1756-L55 Processor putting itself to Remote Test mode?

[MikeRKC]

Hi all,


Any experience of the above?


Happened 3 times now in 1.5 months. Processor is about 10 years old, software major revision is 19.


Machine crash shuts, plc not in run. Open RSLogix5000 go online with processor and it is in Remote Test mode. Change it back to Remote Run in software and it's fine.


I thought you could only go in to Remote Test via Logix 5000?


Have contacted Rockwell UK tech support. They have sent links to a couple of KB articles showing that the processor can be put to Program Mode under certain faults, but the articles don't mention Remote Test mode.


Any thoughts?


Thanks,


Michael

 

[I_Automation]

M first though would be to flash the firmware up to v20 using RSLogix ver 20.05.


My second thought, and about 1/8th of second later, is replace the CPU

 

[alan_505]

I thought the 1756-L55 could only go to v16.

Just checked with RSLogix 5000 and the highest I could get a L55 was v16.

 

[I_Automation]

I thought the 1756-L55 could only go to v16.

Just checked with RSLogix 5000 and the highest I could get a L55 was v16.

I just double checked, plus have a L55 sitting on the shelf I haven't been able to use for anything, or sell on eBay.


L5x's are limited to v16
L6x's can go up to v20
L7x's can go to v34 so far

 

[Ken Roach]

Welcome to the PLCTalk forum community !

I seldom make absolute statements about what is possible with logic controllers, especially because the 1756-L55 had some malfunctions that were exotic, infrequent, and difficult to trace. Those included the memoryc checksum major non-recoverable fault after 3-4 months of continuous operation that affected all 1756-L55 CPUs with v15.03, 15.04, and v16 prior to v16.21, and a (relatively) common backplane interface chip failure that required replacement or remanufacturing.

But I have never heard of a ControlLogix controller spontaneously switching to Remote Test mode. I am unaware of any method for transitioning to Remote Test mode except using a command from RSLogix.

So the most likely explanation is that somebody is inadvertently, or intentionally, switching the controller to that mode.

The first diagnostic I would do is to physically move the keyswitch to RUN mode and remove it from the cabinet, which will prevent any mode except RUN or Major Fault.

 

[I_Automation]

But I have never heard of a ControlLogix controller spontaneously switching to Remote Test mode. I am unaware of any method for transitioning to Remote Test mode except using a command from RSLogix.

Just a weird thought to throw out - but could a glitch in a connected HMI maybe replicate the "Remote Test" signal that should only come from RSLogix?

 

[Ken Roach]

I've worked on systems where a custom HMI or MES software with a reverse-engineered driver caused a memory fault by writing to physical memory. But those were faults, not a function command with an incorrect argument.

There's no reason for any HMI or MES software to attempt to change a controller mode. Malware or sabotage, maybe. But not an ordinary comms driver.

By far the most likely reason that a controller is appearing to spontaneously change modes is the actions of an operator online with RSLogix.

 

[I_Automation]

I would do 2 things.


Every minute log the current time to a special DINT array. Go online with it in Remote Test and that array will tell me the last minute the PLC ran


As a backup program an output to always be on and power a 120V dial clock - that will show the time when the outlet went off because Run Mode stopped.


Also, I would wonder if this PLC was online and some ex-programmer was still able to remote into it for a little retribution.

 

[hardaysknight]

While I have nothing useful to add to this post, I will say that I have personally seen this happen twice.



I have no idea why. I was the only one in the plant with even the capabilities to go online with the processor or even open the cabinet.

 

[I_Automation]

I have no idea why. I was the only one in the plant with even the capabilities to go online with the processor or even open the cabinet.

The sole PLC5 that I still regularly work on (and have talked the customer into upgrading to a ControlLogix CPU but can't schedule the line down) has a PC based iFix HMI that IS online, and that PC has RSLogix5 installed.


Right now I can remote in to that HMI PC 3 different ways and get online with the PLC and do anything I wanted if I really needed a service call and wasn't afraid of prison - without being there or opening the control panel.