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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