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NI9401 module: compatability with SENT (digital communication protocol): internal pull-down and external pull-up

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Good day,

 

Background

I am planning on using cRIO platform to decode a SENT signal (position sensor) and drive our product's motor while recording various analog input data.

 

This is to replace an existing custom embedded solution for future development. 

9063 (basic cRIO with 4 slots)

9401 (decode SENT position feedback signal)

9205 (analog inputs)

9505 (dc motor driver )

 

In our existing designs, we use the SENT J2716 approved electrical interface (in particular, a 51 kOhm pull-up). 

SENT_Network_with_NI9401.png
Though the acceptable values are 10-51Kohm as long as INPUT HIGH minimum / INPUT LOW maximum requirements are met.

 

My concern is the internal pull down of the NI-9401 module.

It has an internal pull-down of 47KΩ according to 

https://forums.ni.com/t5/Multifunction-DAQ/9401-pull-down-value/td-p/2908932

 

Question

With that pull down of 47KΩ, and pull up of 51 KΩ, I will get close to ~2.5V at the input pin of the NI9401 when there is no sensor output.

 

That's getting  close to an ambiguous value (NI 9401 minimum HIGH voltage is 2V) .

 

Is there anyway to disable to internal pull-down resistor?  Or any other recommendations for including an external pull-up with NI-9401 module?

 

I don't see any high speed C-Series modules with configurable pull-up/pull-down resistors. 

 

 

From my understanding, the NI-9401 is the module of choice for this application.

https://forums.ni.com/t5/Example-Program-Drafts/SENT-Communication-Protocol-Reference-Example-in-Lab...

 

Thanks in advance for your help!

 

Matt Lawson

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Solution
Accepted by topic author MattLawson

Hi Matt,

 

To my knowledge, there is no way to disable the internal pull-down resistor on this device. It is intended that the device be used in a system in which the input line is meant to be driven low when there is nothing driving it externally. Is it possible to use the SENT protocol in such a way that the digital line is low by default?

 

Alternatively, you could try wiring a smaller pull-up resistor so that most of the voltage drops across the internal resistor and keeps the reading high. Either way, I would recommend wiring the system up and tinkering with various designs to see what gives you the best performance.

 

Best,

 

Duncan W.

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Hi Duncan,

 

I suppose the input line value when the sensor is not driving HIGH or LOW (high Z) i.e. during a diagostic state or no connection, it is not that critical - as I will be doing the SENT signal decoding myself.  

 

Good idea, if I use a 10KOhm pull-up (minimum recommended by the spec), and the 9401 47kOhm, I will still have ~4V at the input pin. 

 

I will try a few combinations when I receive the module.

 

Thanks!

Matt

 

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