S7 300 SM331 Analog Inputs Problem; Relative DC leval of adjacent inputs.

David Torrens

Member
Join Date
Nov 2004
Location
Live in England Work in Brazil
Posts
13
We are using 6ES7 331-7KF01-0AB0 * Input "Optically Isolated Input Modules.

I have found that if there is a large difference in potential to ground between adjacent inputs then all the inputs in that group read as full scale. This is regardless of what the differential voltage is between the two wires of the input.

Is this a know problem?

I notice that AI 6ES7 331-7KF01-0AB0 is no longer listed in the Siemens catalogue.

However I have an old catalogue and it states "Permissible Input Voltage for Voltage input" 20 Volts.

Now I am not sure what that actually means in practice.

Does it mean that an adjacent input being more than 20 volts different will cause a malfunction OR

does it mean the maximum differential voltage applied to the two input terminals is 20 volts Dc ?

What are the restrictions on the relative voltages to ground of adjacent inputs?
What I would expect is that if I have a system stating that the Opto Isolation has been tested to 500VDC that each input can be at up to 500V DC relative to any other input and the module will work correctly.

:confused: .
 
According to the handbook, the maximum potential difference between the inputs is 2V and the maximum potential between the inputs and the internal ground is 75V DC / 60V AC.

You can download the manual here, or at least I hope you can - as usual with Siemens it's a massive URL and sometimes they don't work. If not go to the Siemens support web site here (that's not quite as long!) and enter the module number in the search field. You pay need to change the language, I think I copied the German page, sorry!
 
Thanks for that. Yes it seems that these "Opto Isolated" modules are not quite so optically isolated as one might like.
Our problem is that I wanted to connect each AI to a separate Engine control system. Each System has its own 24 volt DC supply that is flowating reative to ground.

I will have to use an auxiliary relay instead.

It looks like all the Electronics in the card is isolated sort of together. So each channel has to be at about the same potential to ground as all the others. (Not at all like a "proper" optically isolated I/O card.)

I am finding that Siemens info is very comprehensive once you can find it. The quantity of stuff on there web site seems to be a hindrance. However ther technical support by E Mail is excellent.

BUT they should be able to aford that good technical support as there stuff is so expensive. :(
 
I am finding that Siemens info is very comprehensive once you can find it.

That's sometimes the biggest problem. For some reason their search facility doesn't always turn up things either - I've had this happen with things I've already read and then couldn't find again. Now, I've taken to downloading everything straight away and saving it on my PC, then the only problem is trying to decode the sometimes cryptic titles. But if the worst comes to the worst, then you can always search for content and that always works, even if it sometimes takes a while!
 
I found the relavent page eventually in the Siemens "book" page two hundred and fifty odd. It actually has the cheak to show two voltage sensors connecetd to an analog input apparently floatring relative to ground and then at the start of the section (perhaps an after thought after peole complained) they have added a paragraph .......

See the last page of attached PDF

Reminds me of the opening section of a hitch hikers guide to the galaxy, the bit where he complains that the Planning Application to destroty the earth/ destroy his house was not all that well publicised.......

Anyway folks that is the gen. beware of connection these AI if everything is not at the same voltage....
 

Similar Topics

Hello I normally do AB work but recently I was called in to check a machine with a Simens S7-300 PLC, the machine is not starting. I've noticed a...
Replies
0
Views
3,004
HI I HAVE SF RED LED Fault on SM331 MODULE OF (S7-300 CPU 314)PLC. HOW CAN REMOVE THIS FAULT ON AI SM331 MODULE?🤞🏻
Replies
0
Views
3,155
Hello all, Here is what I am trying to figure out: (Note: The Thermistor itself is fixed there is no way I can change it out so I have to...
Replies
4
Views
3,893
Dear sir, I am using SIMATIC 300, CPU 315-2DP , (6ES7 315-2AF03-0AB0) VIPA 603-1CC21 A1.0 RAM 32KB, Firmware=V4.0.8 The problem Im using MPI...
Replies
1
Views
61
Hello. I have a few machines that use Kinetix 300 (each machine has two drives). Both drives on one of the machine keep losing IP address. They...
Replies
2
Views
75
Back
Top Bottom