When an ISA3 is connected to a line powered EM3xx target, there is potential for causing damage to the ISA3 or the EM3xx target, due to a difference in potential between the ISA3 reference ground and the reference ground of the EM3xx target. The reason is due to lack of isolation between the AC component of the AC Mains reference and the reference ground used for the EM35xx target, for example such as found in custom EM3xx designs using bridge rectifiers. The ISA3 design assumes the EM35xx target reference ground is floating. When the EM35xx target reference ground is not floating, i.e., when it is referenced directly to an earth ground, AC mains neutral wire or AC mains hot wire, care should be taken to verify the isolation of the ground connections between the ISA3 and the EM3xx target in order to prevent damage.
The ISA3 ground is normally connected to the custom EM3xx target via the Packet Trace Port connector and in some rare cases via the DEI connector. The difference in ground reference potential often happens when the ISA3 USB cable connection from the PC or the Ethernet port POE (power over Ethernet) connection provides an alternate ground reference to the ISA3. To solve this problem, we recommend breaking the ground conductors within the Packet Trace Port cable and the DEI cable. A continuity check between the ISA3 reference ground and the EM3xx target should then be made to prove the two device grounds are isolated.