T100MD-888 Timers

[dave333]

I'm using a 888 with a Xinje 562 Touchscreen operating on Modbus RTU.  Everything works fine but my timer values are off by about 12%.  If I set a timer value of 30 seconds (e.g. SetTimerSV2, DM[2]) where the value of 300 is entered into DM[2] via the touchscreen) my timer doesn't go until 34-35 seconds.  I've tried HStimers with same result.  Am I missing something or will I have to go to a counter type timer?

[support]

Since you are using the DM[2] value to update the timer SV, are you executing the SetTimerSV every scan of the ladder program? If you do you should remove it because SetTimerSV writes to the EEPROM which has finite lifespan (100,000 cycles) and it can quickly exceed the EEPROM life span. Also when a EEPROM write occurs the PLC's interrupt shuts down and that would affect the accuracy of the timer because of the loss of interrupts.

What you can do is to compare the DM[2] value with a previous value stored somewhere, and only run the SetTimerSV command when the value of DM[2] has changed. You only need to run this custom function that compares the DM[2] with previous set value periodically e.g. 0.5s clock pulse so that it doesn't take up too much execution time.

[ccdubs]

Hi Support,

I have been writing to eeprom on each cycle of a plc scan, obviously unaware that eeprom has limited life.....what happens when the EEPROM lifespan is exceeded?

Will it be just that memory location that has issues or the whole thing?

I have been using save_EEP DM[51],1. Where DM[51] is only changed once a month or so by manual command.

[support]

If an EEPROM location is "worn out" what it means is that particular location may not store the data you have written to correctly. If you are still able to read the correct data after your write to it (E.g. write a &HAAAA to it and read back, then write a &H5555 to it and read back to check) then it is probably still OK.

The earlier PLC models (before 2002) the EEPROM life cycle is about 100,000. The later model could withstand at least 1 million erase/write cycle.

If you are concerned with the effect of the past write cycles  exceeding its life cycle and could impact future operation, you can change the location you are writing to, or simply replace it with the standard EEPROM (M2017P) or the expanded EEPROM (M2018P).

[ccdubs]

Thanks support,

So to clarify (sorry) I am certain that we have exceeded 1 million cycles and it is unlikely that I will be able to change the code for a while yet. So it is likely I have "worn out" the eeprom on a number of units.

The only time those eeprom addresses are read is on the first scan so it would require a power interruption for the PLC's to reset (unlikely considering they are all battery backed up). If the EEPROM is worn out would there be any issues on an EEPROM read at reboot? Can any other issues be expected during normal operation.

The value being written to eeprom isn't'a changing value, does this extend eeprom life at all?

Finally, assuming the data I am storing in EEPROM is uninmportant how urgently do i need to stop writing to eeprom on each scan.

Thanks again for feedback and responding so late at night!

[support]

Whether you write the same data or different data wouldn't make a difference to the EEPROM since it will always erase the word location before writing to it.

If the EEPROM content is only read on the first scan then it wouldn't matter since you don't need its content until the next time you reset the PLC. I have not experienced a "worn out" eeprom so I can't say for sure if it will become "uneraseable" or "unreadable". But I don't think it will affect the rest of the PLC's operation to have the worn out EEPROM sitting in the socket. Just be sure to check the data read into the PLC after the next restart.

I would still recommend that you take measure to change your program soon, then replace the worn out EEPROM on those PLCs that have been writing to the EEPROM every cycle of the program.