07-04-2012 08:48 PM
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?
07-05-2012 08:57 AM
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.
07-05-2012 08:23 PM
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/179
07-06-2012 06:55 AM - edited 07-06-2012 06:58 AM
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.
07-06-2012 04:21 PM
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.