Hysteresis on T100MD-888 Digital Input

[ccdubs]

Hi all,

Is there a hysteresis on the T100MD-888 digital input? I have an application where a DI will typically have 5V on it and when a switch is closed it gets pulled to 0V. When the switch is released it is pulled back up to 5V.

The interesting thing is when the PLC is started the input at 5V reads OFF, when the switch is closed the input reads ON. But when the switch is reopened the input still reads ON.

Is there anyway that I can make this switch more definitively? Anyone know what the switching levels are for the DI's?

[support]

Yes, the PLC inputs have hysteresis. The input voltage needs to rise above 10.5V in order to turn OFF the input.

If you only have a 5V digital output, you could use a simple transistor to drive the PLC's digital input. You could also use a very low cost IC such as ULN2003A to interface to the PLC's input.

Another way is to connect your 5V digital output to the PLC's analog input and periodically monitor the analog readings of this input and if it is above certain reading, turn on a relay and when it is below certain reading,  turn off a relay. The relay then can be used in the ladder diagram like a digital input for your program.

[ccdubs]

Thanks support.

If I measure the voltage on a disconnected DI on the PLC it only reads 9V so if I had a simple micro connected to the input I would never exceed the 10.5V.

The PLC is being run at 12V and has the link in place. If this makes a difference, what does the input OFF level become? Actually it would be really useful to know the ON, OFF and hysteresis values and if a function of input voltage how this all relates.

I have managed to get around theissue by connecting 2 diodes in series between the DI and the 5V. This pushes the off voltage up to ~6.5V which seems to work OK with the PLC for switching both ON and OFF (FYI, if I use 4 diodes I intermittantly can't switch on).

[support]

Yes, I guess I didn't stately clearly in the last post is that the 10.5V I am referring to the power supply voltage to the PLC. What you are measuring at the PLC's input is after a 2V drop across the input LED. Basically if the PLC input is allowed to float then it would be pulled up to "OFF"logic by the power supply.

The hysteresis voltage level is not exactly the same all the time, but to surely turn on the input you should connect the input to < 1.5V and to definitely turn it off the input should be pulled up to > 8.5V.