Output logic signals with Labview?

[qa9b]

Hi all,

I'm very new to instrument communication & labview. I have an existing setup with two keithley DMMs interfaced by GPIB & VISA to a Labview program. The computer has the standard outfit of USB ports, a serial port, and a GPIB card.

For automation purposes, I need to be able to provide 4 bits of parallel logic to an external circuit. My question is... what's the easiest way to do this? Can I drive individual lines of the serial port high and low? If so, can you recommend a starting place for my learning?

Thanks!

[crossrulz]

Do you have to do any sort of timing with the digital lines?  If not, I would simply go with a USB-6501 or PCI-6503 and use DAQmx.

[johnsold]

To directly answer your question: No. The voltages on a serial port are not compatible with any standard logic family of devices.

As crossrulz pointed out, the best approach is likely to obtain a suitable DAQ device.

Lynn

[qa9b]

The NI USB device definitely looks nifty, but I really do think it's overkill for my situation. I only need 4 data lines and no timing.

Can you further explain why the serial port wouldn't work? From my understanding, it outputs extremely distuingishable logic levels (+-5V or higher). Custom circuitry could translate the levels, or maybe even a full solution like the MAX232 (http://www.maxim-ic.com/datasheet/index.mvp/id/1798) would do the trick; obviously I wouldn't be needing to transmit anything. A $10 solution seems pretty attractive. Is there any other reason why this wouldn't work?

[crossrulz]

I'm not sure the UART will let you control the lines the way you want.  And even if it will, you will spend way more than $100 of your time trying to get it to work.  The obvious economical way is the USB-6501.

Another alternative that I have done is to use the parallel port.  But we ran into port and resource issues when changing computers and it quickly became a mess.  We went to using the DIO in our DAQ card instead and had absolutely no issues.

[Dennis_Knutson]

With a VISA propery node (Serial Settings>Modem Line Settings) you can control the handshake lines (DCD,DTR,RS,RTS).

[johnsold]

Yes, a few dollars worth of parts can convert the RS-232 levels to logic levels.  And, as has been pointed out, the time you spend building the circuit and writing a program to control the handshaking lines costs something.

If you are a student or hobbyist where the value of your time is counted far less than cash out of pocket, that may be a viable approach. In a for profit business buying a DAQ device is more cost effective.

Lynn