CLX and S7

mrmss

Lifetime Supporting Member
Join Date
Aug 2005
Location
Midlands - England
Posts
101
I have a peice of equipment (weigh scale) which I can read from serially and have the data in my contrologix PLC. What would be the best way to get this info to a siemens PLC? would it be fast enough using modbus, or is this too horrible that it would be worth the cost to get a profibus weigh scale so that the siemens can directly get the info?

Is there anything other than Modbus?

I'm still new to all this so patience is much appreciated.

Thanks
 
Hi mrmss,

Modbus is possible as this recent post will tell you:
http://www.plctalk.net/qanda/showthread.php?t=20271

But probably it will not be cheaper than Profibus.

Profibus is allready on the S7 (unless it is a small S7-300 or S7-200) so you have to consider the cost of getting the weigh scale on Profibus against the cost of getting the S7 on Modbus.

The Profibus solution will be more potent all in all.

Sidenote:
I wonder why everybody are using all different kinds of serial interfaces to talk with their weigh scales.
We are using a lot of load cells in our applications, and we allways use a locally placed transmitter box that sends the signal to the PLC as a 4-20 mA signal. Simple, reliable, cheap.
I cant see any need for putting it on a serial connection.
 
Thanks for that.

The serial route was taken because considering commands like tare and zero on the scale we thought it simpler to just have 1 serial connection rather than analogues and relays. With serial we can get the net, gross and issue any commands with 1 interface. The requirement for the Siemens to acquire information from the weigh scale came afterwards.
 
Just for your info:

With the 4-20 mA you have to do the zeroing and taring in the PLC program. But that is actually simpler than having to send commands back and forth.
In my PLC program I have also implemented a "tail-compensation".
It is nothing more than a memory of how much extra weight was achieved after weighing was stopped. At the next weighing sequence the tail is automatically compensated.
In that way a quite accurate weighing is achieved.
 
Siemens CP 340 or 440

mrmss,

If you already have the Siemens PLC and it is a 300 series there is a CP module that you can add that will talk serial for you. It is the CP 340. The 400 series has the CP 440. If you follow the below link you will find them listed. The link is to there ST 70 catalog. Just look at the bookmarks under the different Series. Hope this helps, although it sounds like Jesper has the better idea.

http://www2.automation.siemens.com/simatic/ftp/st70/html_76/st7001_e.pdf
 

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