RE: Re: xAP and embedded devices

["Mark Harrison" <Mark.Harrison@xxxxxxx>]

This is fine if the PIC device is on the ethernet.

However, if the PIC device is going to be on a dedicated serial device,
then I'd suggest that the xAP over ethernet stays as is, but with a
byte-encoder for the serial port.

M.

-----Original Message-----
From: PatrickLidstone [mailto:patrickl@xxxxxxx]
Sent: 15 August 2002 16:24
To: ukha_d@xxxxxxx
Subject: [ukha_d] Re: xAP and embedded devices


--- In ukha_d@y..., ian.bird@c... wrote:
> Hi guys
> 
> I am getting a little concerned that you are all going PC centric 
again and
> forgetting the rather meagre abilities of embedded devices.
> 
> It has taken me at least a year to get where I am with my IR 
project.
> However, I was starting from scratch and the program is currently 
running at
> 1400 code lines (not comments) of assembler. I think it is time to
review
> the target devices again. I don't have the time (or funds) to learn
how to
> program another PIC micro let alone another breed like Rabbit, AVR
etc.
> Ideally I will be using PIC 16F628 chips with 2k program and about
150 bytes
> of spare RAM. Using SNAP a packet has about 7 bytes of overhead 
including 16
> bit CRC which I don't have to store as I can process them as they 
come in.
> The message data I do have to store and rapidly fills my available
RAM. My
> RS485 network works at 19200 and I think these displays run at 9600
max so
> some storage is essential. Please bear this in mind. The cost of 
this chip
> is around 2 to 3 quid, one of its good points.

So should we go back to the original version of the protocol, which 
requires less overhead to process, then? Sensible use of separators 
(say xAP.source=ABC) could provide equivalent hierarchy to the curly 
brace notation. Use of sensible abbreviations could also reduce 
storage requirements (e.g. xAP.src). I'd be reluctant to adopt an op-
code dictionary (e.g. Message key represented by byte sequence 
<0x00><0x01> is xAP.source) because of the problems of
maintaining 
the dictionary in the long term.

Another feature which can help with on-the-fly processing is 
insisting on tag order. Whilst certain name-value pairs may not be 
mandatory, specifying an order may help with state driven parsing?

Patrick



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