Industrial digital enter chips present serialized information by default. Nevertheless, in techniques that require actual time, low latency, or greater pace, it might be preferable to offer level-translated, real-time logic indicators for every industrial digital enter channel.
So, some industrial digital inputs pattern and serialize the state of eight 24-V present sinking inputs below SPI or pin-based (LATCH) timing management, permitting for readout of the eight states through SPI. A serial interface is used to reduce the variety of logic indicators requiring isolation, which is especially useful in excessive channel rely digital enter modules.
Serialization of logic indicators makes use of simultaneous sampling of the indicators in order that the indicators turn into time quantized. Which means real-time data content material is misplaced, which may be of concern in sure techniques. Examples are purposes the place timing variations between switching indicators are of concern, similar to incremental encoders or counters.
These purposes both necessitate the usage of high-speed sampling with high-speed serial readout or the usage of non-serialized parallel information, as supplied by the MAX22195, an industrial digital enter with parallel output. Utilizing the MAX22190/MAX22199 industrial digital enter gadgets with parallel operation offers the advantage of diagnostics and configurability.
This text delves into the traits, limitations, and design concerns relating to methods for producing parallel logic outputs with industrial digital inputs.
Design particulars
The approach relies on repurposing the eight LED outputs to perform as logic indicators. LEDs serve to offer a visible indication of the digital enter’s state—helpful for set up, upkeep, and in service. The traits and specs of business inputs are clearly outlined within the IEC 61131-2 normal, with the output state being binary in nature: both on or off.
The MAX22190/MAX22199 chips function energyless LED drivers that energy the LEDs from the sensor/swap within the subject, not drawing present/energy from an influence provide within the digital enter module. These gadgets restrict the enter present to a degree settable by the REFDI resistor. That is achieved to attain the bottom energy dissipation within the module.
For the frequent Kind 1/Kind 3 digital inputs, the enter present is usually set to a degree of ~2.3 mA (typ) to be bigger than the two.0 mA minimal required by the IEC normal. The ICs channel many of the ~2.3 mA subject enter (IN) present to the LED output pins, and solely ~160 µA are consumed by the chip.
With the LED drivers being present outputs, not voltage, the present must be transformed to voltage for interfacing with different logic gadgets like digital isolators and microcontrollers. Resistors are the only trans-resistance ingredient for this function, as proven in Determine 1.
Determine 1 LED pins are used as voltage-based logic outputs. Supply: Analog Gadgets Inc.
Utilizing the LED output pins on this method is just not documented within the product datasheets. This text investigates the traits and potential limitations.
LED pin traits
When utilizing ground-connected resistors on the LED pins to create voltage outputs, the next must be thought-about:
- What’s the most voltage allowed on the LED pins?
- Is there interplay/suggestions from the LED_ pin to the IN_ pin?
- Particularly, does voltage on the LED pins lead to a change of the enter present, as minimal present ranges are mandated by the IEC requirements?
- Do the LED output currents present undesired transient conduct, similar to overshoots or gradual rise/fall occasions?
- Are the LED outputs appropriate to be used as high-speed logic indicators when the inputs swap at excessive charges?
- Are the LED outputs filtered (as programmable by SPI)?
The MAX22190/MAX22199 datasheets’ absolute most rankings specify the utmost allowed LED pin voltages as +6 V. This means that the LED pins are appropriate to be used as 5 V (and three.3 V) logic outputs, with the caveat that the voltage will not be greater than 6 V.
The affect of the LED pin voltage on different vital traits must be evaluated. Of explicit concern is the change of the enter present with the presence of excessive LED pin voltages, as the present is specified by the requirements. The vital case is with the sector voltage near the 11 V on-state threshold voltage, as outlined for Kind 3 digital inputs.
