Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

[Introduction]When the wave of Industry 4.0 is sweeping, smart sensors are becoming more and more popular in the factory environment. Widespread use of sensors is bringing an important change to handling a lot of IO, either digital or analog, within older controllers. Therefore, building high-density IO modules with controllable size and heat becomes the key. In this article, ADI will focus on digital IO.

Typically, digital IO in PLCs consists of discrete components such as resistors/capacitors or with independent FET drivers. In order to minimize the size of the controller, and the requirement to be able to handle 2 to 4 times the number of channels, these are driving the transition from discrete to integrated solutions.

In addition, the discrete approach has a number of drawbacks, especially when each module handles eight or more channels. In fact, when it comes to high heat/power consumption, a large number of discrete components (in terms of size and mean time between failures (MTBF)), and the need for reliable system specifications, it’s enough to show that a discrete approach is not an option. Row.

Figure 1 shows the technical challenges faced when building high-density digital input (DI) and digital output (DO) modules. In both DI and DO systems, size and heat dissipation issues need to be considered.

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 1. Considerations for Digital Input and Output Modules

For digital inputs, also note that it supports different input types, including 1/2/3 type inputs, and in some cases, 24V and 48V inputs. Reliable operating characteristics are very important in all cases, even open circuit detection is critical.

For digital outputs, the system uses a different FET configuration to drive the load. The accuracy of the drive current is often an important consideration. In many cases, the diagnosis must also be considered.

The following will explore how integrated solutions can help address some of these challenges.

Designing High Channel Density Digital Input Modules

Traditional discrete designs use a resistor divider network to convert the 24V/48V signal into a signal that the microcontroller can use. Discrete RC filters can also be used in the front end. If isolation is required, external optocouplers are sometimes used.

Figure 2 shows a typical discrete approach to building a digital input circuit.

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 2. Traditional Digital Input Design Using Discrete Logic

This type of design is suitable for a certain number of digital inputs, i.e. 4 to 8 per board. Beyond that number, the design quickly becomes impractical. Such a discrete approach poses various problems, including:

● High power dissipation and associated board hot spots.

● One optocoupler is required for each channel.

● Too many parts will result in a low FIT rate and even require larger parts.

What’s more, the discrete design approach means that the input current increases linearly with the input voltage. Assume a 2.2KΩ input resistor and 24V VIN. When the input is 1, for example, at 24V, the input current is 11mA, which equates to a power consumption of 264mW. The power consumption of the 8-channel module is greater than 2W, and the power consumption of the 32-channel module is greater than 8W. See Figure 3 below:

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 3. Estimated power consumption of a digital input module built with discrete logic

From a thermal standpoint alone, this discrete design cannot support multiple channels on a single board.

One of the biggest advantages of the integrated digital input design is the significant reduction in power consumption, which reduces heat dissipation. Most integrated digital input devices allow a configurable input current limit to significantly reduce power consumption.

When the current limit is set to 2.6mA, the power consumption is significantly reduced, about 60mW per channel. 8-channel digital input modules can now be rated below 0.5 watts, as shown in Figure 4 below:

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 4. Estimated Power Savings for Digital Input Modules Using Integrated DI Chips

Another reason against using discrete logic designs is that sometimes DI modules have to support different types of inputs. The standard 24V digital input specifications published by IEC are divided into Type 1, Type 2 and Type 3. Types 1 and 3 are often used in combination because their current and threshold limits are very similar. Type 2 has a 6mA current limit, which is higher. With the discrete approach, a redesign may be required since most discrete values ​​need to be updated.

Integrated digital input products typically support all three types. Essentially, Types 1 and 3 are generally supported by integrated digital input devices. However, to meet the minimum 6mA current requirement for a Type 2 input, two channels need to be used in parallel for one field input. And only adjust the current limiting resistor. This requires a board change, but the change is small.

For example, the current limit of ADI’s DI devices is 3.5mA/channel. So, as shown, when using two channels in parallel, if the system must have a Type 2 input, the REFDI and RIN resistors need to be adjusted. For some newer parts, the current value can also be selected using pins or through software.

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 5. Using 2 Channels in Parallel to Support Type 2 Digital Inputs

To support a 48V digital input signal (not a common requirement), a similar process is required, an external resistor must be added to adjust the voltage threshold on the field side. Set the value of this external resistor so that the pin’s “current limit value * R + threshold value” must meet the voltage threshold specification on the field side (see the device data sheet).

Finally, since the digital input module is connected to the sensor, the design must meet the requirements of reliable operating characteristics. These protection features must be carefully designed when discrete solutions are used. When choosing an integrated digital input device, be sure to determine the following according to industry specifications:

● Wide input voltage range (eg, up to 40V).

● Ability to use field power (7V to 65V).

