Digital Output using PXI-6521.

[Yakleo]

I have recently purchased a PXI-6521. I tested the two normaly closed relays, and they were closed. I then started up DAQ-Assist, and set the relays to true. I checked with a continuity meter, and they were now open. I then added two boolean true constants and a build array, and wired that to the Data input on the DAQ-Assist. The relays did not open.

What am I doing wrong?

Shouldn't the true open the relays?

Message Edited by Yakleo on 06-15-2007 10:49 AM

[Jesse_O]

Hello,

The states of the 6521 for both Form A and Form C relays can be found in the user manual here on page 8.  Based on this page, if you write a 1 for a Form C you connect COM to NO.  If you write a 0 you connect COM to NC.  A similar action is found in the manual for Form A.

A shipping example that would enable you to write to your lines called Write Dig Chan.vi.  This can be found inside of Example Finder (Help >> Find Examples) under hardware input and output >> DAQmx >> Digital Generation.

If this does not help, can you please let me know what happens when you run this example and what you measure when you set port1/line0 state as well as what two connections (including pin numbers) you are measuring between?

Regards,

Jesse O.
Applications Engineering
National Instruments

[Yakleo]

So if I read your post correctly, instead of writing Boolean values to switch the digital outputs, the 6521 takes 1s and 0s?

[Jesse_O]

Hello,

Sorry about that.  When I said write a 1, this also means write a boolean true.  A boolean false would be a 0.  It depends on what DAQmx write option you specify.  This might becomes more understandable if you look at the block diagram, and the different polymorphic instances, of the DAQmx write in the example I mentioned earlier.

Regards,

Jesse O.

[Yakleo]

I guess I am more interested in the speed of PXI and labview and the non latching relays. If the loop time is roughly 500ms, and once every 500ms a short signal is sent to the PXI-6251 to open, I don't think it stays open long enough to be detected. I would need the relay to stay open as long as I am sending true to that digital output.

[Jesse_O]

Hello,

When you write a digital value to the board, this value will hold until you change the value.  For example if I have one part of my application write a high to the 6521 it will remember that I wrote a high and I do not have to keep updating it.  Later in the application if I want to write a low or a 0, it will change the value from high to low and remember the low until I change it to something else. 

If you write a high to the board and expect some state, when you try to measure that state it should be there.  You will not be missing it because the loop was had not refreshed the value for 500ms (unless you are having your loop toggle the value from high to low). 

Regards,

Jesse O.
Applications Engineering
National Instruments

[Yakleo]

I could not get the example to work.  Under Lines, I selected PXI1Slot2/port1/line0

Under Bits, I left as 0.

Under data to write, I pressed the 0 button down.

I get an error message saying Error -200463 occurred.

Specified read or write operation failed, because the number of lines in the data for a channel does not match the number of lines in the channel.

[Jesse_O]

Hello,

That error is caused because in your channel/Lines you are only specifying 1 line, however the data you are trying to write contains 8 lines of data.  In that example there are 8 buttons (in 1 array).  To fix this error you can either specify to use 8 channels by entering PXI1Slot2/port1/line0:7 or by using PXI1Slot2/port1/line0 and right clicking on the array index control to the left of the buttons and selecting Data Operations >> Empty Array.  Next click on the first button to the right of this index control and turn it on or on and then off (depending on what you need).  This array is now a size of one and you can specify a single line.

I hope this helps,

Jesse O.
Applications Engineering
National Instruments

Message Edited by Jesse O on 06-19-2007 03:38 PM

[Yakleo]

This example switches the relays without any problems. Are there any limitations on writing a signal to multiple PXI boards at once?

[Jesse_O]

Hello,

The PXI backplane has the same throughput as a standard computer PCI bus (132Mb/s).  This is shared between all of your devices in the PXI chassis, however a lot of devices like the 6521 will consume very little of this bandwidth.  You can have several 6521s in a chassis and communicate with all of them at the same time.  The bandwidth only comes into play when you are working with high speed devices.  The article linked above on PCI bandwidth has more detail on this.

I hope this helps,

Jesse O.
Applications Engineering
NIC

Message Edited by Jesse O on 06-19-2007 04:09 PM