S7 IM153-1 analog input

Guys,

I have an s7 315-2dp (2ag10) with remote i/o running on an et200m module. On the remote i/o rack there is an analog input module (6ES7 331-7kf02).
My problem occurs when the remote station loses power, in that the analog signal "burns out" high (27648 in Step7) or full scale when displayed on HMI.
As I log the high and low limits is there a way that I can save the value before the power failure occured, ie the last actual true value?

I have an analog input card on the main rack and the same problem does not occur here.

I'm thinking that I need OB40. Any help is appreciated.

Hope this question makes sense.

Many Thanks,
Barry.

What update rate do you require for the analogue input ?

Is the analogue input inside the process image?

Thanks LD

The update rate is not critical as this is only logging temperature in a cold room. The input is outside the process image.

You could check for the presence of the slave on the DP network.
If not available, you mute the logging for the analog values in question.
You may have to play around with some timers, to allow the check for available nodes before logging the values.

In THIS THREAD is a discussion how to check for available DP nodes.
See PLucas post #4 for a simple way to do it.

I'm already looking for the presence of slaves using SFC51. The max and min values are actually used for the upper and lower scale for a dial display in wincc flex. These values are got by comparing the current scan value to the last scan value and stored in db memory area.

Thanks,
Barry

You will need a shift register or the like to have saved analogue values from when the station was up. When the station goes down, revert to a value from the shift register that was good.

It appears that the analogue input conversion is going through a transient before the station stops responding on Profibus.

I am thinking along the lines of what LD says.

I am guessing that it is not that the IM153-1 goes off the bus that causes the out of range analog values.
It is probably when the entire panel with the IM153-1 and analog card is shutdown, that you the IM153-1 'lives' just a little longer than the analog card, enough to pick up the out of range value and transmit it to the DP master.

Maybe an easy fix would be to monitor the 24V on a digital input, and use that to mute the analog values withou having to play with times and shift registers.

Thanks everyone,

That makes perfect sense that the IM153 is "living" a bit longer than the analog card. I have put a second compare in the rung that updates the max value so that if the analog is at it's max valve it cannot update the stored max value. Maybe it will work.

Regards,
Barry.