ISO1212: Isolation forum

Hi Siva,

Unfortunately we do not have a device that will automatically set threshold values internally. You will need to choose the correct values of Rth, Rsense, and potentially Rshunt in order for the ISO1212 to work with your system's input thresholds. This calculator document is useful for calculating the resistor values based on the voltage thresholds of your system.

Respectfully,

Lucas Schulte

Hi Lucas,

Thanks for the info.

The spread sheet and the equations in the datasheet are also confusing,

All 3 inputs have only one threshold values, if the voltage is above this then the signal is valid otherwise is false.

For example: low level for type 3 is between -3V and 11V, and anything above (11V to 30V) is valid according IEC-61131-2.

The formula gives two nominal levels; one (11.01V) for Vin high and other one (9.81V) for vin low, in fact there should be only one threshold level, which is 11V.

(I have inputted 562, 1.25, 10000, & 24 in the spreadsheet)

Thanks

Siva

Siva,

I'm sorry to here that the equations are confusing. Hopefully I can help clear things up.

It is common for digital switching to have a defined high threshold and a separately defined low threshold. This can be seen in the above figure were there is an area that exists in between on and off states. While theoretically a single threshold might work, it would not be at all tolerant to noise in the system. This margin that exists between on and off allows for the toleration of noise. This article can help explain this concept.

Respectfully,

Lucas Schulte

Hi Lucas,

Thanks again for your help.

There is a dead band, and during this time the output won’t change until the input goes in opposite direction from the previous value. Do you mean this is what this IC does?

For example, when the input starts from low level, the output stays low. And when the input reached the higher threshold the output will go high. But if the input signal is decided to go low but not below the lower threshold, the output will stay high. This happens in the opposite direction also. We do this by adding hysteresis in the analogue circuit.

But in our case, the standard states only one threshold, when the input signal is above 11V the output is high, and the output is low when the input signal is below 11V, not 9.81V.

Thanks

Siva

Siva,

The ISO1212 does have hysteresis and it is specified as VHYS in the datasheet. It is typically 1.2V. Does this address your concern?

Respectfully,

Lucas Schulte

Hi Lucas,

No, this device won’t detect the threshold levels mentioned in the IEC61131-2 standard.

Normally we add a hysteresis in the circuit to avoid the noise issues, but in the standard, there is no mention about the hysteresis, or two bands of limits for the higher level and the lower level.

Thanks for your help.

Siva