mysterious sine signal

[yonderstar]

hello ni world

    i'm doing a simple measurement of speed with PCI 6251 and SCB-68.

                                                                    8

                                    sensor 1         ---|          |---

                                    sensor 2         ---|          |---

an object " 8 " will fall and pass the two sensors. 

the sensors functions as follows:

in "normal state" it will send  out 5V but

when the object is in between the emmitor and the reciever of the sensor, the liason will be cut and the voltage will drop to zero.

i have problems with the cabling scheme. because in the "normal state" i should have a horizontal line , indicating 5V, and when the object pass , the line will drop to zero line. something like:

    -----------------------

                                |

                                |

                                |

                                --------------------------------- 

but instead, in "normal state " , i have got a periodical wave that looks almost like a sine function, with a periode of 22ms and a large range.

where does it come from and how do i eliminate it ?

i guess the signal source is a ground referenced source because it has only one out-put voltage, one ground and one power supply.

so i choose RSE in MAX.

i am very confused about how to wire the several grounds. and about the power supply i dont even know where the groud for it is.

i have studied the link in Wang's post

it help to clarify some concept but in practical situations i'm still confused

thanks in advance

[WM_John_Weeks]

You have a TTL-compatible logic signal. To measure the time it takes for the object to fall between the sensors you should be using one of the counters on your DAQ device in two-edge separation mode. I don't use Labview, so I don't know what vi to tell you to use. In the DAQmx C API, you would use DAQmxCreateCITwoEdgeSepChan to set up a channel to do the measurement.

You may need to use a pull-up resistor to make the logic signal reliable. I think the device manual may have some description of how to connect such a resistor if you need it.