Hi Phillip,

HDLC (or some variation thereof) has well and truly stood the test of time
it is very easy to implement, efficient, and most importantly well
understood.

Many protocols base themselves on HDLC particularly its framing structure.
If your product is fairly specific you'll probably just want to use the HDLC
framing rules and your own protocol for the actual data.

An HDLC frame looks like this :-

FLAG            ADDRESS Payload         CRC             FLAG
(8bits)         (0-16Bits)  (8 * N Bits)        (16 Bits)       (8Bits)

Basic rules:-
The flag is a fixed byte (often 0b01111110 or 0x7E)
The flag must be escaped if it appears in the data stream.

Have a search around the web for more details..

Unless you're really tight on time or program space I'lld say go with the
CRC (it's VERY easy to implement) over a simple checksum.

Regards,

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Mike Cornelius                  Internet: mike@bytethis.com.au
Byte This Interactive           Phone:    +61 2 9310-2157
PO Box 1342 Strawberry Hills    FAX:      +61 2 9319-3948
NSW 2012 Australia              URL:      http://www.bytethis.com.au
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Philip Addicott
Sent: Friday, 25 August 2000 3:21 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Protocols for Serial Communications


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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