[Visual-devel] Re: Visual,PLC data update

[Thomas H. <Tho...@we...>]

On Friday 26 September 2003 20:55, you wrote:
> Hi,
>
> I browsed around you hmi pages and will certainly try you program in the
> near future. I have one question about the driver/spreadsheet.
> How does the driver handles the updating of data?
It works through the list of TFI (transfer instructions). When the last one 
is executed. it begins from start.
In read from PLC operations, usually the whole block of data is fetched.
In write to PLC operations, every corresponding cell is checked and only if 
it has changed, the corresponding data is written to the PLC. To achieve a 
consistent state, all cells are marked as changed on program start.
> Depending of the recalculation time of the cells, as fast as possible or
> depending on the requested data from the cells.
As fast as possible. I'm thinking about introducing a delay statement into 
TFI lists for very fast (ethernet based) protocols.
> In other words : if at a certain time spot, only  half of the data
> configured in
>  the cells is displayed in the hmi pages, and the other 50%
> is not used anywhere, is these data then also refreshed?
Yes it is. Another client may need it or you may have a formula that combines 
data from multiple sources or you may want to store the data to a database or 
to a trend data file. Or another program/device may ask for the data via a 
slave driver.
Given all these possibilities, it is easier to re-read all data then doing 
the bookkeeping of what is really needed.
Also many protocols involve so much overhead that it would be slower to read 
two data items separately then a larger block containing both of them 
together with some unused stuff.

Regards,

Thomas
PS: I send a copy to the visual-devel mailinglist, because this may be 
interesting for others, too.