Posted on February 11, 2016 at 15:36Edje,
good idea. Pin 1 and 2 are not in fact there to supply the isolated side with power. They only supply the target side of the front end level translator with power to ensure that the interface signals are at a compatible level with the target. This is needed both for the isolated and non-isolated version. Of course they are connected in my setup. However, your suggestion prompted me to try connecting the 3.3V pin (pin 19) to the target voltage (3.3V in my case) even though the documentation says not to connect this. This pin did indeed not have 3.3V on it, so I thought your idea might apply here. Alas it made no difference. In this context I should mention that the dongle gets noticeably warm in one corner, suggesting that it contains a DC/DC converter to supply the isolated side with power.
Meanwhile my suspicion goes into a different direction. I examined the error log that is generated by the latest version of the ST-Link Utility. This indicates that the target voltage is very low and varies a lot and after thinking a bit about it, the UI suggests that not being able to detect the target voltage is the reason the connection fails. This is of course not true, the target voltage is a solid and stable 3.3V. So, the whole thing looks like a floating A/D converter pin in the ST-Link/V2-ISOL!
This would make sense, since measuring analog voltages across an isolation barrier is a non trivial and costly exercise. I wouldn't expect such a feature in a device of this kind and price range. This is particularly true, since the target voltage is not a vital parameter. Thus at this point I think that the utility should just ignore the target voltage and attempt to talk to the target anyway. There seems to be no way to distinguish the two types of V2, so the utility would need some sort of configuration option to ignore this. Searching high and low, I can't find anything. I also looked inside the ini file for anything suggestive, but no luck...