● Can withstand high ESD (±15kV ESD air gap) and surge (typically 1KV).

It is also useful to provide overvoltage and overtemperature diagnostics so that the MCU can take appropriate action.

Designing High Channel Density Digital Output Modules

A typical discrete digital output design has a FET with driver circuitry that is driven by a microcontroller. FETs can be configured in different ways to drive microcontrollers.

The definition of a high-side load switch is that it is controlled by an external enable signal and connects or disconnects the power supply from a given load. In contrast to the low-side load switch, the high-side switch supplies current to the load, while the low-side switch connects or disconnects the load’s ground connection to draw current from the load. While they both use a single FET, the problem with low-side switches is that there is a potential short circuit between the load and ground. The high-side switch protects the load from shorts to ground. But low-side switches are less expensive to implement. Sometimes the output driver is also configured as a push-pull switch, requiring two MOSFETs. See Figure 6 below:

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 6. Different Configurations Used by Digital Output Drivers

Integrated DO devices can integrate multiple DO channels into a single device. Because of the different FET configurations used for high-side, low-side, and push-pull switches, different devices can be used to implement each type of output driver.

Built-in degaussing of inductive loads

One of the key advantages of an integrated digital output device is the built-in degaussing capability for inductive loads.

An inductive load is any device that contains a coil that, when energized, typically performs some mechanical work, such as solenoid valves, motors, and actuators. The magnetic field caused by the current can move the contacts of a switch in a relay or contactor, to operate a solenoid valve, or to rotate the shaft of an electric motor. In most cases, engineers use high-side switches to control inductive loads, and the challenge is how to discharge the inductance when the switch is open and current is no longer flowing into the load. Negative effects of improper discharge include: possible arcing of relay contacts, large negative voltage spikes that damage sensitive ICs, and high frequency noise or EMI that can affect system performance.

In discrete solutions, the most common solution to discharge inductive loads is to use a freewheeling diode. In this circuit, when the switch is closed, the diode is reverse biased and does not conduct. When the switch is turned on, the negative supply voltage across the Inductor forward biases the diode, causing the stored energy to decay by directing current through the diode until a steady state is reached and the current is zero.

For many applications, especially those in the industrial industry with multiple output channels per IO card, the diode is often large in size, resulting in a significant increase in cost and design size.

Modern digital output devices implement this functionality within the device using an active clamp circuit. For example, ADI employs a patented SafeDemag(TM) feature that allows digital output devices to safely shut down loads without being limited by inductance. For more details, please visit the website to view the application note here.

There are several important factors to consider when selecting a digital output device. The following specifications in the data sheet should be carefully considered:

● Check the maximum continuous current rating and ensure that multiple outputs can be paralleled if needed for higher current drives.

● Ensure that the output device can drive multiple high-current channels (over temperature). Refer to the data sheet to ensure that the on-resistance, supply current, and thermal resistance values ​​are as low as possible.

● The output current drive accuracy specification is also important.

Diagnostic information is very important to recover from some out-of-range operating conditions. First, it is desirable to obtain diagnostic information for each output channel. These include temperature, overcurrent, open circuit and short circuit. From an overall (chip) perspective, important diagnostics include thermal shutdown, VDD undervoltage, and SPI diagnostics. Find some or all of these diagnostics in integrated digital output devices.

Programmable Digital Input/Output Devices

By integrating DIs and DOs on an IC, configurable products can be built. This is an example of a 4 channel product that can be configured as input or output.

Building High Channel Density Digital IO Modules for Next-Generation Industrial Automation Controllers

Figure 7.4 Configurable DI/DO Products for Channel Implementation

It has a DIO core, which means that a single channel can be configured as DI (Type 1/3 or Type 2) or digital output in high-side or push-pull mode. The current limit on DO can be set from 130mA to 1.2A. Built-in degaussing function. To switch between Type 1/3 or Type 2 digital inputs, only one pin is required, no external resistors are required.

Not only are these devices easy to configure, they are also rugged enough to operate in industrial environments. This means high ESD, supply voltage protection up to 60V and line ground surge protection.

Thus, this is an example of what is possible with an integrated approach (configurable DI/DO modules).

in conclusion

When designing high-density digital input or output modules, once the channel density exceeds a certain number, the discrete approach becomes meaningless. Integrated device options must be carefully considered for thermal, reliability and size considerations.

When choosing an integrated DI or DO device, some important data points must be noted, including reliable operating characteristics, diagnostics, and support for multiple input-output configurations.

About Analog Devices

ADI is the world’s leading high-performance analog technology company dedicated to solving the toughest engineering design challenges. With outstanding detection, measurement, power, connection and interpretation technology, build intelligent bridges between the real and digital worlds, thereby helping customers to re-understand the world around them. For details, please visit ADI’s official website www.analog.com/cn.

Author: Yoyokuo