The one that you apply and install properly. Most of the failures I've seen were due to misapplication or improper installation. For example, using relay outputs to flash lights is quite common. You will never know how reliable the PLC is because the relay will wear out.

Switching inductive loads with relay outputs with no arc suppression is another favorite way to reduce PLC life. Ignoring current ratings, etc. Switching inductive loads with transistor outputs with no flyback diode can produce instant failure. Wrong type of contacts for the load, excessive heat or shock. It's really hard to speculate on reliability as hard failures of the electronics are pretty rare. And the manufacturer doesn't publish much for data.

Regards
cww

 

Dear Instrumentation,

Based on my experience, most reliable and cost effective small PLC is
Allen-Bradley Micrologix family (1000 & 1100).

Also, programming software for this family is free (RSLogix micro
starter lite + RSLogix Emulate 500)

Regards

Ikhtiander

 

Curt makes a good point that reliability of PLCs is heavily influenced by proper application.

Often proper application is not real obvious to the inexperienced,
untrained, and/or uneducated.

Part of getting that experience, training, and/or education seems to be
making bad choices along the way.

 

For example, I had about 5 RSLogix 1000s fail with memory issues in a short period of time. One user's experience has been that they are reliable. From mine. could I conclude that they are not? I could, but it would be wrong, because it was an anomaly.

No doubt it caused them a lot of headaches, but I still prefer the little AB bricks and don't doubt they are reliable. I would happily specify Automation Direct if they were not, but the time saved with the superior, if somewhat expensive, software nudges me back. And if not AD then Mitsubishi. They are all reliable.

I prefer transistor outputs for new designs, but relays are sometimes a practical necessity and you accept that they wear out. But I do include suppressors whenever possible and as indicated. There are a lot of orphans out there as well, where the PLC life exceeded the manufacturer's. So take any opinions with a grain of salt.