I teach a PLC class that includes a DeviceNet portion. During this portion, I have my students set node numbers for various DeviceNet smart devices. I have been having problems with a lot of sensors failing recently, particularly the stainless steel prox switches (871TM-D5ED18-D5).

I've heard that flash memory has a limited lifespan with regards to the number of times you can write to it. So I'm wondering if having my students change the addressess and configuration each class is causing these switches to fail. Can someone shed some more light on if this is possible or not?

 

Flash memory typically has a limit of 10,000 to 100,000 write cycles (with the actual limit depending upon the type of flash memory). If people are using a hand-held configuration device to change the node numbers, it is hard to
exceed this limit. However, if the node configuration can be changed from the PLC via a user program, then the limit could be exceeded within a few minutes.

 

I have heard nothing good about using software selected addresses for Devicenet. It seems to work pretty reliably in some cases, in others it is problematic. It is one of the mysteries of life.

I seriously doubt you could make the flash fail in the relatively small number of times you are writing to it.

 

The less expensive flash memory - like what's in our cell phones and many (most?) smart instruments - does have a limited number of "writes". As device address is one of those things we'd like our sensors to remember when power is removed, it is likely stored (and re-written) to flash memory each time it's changed.

I think they are good for c. 10^4 to 10^6 (??) cycles, but maybe this varies by chip. Prox switches might have itty-bitty ones. Did you ask A-B / Rockwell about this?

I know our favorite valve positioners are now shipping with a better technology of some kind (instead of flash). But what can be put in a positioner that sells for $2000 US is maybe beyond what you can put in a $200 prox switch.

This is a good thing for your students to learn. Some of us had to learn it in a real plant which is not as nice.

John Rezabek
ISP Lima LLC

 

Many flash systems now have management that spreads the writes around the array to mitigate this.

Regards
cww