Hi Segger,
I own a J-Link Pro and am generally happy with it. However, I came across an issue in conjunction with powering it via a PoE Splitter, which you seem unaware of. I would recommend to at least extend the relevant descriptions in your Wiki to make users aware of the problem.
The problem lies with galvanic isolation, or the lack of it. Most PoE Splitters I have come across appear to lack galvanic isolation between their upstream PoE power source, and the downstream power port, presumably to save a few cents on the parts bill. Their technical data is often silent on this topic, making it difficult to recognize the problem before purchase.
With no galvanic isolation, the ground of the J-Link probe, and of the connected target, may actually be at 48V. The details depend on the PoE power source, i.e. the switch. You will not usually notice a problem when using a PoE injector, because it will be floating. But when powering a whole equipment farm from a PoE switch, you are asking for trouble. The arrangement may work, but may well be unsafe, i.e. violate safety regulations. Or you may run into ground-related problems that prevent things from working.
The solution is to use a power splitter that includes galvanic isolation between the PoE input and the power output. Those may well be significantly more expensive. Arguably, this is the only standard conforming implementation of such a splitter, but apparently manufacturers don't care.
Of course, it would be even better to include the PoE circuitry in a future version of the J-Link Pro, where you would implement the isolation, and spare your customers the hassle and confusion. That would be the real "Pro" solution.
Please have a look at the Wiki, where I have seen two places where PoE is described without any hint at the problem I described. I suggest adding a few sentences to alert users to it.
Regards
s13n
I own a J-Link Pro and am generally happy with it. However, I came across an issue in conjunction with powering it via a PoE Splitter, which you seem unaware of. I would recommend to at least extend the relevant descriptions in your Wiki to make users aware of the problem.
The problem lies with galvanic isolation, or the lack of it. Most PoE Splitters I have come across appear to lack galvanic isolation between their upstream PoE power source, and the downstream power port, presumably to save a few cents on the parts bill. Their technical data is often silent on this topic, making it difficult to recognize the problem before purchase.
With no galvanic isolation, the ground of the J-Link probe, and of the connected target, may actually be at 48V. The details depend on the PoE power source, i.e. the switch. You will not usually notice a problem when using a PoE injector, because it will be floating. But when powering a whole equipment farm from a PoE switch, you are asking for trouble. The arrangement may work, but may well be unsafe, i.e. violate safety regulations. Or you may run into ground-related problems that prevent things from working.
The solution is to use a power splitter that includes galvanic isolation between the PoE input and the power output. Those may well be significantly more expensive. Arguably, this is the only standard conforming implementation of such a splitter, but apparently manufacturers don't care.
Of course, it would be even better to include the PoE circuitry in a future version of the J-Link Pro, where you would implement the isolation, and spare your customers the hassle and confusion. That would be the real "Pro" solution.
Please have a look at the Wiki, where I have seen two places where PoE is described without any hint at the problem I described. I suggest adding a few sentences to alert users to it.
Regards
s13n