Re: [S7] proposal for changing the way we read STRING values.

[Julian Feinauer]

Hey,

I'm unemotional with your suggestion so you have my +0 there : )

Julian

Am 20.08.20, 13:27 schrieb "Christofer Dutz" <christofer.d...@c-ware.de>:

    Hi Julian,

    what I didn't like with using the Array syntax was that your're not reading 
an array of Strings ... 

    How would you read an array of strings?

    How about using round-braces? 

    STRING(10) 

    This way you could read an array of 10-Char strings:

    STRING(10)[5]

    Chris


    Am 20.08.20, 13:16 schrieb "Julian Feinauer" <j.feina...@pragmaticminds.de>:

        Hi,

        this also was a bit controversial in the past and AFAIR we already hat 
STRING[xx] as the command to not read a String Array but a String with given 
size (ignoring ACT value).
        In the end its about latency vs bandwith I guess. 
        Personally, I would stick to the rule: Read MAX either given by 
STRING[xx] syntax or the default 254 or 256 (dont remember the spec exactly).

        Julian

        Am 20.08.20, 12:46 schrieb "Christofer Dutz" 
<christofer.d...@c-ware.de>:

            Another alternative would be to explicitly limit the number of 
chars read.

            So if we would extend the address syntax for STRING types to 
something like 

            %DB2:30:STRING:10

            It would simply read the 10 chars without checking the size first.

            Chris



            Am 20.08.20, 12:44 schrieb "Christofer Dutz" 
<christofer.d...@c-ware.de>:

                Hi all,

                while investigating: 
https://issues.apache.org/jira/browse/PLC4X-240 I noticed that the reading of 
STRING types seems to be extremely inefficient.

                The reason is that we are currently reading a STRING element by 
reading 258 bytes (1 byte for MAX num of chars, 1 byte for ACT num of chars and 
then 256 bytes containing char data).

                The problem is that with a PDU-Size of 240 you can’t read one 
STRING with only one packet. With a S7 1500 or 400 it might work as they have 
larger PDU sizes. Currently the 0.6 drivers split this up into 2 requests.

                But it’s highly inefficient as usually the content will not be 
anywhere near the 256 chars.

                So my idea is to only request one byte (the ACT size) of the 
String and as soon as that’s returned, to ask for only the chars that are 
actually used. This should drastically reduce the payload on the wire.

                What do you think?

                Chris