Hi guys

Is there some app note somewhere for calculating pull-up resistor values for external inputs. say for example the external switch is 20 meters in a commercial environment (in a 3V3 or 5V system)

I am also interested in the recommended pull-up resistor value for use in industrial environment.

Thanks guys!

 

Hi guys

Is there some app note somewhere for calculating pull-up resistor values for external inputs. say for example the external switch is 20 meters in a commercial environment (in a 3V3 or 5V system)

I am also interested in the recommended pull-up resistor value for use in industrial environment.

Thanks guys!

We need a little more info. Is the external switch mechanical, or solid state? Is it wet or dry contacts?
In any case, you'll most probably need resistors at both sides of the cable you'll be using.

Also, it'll be helpful if you take a look at this link:

https://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronics

 

Yes, there is an app note somewhere but it is probably easier to explain it.
You need to calculate the range of resistance, i.e. the minimum and maximum resistance required.

The max resistance = (Vdd - Vhigh) / Ihigh
where,
Vdd = supply voltage
Vhigh is in input logic-high threshold voltage
Ihigh is the input logic-high leakage current

The min resistance = (Vdd - Vlow) / Ilow
where,
Vdd = supply voltage
Vlow is in input logic-low threshold voltage
Ilow is the driver max output sink current

 

For long distances in industrial environments it becomes more critical because you are up against cable resistance and EMI. When switch contacts are involved, you need higher currents, 20mA being a common value.

Hence 20mA in 5V environment would call for 5V/20mA = 250Ω
200Ω or 220Ω pullup would be used.

For 3V environment, this is reduced to 100Ω to 150Ω.

 

Different devices call for different pull-up values, and even then the values may vary by conditions. For example, I'm working on some I2C stuff today and here is a pull-up resistor chart from one of the devices. Here you can see the pull-up value varies not only by the line capacitance, but also by the data rate. This is just an example of why more details are needed before choosing a pull-up value.

 

From RCA app note ICAN6602:



EDIT: Realized I didn't include the referenced figures:


It's for interfacing between CMOS and TTL, but the reasoning is similar.

 

For long distances in industrial environments it becomes more critical because you are up against cable resistance and EMI. When switch contacts are involved, you need higher currents, 20mA being a common value.

Hence 20mA in 5V environment would call for 5V/20mA = 250Ω
200Ω or 220Ω pullup would be used.

For 3V environment, this is reduced to 100Ω to 150Ω.

What is a common current(mA) value used in a commercial environment.

 

4-20mA interfaces are very common in commercial and industrial installations.

 

4-20mA interfaces are very common in commercial and industrial installations.

Is there a common values used in residential?

 

Dry contact switches require a minimum current called "wetting current" sometimes spelled "whetting current".
Again, 4-20mA is common across residential/commercial/industrial applications.