Analog signal compatiblty

Hi Dear,
we have Abb Dcs AC410, and Siemens master drive with CUVC.
when we connect Dcs analog output directly to analog input of master drive for speed control. Dcs mA goes to 20mA and not control able. In the other way when we use signal Isolator (line Isolator). It work Ok. In our other application with Abb DCS AC410 with Abb Drive Acs800. It is working ok with out isolator why?
Regards,

Possibly
You had an existing lead to earth and when you added this one you introduced a second lead to earth, which the isolator corrected for.

dear Gil47,
Thanks for help. Do you mean interchange conections?
(+ve with -Ve)
Regards,

What you say could be a possibility also.

What I was thinking is, you originally had a connection that had one lead joined or coupled to an earth, that caused no issue, but when you introduced a new connection, it also was connected or coupled to earth this now could show as an error like you described in your first post.

What I really mean I guess is that one fault creates no problem but a second fault in the connections to the cards concerned does.

Remember I am not the one seeing this fault, so all I can do is make calculated guesses