controller) needs to support has grown in complexity, in both size and functionality. The ability to monitor these inputs and react to them promptly is a critical component of most industrial systems.
Over the years, industrial system designers have developed numerous engineering solutions for monitoring inputs from switches, sensors and buttons that are robust enough for harsh industrial operating environments. One common approach, because of its simplicity, is using a combination of discrete components such as transistors and resistors. This approach, however, has numerous shortcomings, especially in more modern designs, which include poor scalability with input channel count, the inability to support a range of switch and sensor types with a single implementation, and a lack of programmable control.
A good example of Multi-input monitoring devices is TI’s TIC12400 family, a Multi-Switch Detector Interface (MDSI), which enables the monitoring and control of many inputs through a single device. Figure 1 shows an example use case for that device.
Multi-input monitoring devices like this can support more than 50 inputs, depending on configuration, with a single device. A similar setup using a discrete implementation would require approximately 200 discrete components, all of which need appropriate selection, sizing and matching in order to meet specific performance targets (see Figure 2). In addition to the increased design complexity, multichannel discrete solutions consume more board space.
The reduction in board space gained from the use of multi-input monitoring devices means that you can include additional functionality in your designs that would otherwise not be possible. A reduction in board space also enables differentiated designs with smaller or unique form factors. Having fewer points of failure within an industrial design helps reduce the overall system failure rate and associated repair and maintenance costs. You can easily scale your input-monitoring solutions while minimizing design complexity and design risk with such input monitoring devices.
Size comparison between discrete and integrated multichannel input detection.
Another challenge when using discrete input-monitoring design approaches is how to seamlessly accommodate and manage a variety of input types. For example, a system with multiple resistor-coded switches, analog sensors and contact switches would require you to complement your discrete design with many additional components. You would need to add analog-to-digital converter (ADC) functionality to help resolve the resistor-coded switch inputs (see Figure 3), as well as the analog sensor inputs. In addition, discrete designs need updating to help source the wetting currents that the contact switches require. You would need to perform additional engineering work in order to ensure that the wetting currents are adjustable over time, as the contact switch resistance changes during the application’s lifetime.
A good integrated solution would have built-in ADCs to accommodate resistor-coded switches and other similar analog inputs, greatly simplifying the input-monitoring circuit for these types of inputs. For inputs that don’t need analog-to-digital conversion, you can a built-in comparator with adjustable thresholds for digital input monitoring. Being able to source selectable wetting currents per input, which further reduces design complexity in supporting contact switches is also a feature that designers may want. The ability of an input monitor to support higher-voltage (up to 24V) inputs from resistor-coded switches, analog sensors and contact switches (in addition to traditional digital inputs) enables you to reduce design complexity, often resulting in substantially lower system costs and faster development times. The TI solution has all of these capabilities for designers.
Resistor-coded switch support used to enable input sharing.
Modern industrial systems are incorporating a higher level of programmability, leading to greater functionality. Discrete input-monitoring and detection design approaches, by their nature, have a low level of programmability, and as a result, limited operational flexibility. In contrast, integrated switch monitoring solutions are highly programmable. Designers will want to programmatically control input-polling times and thus optimize system power dissipation. Achieving granular control of polling times with a discrete solution would require a large investment in engineering time and higher system bill of materials (BOM) costs. See Figure 4 for an automotive application of an MDSI.
In an automotive design nowadays, there are a large number of switches (more than 100, with that number increasing in the future with Electric Vehicles or EVs). The switch status detection is usually done discretely by using microcontroller GPIOs. (See the outlined section in Red)
Simplicity of use that involves just setting a few register values via Serial Peripheral Interface (SPI) control is a feature that designers will like for their system. The potential savings in engineering time can be substantial when adjusting switch-monitoring operating parameters programmatically versus implementing changes in hardware. A system using a device like the TI device can have its operating parameters changed and verified in a matter of days rather than months.
Having many programmable functions, including wetting current settings, wetting current diagnostics and fault detection will enable users to get their design to market quicker with good reliability. The real power of this programmability is that you can experiment with multiple design approaches without having to waste time re-engineering discrete components.
This high level of programmability enables designers to develop systems that are more robust and efficient for your target industrial application. You could potentially derive future system implementations by simply changing firmware rather than starting an entirely new design from scratch. New multi-input monitoring devices like TI’s TIC12400 and TIC10024 allows system designers to scale and differentiate their designs, without compromising on robustness they have come to love from discrete based implementations approaches of the past.
About the author
Atul Patel is a product marketer and new business development manager for industrial markets within TI’s Standard Logic Product Line. Atul has more than 20 years of systems and marketing experience with analog and mixed signal devices covering a board spectrum of markets including Industrial, Automotive and Telecom. He has a bachelor of science degree in computer engineering as well as an MBA from the University of Central Florida.