PLCs and Network Database Access??

Fletch

Member
Join Date
Feb 2003
Posts
1
I am new to the PLC world. I am a PC programmer, but I like the PLC stability features for industrial applications. Can I read and write to Network Databases with PLCs? Any recomendations on hardware and programming software to accomplish this?
 
Look at OPC www.opc.org. OLE for process control.

You can find software packages; OPC servers that take care of the communications to the PLC and make the data available via OLE/Com/Dcom.

I have also seen PC based HMI programs that have direct database access, again these programs use OPC or have direct drivers to take care of the communications to the PLC.
 
Hi Fletch,

most HMI software will allow you to write TO a database, typically there will be three types: cyclical datalogging, on-change datalogging and event-based datalogging.
But be careful: Not all HMI's have all of the above.
For example RSViewME does not have event-based datalogging.
You have to check if it suits your application.

Also: most HMI's will not allow you to automatically write FROM a database to a PLC. At least they will require you to tweak the system to do that.
For example RSView32 can only do this if you write a lot of VBA code.
(personally I hate that, but somebody seems to love it).

An alternative to HMI software might be a software package like RSSQL that is intended to connect databases and PLC's directly. RSSQL can use OPC (I believe) but is of course focused on AB PLC's.

My recommendation:

If you have to connect to only one PLC brand, then use that PLC brands "own" database software (like RSSQL).

If you are looking a bit forward, and might use PLC's from several different brands (maybe at the same time), then use OPC server software to connect to the various PLC's and OPC database client software to connect to the databases (or even wite to databases directly).

best regards,
Jesper M. Pedersen
 
What Kind of Database?
NT Based or UNIX?
Some PLCs have a ASCII Port that can be made to look like a comm port.
You'll have todo ASCII Malipulation with the PLC but depending on the database it may be just as easy. You can check your work from the front of the PLC using an ASCII Terminal.
Simple command structures can be implemented that can be understood by MIS types such as
P = print datatable
t = tare
r = read following value
I created a ASCII port command shell with SLC/PLC 5 that would do almost any thing. Lots of string parsing. You have limits in memory and functionality.
OPC and DDE are great but if your may have to FTP and import the flat file to get it where you want to go.

[email protected]
 
There are many kind of DataBases, related or one table only.

Whith related database will need some application for ex. OLE-DB/ADO AxtiveX (Adodc) control with VisualBasic and some OPC-Server af course.
Application can wait event from OPC-Server when data has changed or
syclicly, anyhow OPC-Server polling PLC in adjusted scanning interval.
Usually PLC is passive for that and not send events when data has changed, OPC-Server do it.

Also it is possible with some Scada developper as InTouch, but needs a lot script-code programming, much more than with VisualBasic.
 

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