Local Fault on SLC 500/05 ASB


Thread Starter


We have SLC 500/05 plc (Allen bradly) here at site. it is found that the asb (ASB 1794) on the rack 3 is declaring local fault with out any provocation.

rack 3 is the last rack and we have 6 analog input cards and 2 analog output cards (respectively) on that rack. we have tried our best to remove that fault but still that fault is popping up.

a few days prior to the commissioning (May 2014) of the plant it started showing "local fault". very first times the local fault indication was blinking. and then it started flickering. when it starts blinking, the status indication on analog cards (7th and 8th slots) starts blinking. and gives weird and random signals on the every channels on the cards and the signals on the 6th card (analog input) won't be read out. and give random values on the SCADA.
remaining 5 analog input cards don't show any problem at all.

the plant ran smoothly for 1-2 weeks. after that we took a shut down and now all of a sudden it declared local fault. the point is that we took a shut down and all the instruments and motors are not on line.

we have put resistors on the scanner and the ASB of the last rack (150 ohms each). the dip switch on the ASB is checked and the baud rate is selected to 57.8.

just to confirm that the chassis and the cards are not faulty, we changed the chassis and cards of slot 6 and 7. still this same problem popped up.

only on rack 3 we face this issue.

what could be the reason of the local fault declaration? how could i get rid of this? can anyone please help me out to resolve this issue?

Bob Peterson

generally when something has been working and then stops after a shutdown it is best to look at what someone changed during the shutdown.

since you mention 1794-ASB, presumably you are dealing with flex I/O for this "rack". however, that does not make any sense based on your statement that you changed the chassis since flex I/O does not have a chassis.

if it is indeed flex I/O, carefully check the connections between the bases. sometimes they do not get put together correctly.

as for "local fault", that makes it sound like flex I/O since it has an led labeled "local fault", but again, since there is no chassis associated with flex I/O I don't understand how you could have changed the chassis.

my guess would be someone screwed around with the modules during shutdown and unintentionally disconnected one or more bases from the flex bus. it is not all that hard to put them back together with the connectors not connected correctly.

Thanx Bob for your response.

Let me clear something first, by chassis i meant the back plane of cards.

The issue is that we still don't known about the root cause. The local fault indication blinks itself without with any provocation.

When ever local fault blinks, the system gives random values on the AO cards on that rack. Last 2 cards are AOs and the rest all 6 cards are AI. The 6th card (AI) also doesn't gov correct value.

Evandro Carlos Vizicato

Please, check if all cards powerup at the same time. The 24-Vdc power need to be applied at the same time to all modules. If the ASB module receive the 24-vdc first, the local fault will blink.