how should the limit switches be connected the umi 7764

[Gops]

Hi
I made the following observations in the UMI 7764

1. The voltage across the limit switch and gnd is 5V. when the switch is not connected.

2. When the limit switch input is given ( the voltage across the 2 ends of the limit switch is 0V) to the forward limit and the gnd in the umi, the voltage measured across the umi terminals is 2.9 V.

3. When the external limit switch is activated, so that there is 5V output from the limit switch is applied to umi, the voltage across the umi terminals is 5V.

4. The read limit vi gives the status of the forward limit as high when run. There is no difference even when the 5V is applied to the UMI.

5. But the polarity of the forward limit changes when the limit and the
gnd terminals in the umi are shorted.

6. I get the 5V from the limit switches via an opto isolator circuit.

Kindly help me in debugging the problem.

[Brent_R]

You should connect a limit swith that will toggle between 0V and 5V (referenced to the UMI ground) to the limit input line on the UMI. Then set the limit polarity in MAX to match the polarity of the switches you are using, either active high or active low. Depending on how you connect the switch, a normally open switch will typically be active low and a normally closed switch will be active high. "Active" in this case means the motor is on the limit.

The default is active low, which explains the behavior that you are seeing in number 5. If you want to use active high and your switch does not drive the limit to 0V under normal conditions (not on limit), then you will need to add a pull down resistor to hold the input low until a limit is reached.

Regards,


Brent R.
Applications Engineer
National Instruments

[Buechsenschuetz]

Maybe the opto isolator you are using does not provide sufficient sink current in low output state to pull the UMI7764 inputs low. Provided that there is no further processing of limit switch inputs in the UMI7764 and that the limit switch inputs are fed directly to the 7344 limit switch inputs, you should keep in mind that these inputs are pulled to a logic high level by a resistor of 3.3kOhms. To provide a low level to the inputs the input signal level should be reduced to less than 0.8V. Your opto isolator should be able to handle a sink current of 1.5mA@less than 0.8V output voltage. Check the data of your opto isolator, especially the output current vs. output voltage plots.

Maybe you will have to increase the current of the opto-isolator's LED, you sho
uld also check the plots of input current vs. output sink current.