"Arijit Bhattacharrya" <arb...@fh...> schrieb am 03.06.04 20:=
27:02:
>=20
> Hi Thomas
>=20
> I am a little bit confused about writing the configuration file
> "s7mpi.tfi" file.=20
Ok, they are somewhat confusing...
> Imagine that I have to write a bit to the memory area M
> 101.0.=20
> I know how to write Dword or byte or float value.=20
Thi is good at least...
> But I am not able
> to understand how I can write or read bits into the PLC memory. It would=
> be a great help if you can clarify the confusion.
You cannot write single bits. You can chosse single bits to correspond to =
Spreadsheed cells. You can change single bits, but not without writing th=
e entire byte they belong to.
In your case, The PLC driver will fetch MB101, change the bit and write ba=
ck MB101. Be careful: reading and writing to PLCs is usually asynchronous =
to their program execution. So if the PLC changes bit M 101.3 between the =
read amd the write, the change will be undone by the driver. This may caus=
e unpredictable behaviour of the PLC program.
So it is a good idea to "reserve" bytes or greater areas entirely for the =
data exchange with VISUAL.
This is not a drawback of VISUAL but true for all SCADA/HMI systems known =
to me and common practice.
The above written is true for all PLC drivers and brands. I thought I did =
already explain it on VISUAL wep site.
The Siemens S7 protocol in particular provides a possibility to manipulate=
single bits which is not supported by VISUALs PLC drivers. I do not think=
it is a good idea to support it because:
1. The communication is not very "economic", exchanging some 10 (30 to 50)=
bytes to set a single bit.
2. A goal of VISUAL is to make it possible to simply exchange e.g. a Sieme=
ns driver by it=B4s Allen Bradley counterpart, simplifying things to machine=
manufacturers to reuse the same HMI for different customers and markets. =
The use of brand specific specialties would brake this concept.
>
> had told shuja, we would be working on sending UDP packets to the ethern=
et
> interface of PLC.
Should prefer TCP packets, as TCP handles data integrity in it=B4s realm.
> The packets would contain the Flag words.
Thomas
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