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interface real world to pc question?

Discussion in 'Electronic Basics' started by russ, Feb 17, 2005.

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  1. russ

    russ Guest

    Hi

    Any help on this would be appreciated.

    I want to read in an analogue voltage from the 'world' into my
    computer, process and store it at regular intervals. I want to do
    this via the serial port. I therefore need an 'interface' where my
    voltage is an input and the output feeds into my serial port. The
    interface and my pc then interact via some C code or something. Does
    anybody recommend any very cheap 'interface' which would do this?
    There are obviously very expensive 'data acquisition cards' on the
    market, but my needs seem very simple in comparison.

    Thanks in advance

    Russ
     
  2. Obvious questions will include:

    What is the range of your input voltage?
    Does it need to be isolated from the PC?
    How fast will the sampling be?
    How much precision is required?
    Single-ended or differential?
    How is the signal conveyed?
    What kind of loading can be tolerated by the driving circuit?
    What accuracy is required?
    What kind of powering for the 'interface' is desired?

    ....

    There is an example of one possible circuit at:

    http://www.elecdesign.com/Files/29/1447/Figure_01.gif

    from this article:

    http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=1447

    That example powers itself from the serial port pins.

    Jon
     
  3. Bob Masta

    Bob Masta Guest

    If you really want to use the serial port, and you are
    running Windows, there is an inexpensive board from
    Dataq for $US 25. If the current model is like the
    older ones, they are 4-channel units that sample at
    a few hundred Hertz max. Comes with software for
    a chart recorder.

    There are also pricier products from a number of
    suppliers, which you could probably locate by Googling
    on "Serial port" and "analog input" or "data acquisition"
    or some such.

    If you are running DOS and don't mind using the parallel
    port instead, you can make a simple 8-bit A/D from
    nothing more than a handful of resistors. It uses the
    "successive approximation" method, just like in "real"
    A/D converters. The LPTX driver for my Daqarta
    package uses such a design, which you can find
    at
    www.daqarta.com/lptx.htm
    Look for the SAR (Successive Approximation Register)
    model.

    Hope this helps!



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  4. Chris

    Chris Guest

    Hi, Russ. You've already gotten some good advice from other posts.
    However, if you're looking for something Q&D that'll do the job with
    absolute minimum fuss at moderate cost, and you can live with fairly
    slow readings (seconds apart instead of milliseconds/microseconds), you
    might want to look at the Radio Shack PC Interface 46-Range Digital
    Multimeter Catalog# 22-812 for $69.99 USD. You get an adequate
    multimeter and software which will allow you to do datalogging without
    programming just by hooking up the included cable from the DMM to your
    serial port and running the program. This will give you the capability
    to read analog DC and AC voltages and currents as well as resistance,
    and temperature with an RTD or optional Extech temperature adapter
    module Catalog# 910-5611 (an additional $39.99 USD).

    Good luck, and have fun
    Chris
     
  5. Jonathan Kirwan was thinking very hard :
    Something else to consider.. a volt meter that has a serial interface
    built in. There are many available from Radio Shack to Fluke.
     
  6. Guest

    Hello to anyone who cares about this Radio Shack Digital Multimeter
    cat# 220-0812. I wrote an interface for myself, that I'd like to share
    with you. The requirements are unix and a copy of gambas, and you get
    the full source code.
    Note that, I found, that, for whatever reason, the instrument sends
    wrong checksums every once in a while - you can see the number of
    resets. I know it's the instrument's or serial connection's fault,
    because when displaying 0.F MOhms overrange continuously, I know
    nothing changes, yet the correct 9A checksum from the device changes to
    12 and 0, and other values, about every dozen 9-byte packet. This also
    messes up the byte-order. Doing a port reset, disconnect reconnect
    resyncs things. When there is a real resistance measured there are
    almost no resets. Another sensitive knob position with lots of resets
    over checksum errors is the temperature-mV one, when values widely
    fluctuate. Still, with a capability of 3 readings per second, you can
    just ignore any off-values and keep the rest, the device still
    functions very well, for the price. You can guess some costs were
    saved. Instead of summing checksum, they could have provided a CRC8,
    and made the packet 10-byte with an STX,ETX or LF char signifying end
    or beginning of packet, so there is never an issue trying to figure out
    which is the starting byte, and there would be no need for resets. Also
    some hardware handshake option would have been nice, because DTR/DSR
    and RTS/CTS are just dummy looped back to each other. But still a fun
    little device, and excellent cost, considering it's from a
    brick-store-in-your-neighbourhood retailer.
    Grab the program at
    http://home.earthlink.net/~sztakacs/gambas-RadioShackDMM.tar.bz2 and
    you may view a screenshot (377KB png image) at
    http://home.earthlink.net/~sztakacs/gambas-radioshackdmm.png
     
  7. Jamie

    Jamie Guest

    maybe you have your parity set incorrectly ?
     
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