Rack failure

tiger45

Member
T
Join Date
Jan 2022
Location
egypt
Posts
5
Hello experts,

I am new with siemens,

I have CPU 317 with 4 racks with im-153 (profibus)

Sometimes every 2 days the fourth rack stopped for one 0.027 sec only then back again to normal.

We concentrate on monitoring on CPU bf and found that it illuminate for one second then go off!!!

We check the tighten of profibus cables connector and check for all racks led, all seems normal.

Start to improve grounding and shielding without progress

Attached is the buffer diagnostics results (date and time is not adjusted)

Noted that this setup was from 3 years without any change.

I think that changing profibus cable may solve the problem but need to ask if there is any troubleshooting recommended before doing that as it will be a very hard job.
OB 86 and OB 122 are included at CPU programe

IMG_20220124_140307.jpg
 
We've run into this before. What speed is your network set at? If it is currently set at 12 Mbps, try slowing it down to 1.5 Mbps if your process allows for it. We have found many troublesome networks fixed by slowing them down.
 
baref00t25 said:
We've run into this before. What speed is your network set at? If it is currently set at 12 Mbps, try slowing it down to 1.5 Mbps if your process allows for it. We have found many troublesome networks fixed by slowing them down.
Click to expand...

Thanks Sir for your reply, the speed is 6 Mbps.
Noted that this setup was built from 7 years and was running normal.
If I do that should I upload the programe to CPU or I can send only the modification only (simatic manager 5.6)
 
We have seen machines that have been running for many years start to have issues with Profibus that has been untouched. Slowing down the bus helps before replacing all the cables. To change the bus speed you do that in the Hardware configurator which then requires a compile and download to the CPU. This will require the CPU to be restarted since it is a hardware change.
 
You might want to consider profiling your Profibus network with an analyser tool such as ProfiTrace.

I've had great luck with this particular analyser on several jobs. It's quite effective at determining the network topology, segment lengths, differential voltages, active nodes, and scope traces for each node.

A tool like this can be very helpful to identify a misbehaving node, excessive oscillations, etc.

Attached are few screenshots from a small network analysis.

-Trevor

profitrace-tested-subjects.PNG profitrace-tested-subjects-fail.PNG profitrace-diff-voltage.PNG profitrace-scope-1.PNG
 
Last edited:
6 Mbps is relatively high.
The Profibus network will be relatively sensitive to disturbances at this speed.
What is your requirement for the TTR (the cycle time of the bus) ?

What are the distances between stations ?
The dropouts may be caused by noise or problems with earthing or equipotential connections between stations.
A short-term remedy, which is a not a real solution, is to adjust the bus profile manually. Change the "token retries" from 1 to 3 makes the Profibus less sensitive. Can also cause the response to take longer.

It is possible that the IM153-1 that drops out has become bad. Try to swap it with one of the other IM153-1 modules (change the address switches accordingly), and see if the problem follows the module or not.

I also recommend profitrace to troubleshoot tricky Profibus problems.
 
JesperMP said:
6 Mbps is relatively high.
The Profibus network will be relatively sensitive to disturbances at this speed.
What is your requirement for the TTR (the cycle time of the bus) ?

What are the distances between stations ?
The dropouts may be caused by noise or problems with earthing or equipotential connections between stations.
A short-term remedy, which is a not a real solution, is to adjust the bus profile manually. Change the "token retries" from 1 to 3 makes the Profibus less sensitive. Can also cause the response to take longer.

It is possible that the IM153-1 that drops out has become bad. Try to swap it with one of the other IM153-1 modules (change the address switches accordingly), and see if the problem follows the module or not.

I also recommend profitrace to troubleshoot tricky Profibus problems.
Click to expand...

Thank you all for your support.

The profibus length is 80 meters.
I don't know the delay time will affect the operation or not as I need to know how to calculate the value of time delay of communication from CPU to this reack.
For IM module, already replace it with a new one without progress.
Attached is the cycle scan time for the CPU.
Earthing is good for the offshore rig that the system is installed in.
unfortionaly i do not have profitrace on location but i will try to get it.

Thanks in advance

1.png
 
You should open the hardware configuration, then open the Profibus network properties, and then open the details of the Bus Parameters.
Post a screenshot.
The interesting value is the "TTR". Not "TTR typ".
And also if is type "DP", "Standard", or "User Defined".

Do you know who specified the 6Mbps and why ?
I would discuss the need for this bus speed, and if it is possible to drop it to 1.5 Mbps.
 
You say that "earthing is good". But you have to check that there is an equipotential bonding cable between all stations as well. This is just as important, and it is NOT the same as the general earthing connection.
 
JesperMP said:
You should open the hardware configuration, then open the Profibus network properties, and then open the details of the Bus Parameters.
Post a screenshot.
The interesting value is the "TTR". Not "TTR typ".
And also if is type "DP", "Standard", or "User Defined".

Do you know who specified the 6Mbps and why ?
I would discuss the need for this bus speed, and if it is possible to drop it to 1.5 Mbps.
Click to expand...

Attached are the bus parameters and DP as well.
I dunno why they programmed it with 6 MB, this rack holds all switches i/ps to operate motors, emergency stop and reference speed for motors.
if i dropped the speed to 1.5 MB, will it affect the scan cycle of CPU to scan all inputs or not?

thanks Sir for your time.

2.png 3.png
 
At the moment you have TTR = 9.6 ms.
If you have very fast (short) pulses you have to sense, or fast motion to control, then it may be necessary with a fast update rate.
If you dont have anything particularly fast, then you can drop the speed.
Does the master drives control something that require tight control, i.e. motion ?

Save the STEP7 Project under a new name, then change the bus speed to 1.5Mbps, compile the hardware and check the TTR again.
edit: But dont download the modified HW configuration before you know the consequences. Maybe you need approval from someone.
 
Last edited:
I notice that "retry limit" is set to 3.
The default value is 1. Someone has already made the change that I suggested in post #6.
That is a sign that the Profibus have had problems from the beginning.
 
Look for carbon score marks on the racks. Sometimes those grounding build carbon up on the din rail. I only seen this on point io stuff. Reducing of speed of profibus might be necessary like others have said. Check the profibus connector terminating resistors, make sure the switch did not accidently get hit. Where the switch is a passthru make sure its in passthru or if its end of the line that the terminating resistor is properly terminated. I have had problems where ground was not passthru the cable and then had to reterminate the connectors to make sure the ground shield is passed thru all the connectors.
 
Jim3846 said:
I have had problems where ground was not passthru the cable and then had to reterminate the connectors to make sure the ground shield is passed thru all the connectors.
Click to expand...
Even though the shield gets connected to ground by the DB9 connector, a more secure shield termination is by using shield termination clamps that fits on a dedicated earth busbar, like this one.
https://encrypted-tbn1.gstatic.com/...ZEP2aXojU6oxJcW3OoOGR3xOUlW1EPVKHyjyxz0DlXCW4

We use them whenever a shielded cable has to pass between cabinets.

If there are noise problems, it is one of the things I would do.
 
It is enough to put near that Profibus cable some relatively bigger power cable and cause problems. Sorry to say this, but I've been several times in that part of the world and I've seen some really bad practice - for example I've never seen communication cables in separate duct and cable tray and always I've seen unshielded cables from VFD to motor.
 

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