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'HDLC'
1996\08\03@230349 by Mark K Sullivan

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I'm working on a PIC implementation of HDLC.  Does anyone know if the CRC
includes the bit-stuffed zeroes?

- Mark Sullivan -

1996\08\04@045129 by Ray Gardiner

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>I'm working on a PIC implementation of HDLC.  Does anyone know if the CRC
>includes the bit-stuffed zeroes?
>
>- Mark Sullivan -

I my memory serves me correctly, Flags MUST be unique, and so
everything except flags must be bit-stuffed. To avoid a CRC
accidently looking like a Flag sequence.


Ray Gardiner, 104 Macintosh Street, Shepparton, Victoria 3630,  Australia
spam_OUTrayTakeThisOuTspamnetspace.net.au

1996\08\04@050823 by Kalle Pihlajasaari

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Hi Mark,

> I'm working on a PIC implementation of HDLC.  Does anyone know if the CRC
> includes the bit-stuffed zeroes?

The Intel and Zilog books make no explicit mention but by inference
one can deduce that the CRC does NOT take the stuffed zeroes into
account as it gets send over the same channel and may itself require
zero stuffing. These 0s all have to be removed before the CRC can
be used so I expect it will be before the 0 stuffer and after
the 0 remover.

PS. What do you have in mind for your project and how FAST are you planning
to go with what chip ?  I an looking at doing a project which would require
synchronous comms with minimal overhead and have been thinking of using
the mono-sync protocol without any CRC to make things go FAST, and easy
for me but if I can do HDLC I might go for the standard.

Cheers
--
Kalle Pihlajasaari     .....kalleKILLspamspam@spam@data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

1996\08\04@133231 by Mark K Sullivan

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Thanks to Kalle and Ray for advice.  I do know that the CRC itself is bit
sutffed and must be sent MSB first in contrast to the data which is LSB first.
I'll proceed on the assumption that Kalle is right about excluding stuffed
zeroes from the CRC.

>What do you have in mind for your project and how FAST are you planning
>to go with what chip ?

I am able to serialize and bit stuff at 18 instruction cycles per bit and to
serialize, bit stuff, and accumulate the CRC at 27 cycles per bit.  I have no
doubt this can be improved upon, at least by a few cycles.  I am only trying to
transmit at 19,200 bps so it is no sweat with a 3.6864 MHz clock in the 16C54.
That works out to 48 cycles per bit.  I haven't gotten to the receive code, yet,
but I don't anticipate any big surprises.

- Mark Sullivan -

1996\08\05@183400 by Brian Boles

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    The stuffed zeros are not included in the CRC.  In other words, the
    CRC is computed on the unstuffed message first, then appended to the
    message, then the whole message is subject to bit stuffing.

    Rgds, Brian.                           bbolesspamKILLspammicrochip.com



______________________________ Reply Separator _________________________________
Subject: HDLC
Author:  Mark K Sullivan <.....msullivanKILLspamspam.....VAX.NIOBRARA.COM> at Internet_Exchange
Date:    8/3/96 10:03 PM


I'm working on a PIC implementation of HDLC.  Does anyone know if the CRC
includes the bit-stuffed zeroes?

- Mark Sullivan -

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