Re: Parsing binary data from an RS232 connection
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- Subject: Re: Parsing binary data from an RS232 connection
- From: Sean Conner <sean@...>
- Date: Fri, 6 Dec 2019 21:13:39 -0500
It was thus said that the Great Russell Haley once stated:
> Hi,
>
> I am writing a little Lua program to parse binary data received via serial
> communications. I'm wondering if someone on the mailing list has a novel
> approach to parsing the data? The communications protocol is something like
> this:
>
> <HEADER> - 4 bytes
> <MSG_TYPE> - 1 byte
> <SEQUENCE> - 2 bytes
> <PAYLOAD_LENGTH> - 2 bytes
> <PAYLOAD> - X bytes (variable based on message type)
> <CHECKSUM> - 4 bytes
>
> I see two options for parsing messages received via serial:
> 1) Process each character. Lots of examples here:
> https://stackoverflow.com/questions/829063/how-to-iterate-individual-characters-in-lua-string.
> Once the header is found, just count until I find the payload length (+
> checksum) and then count that number of characters
> 2) Using pattern matching and captures to search for the <HEADER> + 5
> bytes. Then parse out the payload length (+ checksum) and take that many
> characters. This seems potentially very fast, but creates a great deal of
> complexity if I don't get the entire message in one serial port read.
> Something like this (where 'ABCD' represents the header):
> local m = input:match('ABCD(.+5)')
> if m then
> local typ, seq, pl_len = string.unpack('>B>H>H',m)
> local msg_len =
> if pl_len > 0 then
> ...
> end
>
> Does anyone have a suggestion for parsing the data or even an existing
> project? Any and all input is welcome.
How about:
data = serial:read(9)
header,type,seq,len = string.unpack(">I4 I1 I2 I2")
data = serial:read(len)
crc = string.unpack(">I4",serial:read(4))
Seems straightforward to me
-spc (Just add some error checking ... )