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Question about Serial communication (RS422 standard) and conversion

  1. Nov 10, 2009 #1
    This is my first post.
    I am an aerospace engineer so these very technical computer questions still elude me.

    My question is mostly about RS422 standard.
    I have a piece of equipment (LIDAR) that is able to transmit data via a 9 pin serial port. If you put a jumper across two of the pins it knows to transmit as RS422, otherwise it transmits RS232. I make all the cables myself, so that is no problem.
    The reason that RS422 is better for my application is that is transmits at a much higher data rate than i can get from 232. I don't care about cable length because my cables are only a few feet long.
    My computer has a 9 pin serial port that can send and recieve data as RS232. I have multiple USB2 ports as well as a PC Card slot.
    I have been researching RS422 converters, and this is where my question comes in. If I use a 422 to 232 converter, am I still limited in the BAUD rate of the 232 port on my computer?
    Which RS422 converter types would provide me with the fastest BAUD rate possible? Which would impede the data rate the least? 232, USB, or PC card.

    One other question that is a little more obscure. I have programmed a device driver in LabView that uses the LIDAR as one of its devices. Will I have to completely re-code the Tx and Rx parts of the code if I use a 232 to 422 converter, or will the converter take care of that for me?

  2. jcsd
  3. Nov 11, 2009 #2


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    Welcome to PhysicsForums, BigSteve!

    You may be able to chop up this knot by buying an RS-422 PCI card, or at least a RS-422 to USB interface card, thus avoiding the need to go back to RS-232. When I was with a robotics project, we used the USB-COMi from these guys:

    It was built around FTDI's USB-serial conversion chips (one of them plus probably a MAX485), so you may be able to find one that doesn't have to ship in from England (though to be fair, it worked really well--at least under Linux, and with the really good CS guys we had working on it). After a little bit of Googling, these guys seem to sell the exact same thing:

    One of the big differences between RS-422 and RS-232 is that in RS-422, there's differential signaling (half the signal on one pin, half on the other), giving it better noise rejection, and making it much more suitable for long-haul transmission (few hundred feet, if I recall correctly) at much faster data rates.

    If LabView has the SICK built-in (I assume it's the SICK), then it should have something to handle the RS-422 side of things (probably a configuration option somewhere). Which actually should give you the fastest possible BAUD rate (I seem to recall that, at least in Windows XP, the serial port was restricted to about 125 kbps)
  4. Nov 11, 2009 #3
    Thanks man.
    I wanted to use USB but I thought it might bottle neck the higher data rate.
    So,I'll definitely look into the product you suggested.
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