Original question:
Hi Team,
My customer has question about ISO1211 formula. Below is IEC 61131-2 type-1 standard. For type 1, the turning point is 15V between VIH and VIL. If use our below formula, VIH is not equal to VIL. If customer want to use 15V, what's formula we can use?
Example :
VIH (15V) = 8.25 + Rthr * 2.25mA*562 / 562 >> Rthr is 3Kohm.
If we input 3Kohm in to VIL formula, we can't get 15V. Which formula we can use?
Hi Bo,
Thanks for reaching out.
Please note that the standard IEC 61132-2 requires that Type 1 inputs should recognize an input voltage >15V as HIGH and inputs <5V as LOW. The standard has undefined area between 5V and 15V so that the Schmitt thresholds can be set in this region by the systems.
Having both VIH and VIL at 15V is not a good design as if there is a high frequency noise riding on a 15V input could cause the output to go HIGH/LOW repeatedly based on noise frequency. i.e., if the input is 15V and the noise of 200mV peak-to-peak makes the input signal swing between 14.9V to 15.1V with a frequency of 1MHz, then the output will switch HIGH and LOW at a rate of 1MHz. This is not desirable.
Hence, the VIH and VIL thresholds are set with some hysteresis between them so that there is sufficient noise margin. Hence, please use RTHR = 2.5kΩ and RSENSE = 562Ω to meet Type 1 requirements (as recommend in section 9.2.1.2.1 in datasheet, snapshot below). These values will achieve a VIL of 12.7V and VIH of 13.9V. These thresholds will guarantee that any signal <5V is treated as LOW and any signal >15V is treated as HIGH. Choosing RTHR = 2.5kΩ will also help protect the solution against surge by limiting the input current, please see section 9.2.1.2.5 in datasheet for more details.
I hope this helps you understand the thresholds, thanks.
Regards,
Koteshwar Rao
