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Learning to crawl - Newbie question about serial communications

Discussion in 'Electronic Basics' started by schoolathome, Apr 25, 2004.

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  1. schoolathome

    schoolathome Guest

    Greetings,

    I am contemplating building an idea I have had for some time, and in the
    process learn more than just basic electronics. Without going into great
    detail at this phase, I would like to implement a serial bus in order to
    communicate with multiple receivers each with it's own binary address. I
    have been looking at max232, a RS-4XX equivalents, and PIC controllers. I am
    overwhelmed and at this point, as each stop just short of what I "think" I
    need.

    I would like to have a PC send instructions and read responses/status back
    from these modules. I can think of many ways to accomplish my goal, but the
    serial bus is something I seem to have under-estimated.....as usual.

    Any public references, websites, or schematics would be helpful in
    understanding how I can implement this concept and possible incorporate it
    into my design.

    Regards

    Doorman352
     
  2. Rich Webb

    Rich Webb Guest

    Take a look at the RS-485 protocol instead of RS-232. Other options
    might be Microwire (trademark of National Semi, IIRC) or SPI or "three
    wire interface"; I2C (trademark Philips) or "two wire interface", or
    more specialized protocols like CAN. All of these support multiple drops
    better than "regular" RS-232.

    Microwire is pretty easy to bit-bang for parts without built-in SPI
    ports. You'll need a separate enable line for each remote component.

    What kind of data rate are you looking at?
    A microcontroller board as "glue" between the PC's standard RS-232 and
    whatever multi-drop protocol you implement between the other components.
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Learning to crawl - Newbie question about serial communications
    A good place to strart is Jan Axelson's Lakeview Research Serial Links web
    page...

    http://www.lvr.com/serport.htm

    The book "Serial Port Complete" by Axelson is a great resource. It starts from
    square 1, and it's available from her website, as well as amazon and most of
    the hobbyist sources like Jameco, &c. Well worth the price.

    Good luck
    Chris
     
  4. Doorman352

    Doorman352 Guest

    Rich,
    Thank you for your prompt response.

    RS-485 does seem to offer the most bang for me. I haven't run accross
    any schematics showing a bus scheme, and so I don't know what is
    involved..... Any sources?

    As for data rates, they aren't critical since the communication won't be
    continuos. The thought was to provide RS-232 to X conversion on my board
    with X being the "data bus". I keep running into various techniques that use
    serial "ports" embedded in chips, but I can't figure out how I would manage
    the data stream. One thought was to use a shift register to create a
    parallel bus that I could check bit values, as I am using a binary coding
    for staus registers..... (K.I.S.S. - Keep it simple stupid).

    Do you have any experience working with this interface in embedded
    applications?

    Doorman352
     
  5. Gareth

    Gareth Guest

    Have a look at the Maxim web site, they have application notes to help
    you use their ICs and I'm sure there is information there on RS232,
    RS422 and RS485. See:

    www.maxim-ic.com

    You said in your original post that you want multiple devices, but I
    would suggest that you start with a PC and a single device and get that
    going first. You can then move on once you are confident that you have
    the basics working.

    A PC to single PIC is quite easy to do. The PIC has a serial port on it
    which outputs logic levels. The shift registers are inside the PIC so
    you don't need to add any, but RS232 has some strange voltages so you
    need a MAX232 (or something similar) to convert the voltage levels to
    something a PC will recognise. If you then get the right baud rate and
    data format the PC and PIC should be able to communicate.

    For multiple devices the tricky bit is probably ensuring that only one
    device is transmitting at any one time and that the devices which are
    not transmitting have their outputs set to high impedance. To do this
    RS485 drivers will have an Disable pin which you can control from the
    PIC. Only one transmitter on the bus must be enabled at any one time.

    One way to arrange your bus would be to have the PC as the bus
    controller and have the PICs transmit only when the PC asks them to,
    That way you can be sure that you wont end up with two devices trying to
    talk at once.

    The Transmit on the PC would go to the Receive on all the PICs, and
    another connection from the Transmit on all the PICs would go to the
    Receive on the PC. The PC would transmit a data packet which would
    contain an address and instruction. All the PICs would receive this
    data packet, but if the address was not theirs they would just ignore
    it. If the PIC is to respond to the PC it would Enable its RS485
    transmitter, send the data then disable the transmitter as soon as it
    had sent the data. The PC would then know that the bus is be clear once
    it had received the PIC's response. There should be a time out as well
    so that the PC doesn't wait for ever for a response that, for some
    reason, doesn't get through.

    The RS485 bus will simply be two twisted pairs of wire - one pair for
    the PC to transmit to the PICs and one pair for the PICs to respond to
    the PC. To convert from RS485 to RS232 to get the data from your bus
    into the PC you could just use the RS485 receiver to get to logic levels
    then use an RS232 driver to go from logic levels to RS232, and obviously
    vice versa to get data from the PC to your bus.

    Hope that helps,

    Gareth.

    --
     
  6. Rich Webb

    Rich Webb Guest

    As Gareth mentioned, Maxim has some good app notes. Search their site
    with "485" and you'll find lots of info. Also, a Google search for
    "introduction 485" has many good hits.
    The serial-parallel conversion is in the UART/USART "peripheral" inside
    the microcontroller chip. Typically you'd throw a byte at a transmit
    register and go away to do other work. When the transmit register is
    ready for another byte it asserts an interrupt and/or sets a flag.
    Either service the interrupt and send the next byte or periodically poll
    the ready bit and respond to it.

    My preference is to offload as much of the low level grunt work from the
    PC as possible (not least so that things continue to operate if/when
    Windows crashes). In this case I'd probably look at running everything
    from either an ATmega162 or ATmega64 master. Both have two UARTs and so
    could easily be made to speak to a PC out one port and the other
    controllers out the other (adding appropriate line drivers/receivers, of
    course). Throw on an LCD character display and a small keypad and you
    have an autonomous device that can work without a PC but that can send
    status & accept commands if the PC is awake.
    Long, long ago. All of the multiple-devices-on-a-bus stuff recently has
    been SPI/Microwire. SPI is single-ended and runs at logic levels, so it
    can be used simply port-to-port for neighboring chips or run (for short
    distances) off-board with ordinary (cheap) buffers.
     
  7. A friend of mine just implemented a PC-to-PIC interface with RS232. It
    works great. He wrote the PIC code using the CCS C compliler. If you
    plan to use the CCS C compiler for your PIC, I'm sure he would be glad
    to share the code with you. Let me know.

    BRW
     
  8. Hi everybody,

    I am wondering which port; serial or parallel will be a good startup
    for a beginner like me.

    I saw lot of PC interfacing work done by parallel port, so should I
    move forward with this one?

    Thanks

    Regards,
    Animesh Maurya
     
  9.  
  10. Hi Steve,

    Thanks for the direction.
    What does it means, is parallel port so vulnerable to blow-up your PC !
     
  11. Gareth

    Gareth Guest

    I think it depends what you are doing. For example if you are using a
    microcontroller with a serial port on it the serial port would be easy
    to use. If you just want to switch a few things on and off from your PC
    the parallel port will probably be easier.

    One thing to watch out for though with the parallel port is that most
    modern PCs seem to output ~3.3 Volts for a logic high rather than the 5V
    you might expect.

    Gareth.

    --
     
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