Your Pick can be one of many.
        Pick SOME NAMED STANDARD and stick to it (sorry for the shout).
Modbus, STD-12, et., al.
        IMHO. if you let someone bastardize some Standard , I think you are
doomed for life.

        Regards
        Keith

        -----Original Message-----
        From:   Ray Gardiner [SMTP:ray@DSP.COM.AU]
        Sent:   Friday, August 25, 2000 12:00 PM
        To:     PICLIST@MITVMA.MIT.EDU
        Subject:        Re: [PIC]: Protocols for Serial Communications

        Hi Philip,
        Someone once said, the nice thing about standards, is that there are
so
        many to choose from.

        Since what you want is a point to point link between the PC and a
PIC based
        controller, I recommend that you consider the following approach.

        Use modbus framing. This is simple and well documented. see
www.modbus.org
        Implement only two transaction types. ReadHoldingRegister and
        WriteHoldingRegister. This allow you to simply read and write any
        memory that you choose on the target (PIC) Additionally you can
extend the
        networking to multiple PIC's by going to RS485 HDX.

        see:--

            http://www.modicon.com/techpubs/toc7.html

        for more detail on how to do other functions, crc calculation etc.

        The basic modbus frame for reading is 8 bytes as follows.

                <NetAddress>         == 0x01    // ok, you don't really need
it but
                                                   // it makes for a
convenient sync flag
                                                    // anyway
                <FunctionCode>       == 0x03    // 0x03 == read holding
register
                <DataAddressHi>      == 0x??    // address in PIC data space
                <DataAddressLo>      == 0x??
                <NumberofRegistersHi>== 0x??    // number of registers to
read
               <NUmberofRegistesLo> == 0x??    //
                <16bitCRClobyte>     == 0x??
                <16bitCRChibyte>     == 0x??

        The response will be something like...
                <NetAddress>         == 0x01
                <FunctionCode>       == 0x03
                <DataCount>          == 0x??
                <Data>               == 0x??
                <Data>               == 0x??
                <Data>               == 0x??
               .... for DataCount Bytes ...
                <16bitCRClobyte>     == 0x??
                <16bitCRChibyte>     == 0x??

        The 16 bit CRC is sufficient for most applications. But... IF you
are
        transmitting over a noisy link (eg radio), you could consider
implementing
        some kind of FEC (forward error correction) using hamming codes or
        reed-solomon. This means assembling the packet and then encoding
with
        the appropriate FEC. Then decoding and correcting errors before
passing
        up to the modbus receive routine. If you have low error rates then
you
        don't really need it.



        >> I have been working towards a stable serial link with a PC. I am
not
        >sending
        >> a lot of data, things like date,time stamps plus the odd 6 or 8
byte
        >codes.
        >> However it has become pretty clear that good serial coms has a
lot more to
        >> it than banning bits in and out of a PIC, bits DO go missing or
get
        >> corrupted. It also seems that the pins for hardware handshaking
are never
        >> used so there must be protocols involved, so here are my
questions.
        >>
        >> Is there a standard protocol or at least a guideline that one
could work
        >> from?
        >>
        >> Do I need to use CRC or is a xor check sum ok?
        >>
        >> Thanking you in advance.
        >> Philip

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