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'[PIC]: Embedded Communication System'
2002\10\13@124838 by Donovan Parks

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Hello,

I have a fairly simple I2C network that consists of 5 PICs (one master and
4 slaves).  What I am wondering is if there are some standard communication
protocols used on embedded systems.  I've tried a web search, but I have
the feeling my terminology is wrong (as I am not finding what I want).

What I am looking for is a communication protocol that specifies the
package structure, how to handle errors, what checksum/crc to use, etc...

Or is it more common just to roll your own protocol in an embedded network?
(I have found the X-modem protocol, but it is far more than I need).  Can
anyone recommend a good source that discusses issues involved with creating
communication protocol?

Thanks.

Regards,
Donovan Parks

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2002\10\13@132613 by Wagner Lipnharski

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Donovan Parks wrote:
{Quote hidden}

It doesn't matter so much if you chose one or another,
all of them would be based on simple things;

Packet preamble identification (sync) byte or bits.
Destination address byte(s)
Sender address byte(s)
Packet Frame number in 8 or 16 bits
Type of Frame - Data, Request or Confirmation.
Size of dataframe (in bytes)
DataFrame
CheckByte(s)

With this in mind, you can develop your own protocol.

Preamble ID can be anything you create, some sugestions goes to 55h, AAh,
or any combination of those. You can use one byte or more to ensure the
receiver will not start to receive a frame in middle of it.

Destination address should match destination hardwire or softwire station
setup.

Sender address should be used by the destination to answer the frame.

Packet Frame Number will be used to identify an answer, request or
confirmation.

Type of Frame - So the destination will know what this frame is about,
data, request, etc. and deal accordingly. A single byte should do it.  You
can think to share just one byte for Type of Frame and Packet Frame Number
if you want - 4 bits each.

Size of dataframe - 1 or 2 bytes, you should define the max length of your
data transmission.  The receiver will know exactly when the data part ends
and crc or other check bytes start.

Dataframe - a bunch of data - binary or packed bcd - the Type of Frame can
identify the data type.

CheckByte(s) - CRC16 is recommended, easy and fast.

Confirmation should be always done to the sender by the receiver with frame
number, so the sender will know the receiver got the data.  A retry count
should be accounted, so the sender will just consider destination not
operational after a certain retry attempts, or by reception error, or by
timeout (destination is really offline).

Frame number should be a dedicated sequence between sender and receiver, so
the receiver will always expect an incrementing number in a multiple frame
reception.  A broken sequence will flag a missing frame and should start a
destination request for that particular missing frame to the sender.

Easy simple protocol is what is the best.  You can accomodate exactly what
you need, how you need and as simple as you need.

Except if you want to explore the market with available protocols for a
better marketing approach.

Wagner Lipnharski - email:  .....wagnerKILLspamspam@spam@ustr.net
UST Research Inc. - Development Director
http://www.ustr.net - Orlando Florida 32837
Licensed Consultant Atmel AVR _/_/_/_/_/_/

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2002\10\13@184155 by Christopher Graham

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Here's a MicroChip paper on a I2C network protocol including flow charts and
source code in C.  Hope this helps.

http://www.microchip.com/download/appnote/pic16/00736a.pdf

- Chris Graham


> Hello,
>
> I have a fairly simple I2C network that consists of 5 PICs (one master and
> 4 slaves).  What I am wondering is if there are some standard
communication
> protocols used on embedded systems.  I've tried a web search, but I have
> the feeling my terminology is wrong (as I am not finding what I want).
>
> What I am looking for is a communication protocol that specifies the
> package structure, how to handle errors, what checksum/crc to use, etc...
>
> Or is it more common just to roll your own protocol in an embedded
network?
> (I have found the X-modem protocol, but it is far more than I need).  Can
> anyone recommend a good source that discusses issues involved with
creating
>  communication protocol?
>
> Thanks.
>
> Regards,
> Donovan Parks

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2002\10\14@133243 by Donovan Parks

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Thanks Warren and Christopher.  This should help me on my way.  If anyone
knows a good resource that discuss communication systems I'd love to known.
Any link talking about how to format frames, how to handle errors, etc...
More information the better.

Donovan


{Quote hidden}

frame
> number, so the sender will know the receiver got the data.  A retry count
> should be accounted, so the sender will just consider destination not
> operational after a certain retry attempts, or by reception error, or by
> timeout (destination is really offline).
>
> Frame number should be a dedicated sequence between sender and receiver,
so
{Quote hidden}

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2002\10\14@193538 by cdb

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What about the Snap protocol - can't remember the website, but it is
googalable and demonstartion code is available for download - the
site is a Swedish or Danish one AFAIRC.

Colin
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