Instrument Control (GPIB, Serial, VISA, IVI)

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I want to crank up the speed > 1Mbaud on RS232

I'm using a PXI-8430/4 serial card and just found out my UUT speaks serial at 1.04 Mbaud.  The VISA driver limits my high-speed PXI card baud to 1,000,000 and no more.  I was wondering if there was a sneaky way to circumvent the driver and gain extra baud rate.
 
I have already verified the integrity of the serial link hardware with a loopback right before the UUT.  I know the UUT works and the UUT speed.  Don't bother suggesting to change the UUT speed.  Also, I'm using a nice short (~5 foot) cable.
 
When communicating using LabVIEW terminal VIs I get what looks like email spam as the UUT spits out its message.  The kind you can read but most of the words are misspelled.  This is a symptom of slightly mismatched baud rates.
I would like to poke or write to some low-level register to get around the magic 1Mbaud rate to communicate with the UUT if possible.  This would save me mucho test time and make me a super engineer in the eyes of my comrades.
 
Thanks
Ralph
Navini Networks, Inc.
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I don't think VISA has anything to do with the maximum setting of the baud rate. It supports what the serial board supports and the 8430 has a max of 1 MB.
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Hi Ralph,

Dennis is correct, the PXI-8430 is limited to a maximum of 1 Mb/s.  Here's a KnowledgeBase entry that explains why:

http://digital.ni.com/public.nsf/websearch/BA0652D1E13DBBC58625649F006561C2?OpenDocument

As noted at the end of the KB, the only way to "cheat" the baud rate is to modify the oscillator on the board.  This is not something that is supported by National Instruments.  You might find out what transceiver and oscillator frequency your DUT is using, and see if it might be possible to match those rates by matching the oscillator.

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Thanks a bunch for the web-link with descriptive baud rate calculations vs. crystal frequencies.  With a 66 MHz (commonly available freq (in fact that is what is used on the UUT)) crystal installed in my test setup, I should be able to hit 1.04 Mbaud within +/- few tenths of a percent.  Should be close enough to disable my "spam generator" and let me communicate error-free.  Faster comm = shorter test time!

With the same crystal I'll be able to hit 115.729 kbaud which is close to the other baud rate (115.2 kbaud) I would like to nail.  If that doesn't get me close enough, I'll just get another PXI-843/4 and leave one unmodified.

Many thanks,

Ralph

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Hi Ralph,

Before you go changing the crystal, you might want to make you you have tried the latest serial driver:

http://digital.ni.com/softlib.nsf/websearch/A1F4E0BFDB3ADD228625711B005BAF63?opendocument&node=13207...

This version might let you cheat the baud rate up a little.  I would definitely try this before making any drastic, and possibly damaging, hardware modifications.

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Nevermind.  Please disregard that last post.  I tried an 8430 here with NI-Serial 3.0 and it limited the baud rate to 1 MBaud.

I still want to emphasize that modifying your 8430 will void any waranty, and you do so at your own risk.

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The limitation of the baud rate is something that is set by the serial driver based on the transceivers maximum rated speed. The board and the crystal on the 843x boards are capable of higher speeds, but we limit the 232 boards to 1 Mbaud and the 485 boards to 3 Mbaud. So please do not change the crystal. It isn't the limitation.

If you would like to try a higher baud at your own risk you can change the Maximum baud rate in the Windows registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\mf\...\Device Parameters. There should be a key for each port for each NI serial board that is in your system. After changing the MaxBaudRate you will probably need to disable and re-enable the ports in the Windows Device Manager for the changes to take effect.

Also note that modifying the registry can be dangerous if you change or delete the wrong thing and NI does not support using the boards at rates higher than they spec, so proceed at your own risk.

I hope this works out for you. I'm all about helping others become super engineers.

-Josh
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