Determine 2 exhibits the measured subject enter present dependence on the LED pin voltage for 3 subject enter voltages near the 11-V degree: 9 V, 10 V, and 11 V. The ten-V and 9-V ranges had been chosen as these are throughout the transition area for Kind 3 inputs, and their enter currents haven’t any outlined minimal, whereas the minimal for the 11 V enter case is 2 mA.
Determine 2 Discipline enter present depends on the LED pin voltage. Supply: Analog Gadgets Inc.
With the sector voltage on the 11-V threshold, the blue curve exhibits that the enter present begins lowering when the LED voltage is greater than ~5.8 V. The present lower is simply 0.6% at 6 V. For instances of 9 V and 10 V, that are within the transition the place the currents usually are not outlined, the measurements present that the enter present remains to be above 2 mA for as much as 5.5-V inputs.
In conclusion, this exhibits that the MAX22190/MAX22199 will produce 5-V LED logic outputs (in addition to decrease voltage logic like 3.3 V) and nonetheless be appropriate with Kind 3 digital inputs. For Kind 1 digital inputs, the case is trivial for the reason that on-threshold is far greater at 15 V, which means that the LED pins may even present 5-V logic ranges with none affect on the sector enter present.
Parallel operation instance
Determine 3 exhibits a 10-kHz subject enter (yellow curve) with the ensuing LED output voltage in blue. A 1.5-kΩ resistor was used on the LED output, which offers a 3.3 V logic sign. Glitch filtering was disabled (default bypass mode).
Determine 3 In 10-kHz switching, Channel 1 has subject enter and Channel 2 has LED output. Supply: Analog Gadgets Inc.
Concerning the transient conduct of the LED output present below switching circumstances, Determine 3 exhibits a case of 10-kHz switching. A 1.5-kΩ resistor was used to transform present to voltage. The scope shot illustrates that the LED outputs don’t produce transient overshoots or undershoots that would injury logic enter gadgets. The rise and fall occasions are quick and don’t result in sign distortion.
Utilizing the SPI interface
The MAX22190/MAX22199 gadgets function SPI-programmable filters to allow per-channel glitch/noise filtering. Eight filter time constants as much as the 20-ms degree can be found in addition to a filter bypass for high-speed purposes. The chosen noise filtering additionally applies to the LED outputs to make the visible illustration according to {the electrical} indicators.
Diagnostics are supplied through SPI, like low energy provide voltage alarms, overtemperature warnings, short-circuit detection on the REFDI and REFWB pins, and as wire-break detection of the sector inputs.
The facility-up default state of the register bits is:
- All eight inputs are enabled
- All enter filters are bypassed
- Wire-break detection is disabled
- Brief-circuit detection of the REFDI and REFWB (solely MAX22199) pins is disabled
Therefore, the SPI interface doesn’t must be utilized in purposes that don’t require glitch filtering (for instance, for high-speed indicators) and diagnostics. In instances the place the per-channel selectable glitch/noise filtering is required or diagnostic detection is needed, SPI can be utilized.
The LED output waveform doesn’t present overshoots or different undesired irregularities similar to various voltage within the on-state. This illustrates that the LED outputs can be utilized as voltage outputs. Its traits and limitations are investigated.
Glitch filtering
The MAX22190 and MAX22199 gadgets present per-channel selectable glitch filtering. The next content material demonstrates the impact of the glitch filters on the LED outputs by instance of a 200-Hz switching sign with filter time set to 800 µs. Outlined glitch widths had been emulated by altering the obligation cycle. Each constructive and unfavorable glitches had been investigated.
Determine 4 exhibits an instance of 750-µs constructive pulses being filtered out by the 800-µs glitch filter. So, constructive glitch filtering works each for the LED outputs in addition to the SPI information.
Determine 4 Right here is an instance of constructive glitch filtering. Supply: Analog Gadgets Inc.
Unfavourable glitches are, nonetheless, not filtered out on the LED outputs, as proven in Determine 5, the place a 750-µs falling pulse propagates to the LED output. This differs from utilizing the SPI readout, for which each constructive and unfavorable glitches are efficiently filtered.
