Electronics > Repair
HP 3478A: How to read/write cal SRAM
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lmester:

--- Quote from: fenugrec on January 22, 2020, 03:25:18 pm ---
--- Quote from: lmester on January 22, 2020, 02:28:29 pm ---Would you or fenugrec be able to help me get the gain data properly re-packed? Also, let me know if I need to make changes in the unpack algorithm.

--- End quote ---

Sure, but as I mentioned we may be slightly wrong (or at least uncertain) about the decoding; it would be really good to do some kind of test as I described, before spending too much time on a possibly-wrong encoding implementation.

--- End quote ---

That sounds good. Apply a fixed input to the meter and then enter various gain values. Is this correct? The most stable reference for me would be a resistor. I assume it won't matter which function or range is used. You only want the meter response to changes in the gain value.

fenugrec:

--- Quote from: lmester on January 23, 2020, 12:36:13 pm ---[...] I assume it won't matter which function or range is used. You only want the meter response to changes in the gain value.

--- End quote ---

Correct. And to make the math easier, set the Offset for that range to 0 too. Your resistor can be fine as long as you can ensure a tight connection for the duration of the tests; in 5-digit mode the last digit(s) should be pretty stable and repeatable. If not, @Miti can probably help (again !)

Gain constants: I think these should cover all edge cases:

--- Code: ---00000
00700
00800
00900
00a00
00f00
90700
77777
88888
99999

--- End code ---
Miti:
And how do I send these raw gains? I would have to read the calibration constants, modify the gain for the 3V range, calculate the checksum, upload the new constants?
lmester:

--- Quote from: Miti on January 23, 2020, 10:53:37 pm ---And how do I send these raw gains? I would have to read the calibration constants, modify the gain for the 3V range, calculate the checksum, upload the new constants?

--- End quote ---

That's what I'm testing now. I modified my cal edit program to accept a raw gain value. I enter 0 for the offset and 5 hex digits for the gain. The program calculates the checksum and sends the new cal entry to the meter. I then log the new reading from the meter.

If you're interested I can put the modified program up on my web site.

I may need to hold off on this and just let the meter run overnight. It's been on for several hours. The reading started at .68324 K Ohms it's slooowly decreased to .68311. I'm assuming that the meter and the resistor that I'm measuring have not yet reached a stable temperature.

Just looked over at the meter reading. It's been stable during the time I was typing this.  I'll try to get some more data.
Miti:

--- Quote from: lmester on January 24, 2020, 12:15:48 am ---That's what I'm testing now. I modified my cal edit program to accept a raw gain value. I enter 0 for the offset and 5 hex digits for the gain. The program calculates the checksum and sends the new cal entry to the meter. I then log the new reading from the meter.

If you're interested I can put the modified program up on my web site.

--- End quote ---

Sure I'm interested. Even if you do this experiment, I still intend to do it in the weekend.
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