Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
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joeqsmith:

--- Quote from: garrettm on February 08, 2021, 06:00:51 am ---Using 1kV range, zero off and internal triggering at the DMM and a 10 second gate time at the counter, I got the following results using the Scan Advance.
...

--- End quote ---

Thank you for taking the time to look into it.  It makes more sense to me now after reading your posts.   
SilverSolder:

--- Quote from: joeqsmith on February 08, 2021, 12:36:39 pm ---
--- Quote ---[...]
The code appears to start in U24 (at address 0000 in that EPROM), and continues into U23.  A 'gotcha'!  :D

--- End quote ---

They have 2K of RAM.  I wonder are they jumping there?   The RAM may have been faster so they may have copied critical routines from ROM to RAM.   Just a guess.

--- End quote ---

That's a good thought - but the 8080 reads the EPROM at a very moderate pace...  even slow EPROMs are already as fast as the 8080 is ever going to be able to read anything, is my (limited) understanding.  When I get some more time, I'll try to decode what is going on around those calls, to see if it is intentional or just a problem with the disassembly process.  This is not something I have ever attempted before...  may get stuck in the snow occasionally!  :D
SilverSolder:

--- Quote from: dietert1 on February 08, 2021, 10:05:54 am ---[...] And i don't yet understand the meaning of "T" or "T1" continuous reading modes.  [...]

--- End quote ---

You got to 125 readings per second, very nice!  :D

The T command appears to control whether the ADC is triggered synchronously with the mains waveform, or whether it just triggers immediately on receiving the '?' character (faster).

So, we would expect the full 500 readings per second speed to be theoretically achievable in asynchronous mode, and 240 readings per second to be theoretically achievable in Synchronous mode (4 readings per mains cycle, as the meter can trigger on 4 flanks of the curve).  (200 readings per second in Germany with 50Hz mains!)

To this, we have to add any latency in our GPIB setup - which doesn't have to be much to cause issues at this repetition frequency!


dietert1:

--- Quote from: joeqsmith on February 08, 2021, 12:34:34 pm ---...
I would like to see the commands you are sending.  Maybe I was doing something wrong when trying to get the serial port to work in high speed mode.

--- End quote ---

Configuration string is "R2S7F1!T," and then for each reading the trigger string "?". As i wrote, "S7" is ignored and continuous triggering isn't working as expected. The DVM front panel is dark except the power LED. Meanwhile i noticed that total data taking time for 1600 samples is between 10015 and 11531 msec, so there seems to be "interference" between the USB 1 msec service period and ADC timing. Meanwhile the lower limit would be 10015 msec / 1600 = 6.3 msec per reading. This cycle includes four GPIB transactions (send two triggers and then read two results).

Standard deviation of the readings are 21 and 19 uV for the two instruments. When i look at the averages of those 1600 readings, their standard deviation is about 1 uV over 10 or 20 minutes. This includes the noise of the source (ovenised JVR).

Regards, Dieter
joeqsmith:

--- Quote from: SilverSolder on February 08, 2021, 03:20:47 pm ---
--- Quote from: joeqsmith on February 08, 2021, 12:36:39 pm ---
--- Quote ---[...]
The code appears to start in U24 (at address 0000 in that EPROM), and continues into U23.  A 'gotcha'!  :D

--- End quote ---

They have 2K of RAM.  I wonder are they jumping there?   The RAM may have been faster so they may have copied critical routines from ROM to RAM.   Just a guess.

--- End quote ---

That's a good thought - but the 8080 reads the EPROM at a very moderate pace...  even slow EPROMs are already as fast as the 8080 is ever going to be able to read anything, is my (limited) understanding.  When I get some more time, I'll try to decode what is going on around those calls, to see if it is intentional or just a problem with the disassembly process.  This is not something I have ever attempted before...  may get stuck in the snow occasionally!  :D

--- End quote ---

I had to learn the 8080 at one time in my life but the only thing I remember is having to learn it and stories of may classes.   Looks like the data sheets have been scanned and are on-line. 

There may have been other reasons to jump to RAM.  For example, they may have had some self modifying code to save space. 

I did program up a few PROMs.  These are Toshiba 27128-20s.  Ethically, something just doesn't seem right about using them in the meter.  I just mirrored the image to both the lower and upper bank.   


--- Quote from: dietert1 on February 08, 2021, 04:46:46 pm ---Configuration string is "R2S7F1!T," and then for each reading the trigger string "?". As i wrote, "S7" is ignored and continuous triggering isn't working as expected. The DVM front panel is dark except the power LED. Meanwhile i noticed that total data taking time for 1600 samples is between 10015 and 11531 msec, so there seems to be "interference" between the USB 1 msec service period and ADC timing. Meanwhile the lower limit would be 10015 msec / 1600 = 6.3 msec per reading. This cycle includes four GPIB transactions (send two triggers and then read two results).

Standard deviation of the readings are 21 and 19 uV for the two instruments. When i look at the averages of those 1600 readings, their standard deviation is about 1 uV over 10 or 20 minutes. This includes the noise of the source (ovenised JVR).

Regards, Dieter

--- End quote ---
Thank you very much for providing this information.  I will give it a try and report back what I find.
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