Determine 5 This picture exhibits unfavorable glitch filtering. Supply: Analog Gadgets Inc.
Determine 6 exhibits the LED output sign with an 800-µs glitch filter enabled and enter switching with a 50% obligation cycle. The rising edges are delayed by ~770 µs whereas the falling edges present no delay. This illustrates that the filters don’t work correctly with the LED outputs.
Determine 6 This picture highlights the filtering impact on LED output. Supply: Analog Gadgets Inc.
Excessive frequency switching
For purposes with excessive switching frequencies, low propagation, or low skew necessities, glitch filtering can be disabled. In bypass mode (glitch filters) and 100-kHz enter, the LED output ends in the waveforms proven in Determine 7.
Determine 7 The 100-kHz enter switching is proven with filter bypass. Supply: Analog Gadgets Inc.
Whereas the falling edges present low propagation delay of ~60 ns, the rising edges have important propagation delay in addition to jitter. The rising edge jitter is within the vary of ±0.5 µs with a median propagation delay of ~1 µs. The rising delay and jitter are as a result of ~1 MHz sampling documented within the datasheet. Sampling doesn’t happen on the falling edges, therefore the quick response.
This illustrates that the LED outputs have rise time/fall time skews of as much as ~1.5 µs with jitter. Channel-to-channel skew is low on the falling edges however a lot greater on the rising edges. This might restrict the usage of the LED outputs in some purposes.
Design concerns
This part discusses some concerns required when utilizing the LED output pins as voltage outputs.
Be sure that the MAX22190/MAX22199 current-drive LED outputs are voltage restricted to not exceed the secure ranges of the logic inputs that they drive. Whereas the REFDI resistor units the sector enter present to a typical present degree, the precise enter present has a tolerance of ±10.6%, as specified within the datasheets. Thus, the voltage throughout the resistor shall be within the ±10.6% vary.
Logic inputs usually have tightly specified absolute most rankings, like VL + 0.3 V, the place VL is the logic provide voltage. When interfacing two logic indicators, a typical VL provide is commonly used to make sure matching as normal logic outputs have push-pull or open-drain outputs whose most output voltage is outlined/restricted by a logic provide, VL.
One could make the everyday LED pin’s output voltage decrease to make sure that absolute most rankings usually are not exceeded for the enter. Alternatively, one can take into account that the LED pin’s ~2.3 mA output present is not going to injury a logic enter, as these are generally specified for tolerating a lot greater latch-up currents, within the 50 mA to 100 mA vary. This must be verified for the system into account. The third, much less engaging, possibility is to restrict the voltage by clamping.
Commonplace logic outputs are push-pull and thus low impedance, offering excessive flexibility in driving logic inputs. In distinction, the LED outputs are open-drain outputs the place the pull-down resistor with parasitic capacitance determines the switching speeds.
With out further capacitors, switching charges of 100 kHz and better are possible.
The MAX22190/MAX22199 industrial digital inputs can be utilized as an octal enter having eight parallel outputs, regardless of being documented for serialized information operation. To this function, the LED drivers, initially supposed for visible state indication, are repurposed as voltage-based or current-based logic outputs. When utilizing parallel operation on this method, the usage of the SPI interface is elective and offers all of the diagnostics in addition to system configurability with some limitations.
Wei Shi is an purposes engineer supervisor within the Industrial Automation enterprise unit of Analog Gadgets primarily based in San Jose, California. She joined Maxim Built-in (now a part of Analog Gadgets) in 2012 as an purposes engineer.
Reinhardt Wagner was a distinguished engineer with Analog Gadgets in Munich, Germany. His 21-year tenure primarily concerned the product definition of latest industrial chips within the areas of communication and enter/output gadgets.
Editor’s Be aware
This text was written in cooperation with Chin Chia Leong, senior workers engineer for {hardware} at Rockwell Automation.
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