1794-l34 ab flex major fault

having a issue and just wondering if anyone else has run into something like this. have a l34 with 2 local busses. anytime we power down the unit it goes into a major fault and dumps the program. After doing alot of troubleshooting it seems to be related to the local2 bus. Ive swapped I/O modules and bases around and stuff and it seems to do it no matter what is hooked up to the local2 flexbus adapter. Ive tried changing the processor, the cable between the processor and the flexbus adapter, and the flexbus adapter it self. I cant really pin the problem down to anything since I have changed those three devices. I saw in knowledgebase that they have an issue with a 1794-it8 module that causes this, but we dont have that module in tis system. plus the fix for it is to not check the box that makes the controller go into a major fault if comm between the modules is lost. we dont have that checked for any module. The funy thing is that it will do it without any logic in the controler. if I pull the battery and power down the unit, it will come up ok but if I power down again it goes into fault, no matter if I dump a program or leave the controller blank. Im pulling my hair out over this one.

a long time ago..

we had a flexlogix processor (it was the L33 though) acting really oddly. The processor was running out of memory.
I know it's a longshot, but ... take a peak.

thanks for the reply. the I/O memory shows 80k out of 108k free. the data/logic memory shows 186k out of 468k free. also, the processor rev is 15.6, just for reference.

I am trying to follow your description of the events that lead up to the fault.

The funny thing is that it will do it without any logic in the controller. If I pull the battery and power down the unit, it will come up OK but if I power down again it goes into fault, no matter if I dump a program or leave the controller blank.

Click to expand...

Are you saying that the controller only (and always) faults if there is a battery installed during power cycle ?

When you attempt to go online with the faulted controller, RSLogix 5000 should allow you to view the Minor Faults and Major Faults status of the controller. Can you describe the Fault Codes that you are seeing ?

What exactly led you to think that the problem is related to the Local2 bus ?

Is the power for the I/O modules and the power for the 1794-L34 controller coming from the same 24VDC power supply ?

Eddie Willers said:

I am trying to follow your description of the events that lead up to the fault.

Are you saying that the controller only (and always) faults if there is a battery installed during power cycle ?

yes, I guess because the processor is cleared when you power down and power back up without the battery.

When you attempt to go online with the faulted controller, RSLogix 5000 should allow you to view the Minor Faults and Major Faults status of the controller. Can you describe the Fault Codes that you are seeing ?

unless I am doing something wrong it will not allow me to communicate with the processor.


What exactly led you to think that the problem is related to the Local2 bus ?

If I remove the local2 bus from the processor it does not go into fault. It is ok if I leave just the local 2 flexbus adapter it is ok, but as soon as I plug a single I/O module or any combination of I/O modules the problem occurs

Is the power for the I/O modules and the power for the 1794-L34 controller coming from the same 24VDC power supply ?

Click to expand...

each module is on a separate breaker.

When you try to go online with a faulted controller, RSLogix 5000 presents a "Connected to Go Online" dialog box. By default you're on the "Options" tab that allows you to browse to a project to download, or you can select one of the other tabs.

The Major Faults and Minor Faults tabs can be very informative when the controller is in this state.



The fact that each module is on a separate breaker leads to the question: what kind of modules are on the Local2 bus ? Are they different from the modules on the Local1 bus ? Is the power wiring different ? Are the modules on the Local buses powered down at the same time as the controller, or do they remain powered ?

I'm trying to help you determine if the fault occurs during power down, or during power up. It would also be helpful if you could post your *.ACD file, since it would answer quite a few questions.

Is this a system that was built and installed in-house ? Is it a new installation ? Did it ever work as desired ?

One of the reasons for carefully browsing the controller with RSWho and looking carefully at that dialog when connecting to go online is to determine if the controller really did clear the user program or if it simply needs you to clear the faults or select a file when you go online.

Eddie Willers said:

When you try to go online with a faulted controller, RSLogix 5000 presents a "Connected to Go Online" dialog box. By default you're on the "Options" tab that allows you to browse to a project to download, or you can select one of the other tabs.

The Major Faults and Minor Faults tabs can be very informative when the controller is in this state.

The fact that each module is on a separate breaker leads to the question: what kind of modules are on the Local2 bus ? Are they different from the modules on the Local1 bus ? Is the power wiring different ? Are the modules on the Local buses powered down at the same time as the controller, or do they remain powered ?

I'm trying to help you determine if the fault occurs during power down, or during power up. It would also be helpful if you could post your *.ACD file, since it would answer quite a few questions.

Is this a system that was built and installed in-house ? Is it a new installation ? Did it ever work as desired ?

One of the reasons for carefully browsing the controller with RSWho and looking carefully at that dialog when connecting to go online is to determine if the controller really did clear the user program or if it simply needs you to clear the faults or select a file when you go online.

Click to expand...

i cannot communicate with the controller while it is in fault mode. the ok led is solid red. i hav to recycle the power for it to allow me to communicate, and then when i check the fault tables, they both show clear.

local2 bus consists of 4 analog inputs and a analog output module. llocal 1 has digital ins and outs.

just the controller is being powered down.

I am not sure if it against company policy to post the loic so i cant do that, but the project was built by a engineering firm. i think this problem has been happening since it was installed but this issue just came to light.

Well, a solid red OK LED is a major problem; even a controller with a major fault (like an index overflow, or an I/O connection fault) will allow you to communicate with it during fault mode.

I remain unclear on the conditions under which you cycle power and clear the fault. It appears to me that you are saying "I turn off power to the FlexLogix controller with the battery connected and a program loaded, and when I turn power back on the controller has an unrecoverable Major Fault that results in a solid red OK LED. When I cycle power again, also without removing the battery, the controller recovers but the user program has been cleared and there is no Major or Minor fault indication".

If that is incorrect with regard to sequence, results, or battery condition, please describe the precise sequence of power application.

It is curious that the Local2 I/O port is the one that seems to be related to the controller crash. Analog modules generally get their operational power from outside the FlexBus, in contrast to discrete modules that get their power from the Flexbus connection.

This really sounds like a job for Rockwell Automation technical support. You might also ask about the Fault Dump Utility, which will extract information on a controller operating system crash, as long as the battery has not been removed.

Also check the firmware revision of this controller; for 1794-L34's with v15 you need to be running at least 15.06.

yeah you have the sequence down.


I am working with tech support now. I tried the fault dump utility and it says that there is no file available. firmware is 15.06.1

found the issue.

I received your application and looked it over and found some interesting things.

The first thing that jumped out at me was that you had Flex HART on Local 2 bus. We do not support the HART protocol on any local bus, only ControlNet (with ACN) and Ethernet (with AENT v3.1 FW or higher). See this tech note for reference:

http://rockwellautomation.custhelp.com/cgi-bin/rockwellautomation.cfg/php/enduser/std_adp.php?p_faqid=33477

Now, having said that, putting HART modules on Local 2 should still work as a plain analog module (no HART data) and not crash the controller, so I spoke with engineering and found that indeed this is an anomaly that was found and occurs using HART modules on Local 2 bus. There is no work around currently known other than keeping the power on or getting a ControlNet card for the L34 and a 1794-ACN(R)15 and connecting the flex modules to that adapter. Of course if the adapter is used, HART will then work also.