[volt-nuts] 3458a RAM batteries - longer life if unit is continually powered?

[Tom Knox]

Date Codes can be misleading because most of this style of chip does not connect to the battery until the first power up cycle. So evl

Thomas Knox



> Date: Thu, 27 Mar 2014 17:33:17 +0100
> From: frank.stellmach at freenet.de
> To: volt-nuts at febo.com
> Subject: [volt-nuts] 3458a RAM batteries - longer life if unit is	continually powered?
> 
> Hi,
> 
> the DALLAS datasheet specifies 10 years data retention. Quite obviously, 
> that's the unpowered time.
> 
> That time includes self discharge and RAM sustaining currents.
> 
> self discharge and RAM supply current will be of the same order, i.e. < 
> 1µA, I assume from experience.
> And I assume also, that those currents will vary over samples, i.e. over 
> samples of battery and RAM.
> 
> Recently, there was the case, where the CAL RAM was still alive after 24 
> years (instrument vintage 1989, same DALLAS date code!), but with 
> unknown powering history.
> 
> Additionally, a typical 10 years of lifetime is given also for those 
> batteries.
> 
> Therefore, in practise there will be no pronounced difference between 
> powered / unpowered state.
> On the other hand, DALLAS implemented an electronic seal, so that the 
> RAM inside the package is initially powered only after first power up. 
> But there's no parameter in the datasheet, specifying data retention 
> according to date code or to initial power up.
> 
> 
> The waste of energy just to extend the batteries lifetime, does not pay off.
> On the contrary, the MTBF time of the instrument will be consumed, the 
> display will get bad, and here in Germany, this energy consumption would 
> cost a fortune (meanwhile: 25+ Cents / kWh, crazy).
> 
> 
> Yes, as long the instrument is powered, the CAL RAM will not lose its 
> content.. but what about unexpected power outages?
> 
> 
> If you save the CAL RAM and repogram a fresh one after failure, there's 
> no need to send it to agilent.
> 
> 
> No, not any RAM error needs recalibration.
> Those error messages are not documented.
> 
> So I've scanned the text fragments in the firmware file:
> 
> RAM TEST 1
> RAM TEST 2
> 
> probably refer to either to the configuration RAMs (2x32kB), andthe CAL 
> RAM (8kB).
> 
> Eror messages are:
> NONVOLATILE RAM HiGH /LOW
> CAL RAM FAILURE
> 
> and known ones from other users:
> 
> RAM TEST 1 HIGH / LOW
> 
> So I assume, that a real failure of the cal constants only apply on 
> message: CAL RAM FAILURE and perhaps RAM TEST 2 ...., as RAM TEST 1 
> should refer to the configuration RAMs only, HIGH to the high byte RAM, 
> LOW to the low byte RAM.
> 
> Errors of the cal RAM will manifest in additional ACAL or CAL failures.
> 
> 
> Frank
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