HY VFD DCM connection

[Doddy]

Final comment from me then I'm going back under the rock for 6 months. The control inputs to the HY VFD are fed each though onto-isolators on the control board internal to the VFD. You already have galvanic isolation... until the point that Neale asked about - providing a common 0v line between the VFD DCM and the UCBB 0v. I've not stripped my HY down for a few years, but there's descriptions on the net of all internal supplies (including the 24V sensor input bias supply, and presumably the logic supplies) from a single transformer (presumably independent secondaries) - so that helps to explain the isolated nature of the supplies that Neale has observed. Yes, you have to expect that you will be presented 24V (minus any Vf on the opto-isolator LEDs) and that could destroy any control board directly coupled to the VFD UNLESS that control board is designed with an open collector/open drain output current sink, as is the case with the UCBB - see first reply - or for the isolated outputs from the AXBB-E. Both of these controllers are intelligently designed around the intent to drive such 24V signalling control systems 9such as the VFD).

Anyway, I'm off.