While testing measurement recording I realized it is not really possible to read data at exact short intervals using external triggering mode. Sampling rate for DM3058 ranges from 400 ms in SLOW reading rate to 8 ms in FAST reading rate. But in case of external triggering it does not mean that measurement is done exactly at external trigger pulse. It seems that sampling runs on its own internal clock and on external trigger pulse multimeter just returns value of measurement that it currently is sampling. But it does not correspond to exact time when trigger pulse was sent. It does not really matter for logging lengthy battery discharge curve every few seconds or so. But it seems not possible to accurately log measurements anywhere close to sampling rate of DM3058.
I decided that for short intervals I might try using use internal triggering mode of multimeter and read out measurements when they are ready. It is possible to set custom internal trigger period in the range from minimum sampling time up to 2000 ms. But as I found out internal trigger period of the multimeter is not accurate either, not at least on my device. In fact it does not trigger at exact interval that is set using menu or serial command. Measurement triggers at some other time period that most likely depends on internal software or hardware constraints. I tested it with measuring slowly changing sine wave voltage at various intervals and different measurement rate modes. Sine wave period was fixed to 2 seconds. By saving built in measurement history (up to 2000 readings on DM3058) to USB flash I examined voltage wave forms using excel graph. I checked how many measurements were taken over one full sine wave period. I found out that while some trigger interval settings were logged as expected, most of them did use some other interval value that did change with selected measurements mode.
I though I would just post this to Rigol tech support helpdesk, as these settings are set according to user manual and description but they don’t seem to work for me. I did not have very high hopes for it but response was unexpectedly helpless – see screenshot below. I wonder if any one from Rigol did actually try to read my message or did Mr. Christian Rau just wrote it of as ‘solved’ without bothering himself. It is so ignorant and somewhat rude, that I found it amusing. They made my day.