Life Expectancy of MX-RTC Real time clock on T100MD888+

[benb2000]

Have been running for almost 2 years, but now experiencing data retention problems of the SDRAM. Is 2 years an acceptable life expectancy?

Voltage on pin 28 of the MX-RTC when powered off is 2.927 Vdc.

Curiouesly when observing the voltage on Pin 28, it suddenly dropped to 0.0v, and then reapplying the power to the PLC, the program variables were gine. ( as expected).

Powering down again, the voltage at pin 28, returned to 2.927 V.

Is this maybe a socket problem? I have used your suggestion of using WD40 on the pis of the SDRAM as well as the pins of the MX-RTC.

Your input is appreciated

[benb2000]

Just another little tidbit, it turns out that if the tye wrap that hold the MX-RTC sandwich onto the PLC is cinched tightly, the voltage on Pin 28 goes to 0.0 volts. Pulling down too hard on this sandwich seems to disconnect pin 28.

I have two PLC's complete with MX-RTC sitting on the shelf as spares. One had SW1-1 on and one had SW1-1 off, the one with SW1-1 on has a dead battery, Pin 28 reads 0.20 Vdc, and the other with SW1-1 off, pin 28 reads 2.905 Vdc.

So a lesson learned, DO NOT leave a MX-RTC plugged in to the PLC, with the SDRAM in place and SW1-1 ON.


I have been disappointed with these sockets from the beginning.

The PLC's work well, but the MX-RTC design could have been more robust. We currently have 27 units, that are approaching 2 years old .

Any comments are appreciated

[support]

Usually the Lithium battery in the MX-RTC should last about 5 years assuming normal usage. It doesn't matter if DIP SW 1-1 is ON or not. Is your PLC power in the OFF state most of the time?

One possibility of a drain MX-RTC battery may be caused by a bad SRAM which consume more power than it is supposed to when powered down. Does your SRAM has the "LL" suffix which is for ultra low standby power?

Note that the PIN 28 on the PLC SRAM socket bring power into the MX-RTC. Internally the MX-RTC switches between +5V power or the lithium battery to the SRAM that piggy-back on it, depending on whether there is +5V present at the socket pin 28. If pressing down on the SRAM (tightening the cable tie) causes you to lose voltage on pin 28, it means a contact problem at the contact between pin 28 of the SRAM and the MX-RTC pin. A contact cleaner such as WD-40 will work provided the socket spring is still able to hold the pin properly. If need be, use a little tool to push on the socket leaf spring to provide better grip on the SRAM pin.

[benb2000]

The SDRAM part number on all units  is UT62256CPC-70LL, as originally supplied by Tri-plc, no substitutions have been made.

How do we explain the loss of battery voltage on an MX-RTC stack, that has not been powered up? Bad SDRAM? Bad MX-RTC?Should I be elegible for a replacement? It has not been 5 years, it's less than 2 ...

As mentioned, the units in the field have a duty cylce of about 25 %, on during the day, off at night.

You haven't mentioned the data retention cut off voltage. Is 2.905 Vdc acceptabe? Or might it be that this unit is in a portable case, and when handled, the pin #28, loses contact momentarily, then the variables are lost.

If I write these variables to the EEPROM, does the life of the EEPROM get consumed just by reading it? Or it the life of the EEprom regulated by writes only.

[support]

If the MX-RTC has never been installed or powered up, then the battery freshness-seal circuit inside MX-RTC will disable the connection of the internal battery to the 28 pin. Please install and power up the MX-RTC once and then remove power to the PLC, then measure the voltage on pin 28 again. If you see 3V now that means that the MX-RTC is still good. Please check. If after powering up and down the MX-RTC is still not providing 3V that means it is bad and a replacement can be arranged with your dealer.

2.9V is still a good voltage to keep the content of the MX-RTC.

Data written to EEPROM are non-volatile. Data EEPROM can be read unlimited number of times. The write cycle life of 100,000 times is only applicable to the write cycle.

[benb2000]

I powered up the dead MX-RTC and it returned to 0.20 volts after power was removed.

My local rep, has offered already offered a replacement

The MX-RTC has a DS-1216C marked on it. Dallas semiconductor has a product that marry's SDRAM, timekeeping, and battery backup in one package, a DS1244Y-70. Would this work in the T100MD-888+ PLC? This would eliminate one socket junction.

[support]

You can try it but we are unable to confirm if the alternative you mentioned will work. Previous attempts by others to use alternative RTC solution did not yield postive result.

Our dealer mentioned that your application is in a vehicle in oil and gas industry and subject to severe vibration. This requires you to take special actions in term of shock absorbing mounting of the PCB. One option is to eliminate the socket and solder the MX-RTC directly to the PCB. Special arrangement can be made with the factory to purchase the PLC with MX-RTC soldered directly to the PCB without using the IC socket. This should reduce one potential problem area. However, the draw back is that the MX-RTC cannot be replaced easily by users but it increases the reliability for application in severe environment.

[benb2000]

I'll add another quirk to the puzzle... We have 3 of the finished instruments in our warehouse, two that have been there since we built the units, December of 2005. and one that was checked on March 30, 2006. These are units that have been powered up several times, run for 24 to 48 hours to check programming and the Time/date, before being warehoused.

Checking all 3 today, I found that all three had no voltage on pin 28 of the MX-RTC. Upon powering up, the program didnot function properly, all DM[] data was gone, and the date was 1/1/1996 0:0:0.

I reloaded the program, set the date, powered off, and measured the voltage on pin 28. In all three cases it was about 3.25Vdc and holding. The measuremets were made with a Wavetek 4.5 digit multimeter, with extra fine tips, and measuring on the SDRAM pins. The measurenments were made several times

Did I put enough pressure on the SDRAM pins to re establish contact with the MX-RTC socket? Without question there was no voltage there until after poweing up and powering down.

Is there an unpublished quirk to the DS1216C that it re-establishes the "Freshness Seal" if the unit is not powered up for a period of time? No literature at Maxim/Dallas indicates anything like this.