Yet another grounding question.

[jwantulok]

I am building a MachIII machine that has an extremely isolated design.
240 3 phase through isolation transformer makes 120VAC neither secondary leg is grounded. Smooth Stepper with Isolated BOB. PLC handles most logic w/isolated serial adapter to PC. All PLC I/O is Isolated and separate subsystem power supplies are not tied together. VFD I/O is isolated. VFD is Controlled through isolated RS485. No major sub-systems share any electrical commonality with each other, except that all protective and shield grounds are connected to a common star earth/chassis ground.

Question for Al the Man:

Would there still be some benefit from grounding the dc commons, or would that defeat some of my carefully planned opto-isolation? Not fully sure of the various pros and cons with each approach...

[Al_The_Man]

If you have a local 120v supply via a transformer you have the option of grounding one side of the 120v to the service ground immediately at the secondary terminals and setting up a neutral.
I usually do this as a matter of course.
The PC power supply and M.B. common is going to be at Earth ground by virtue of the MB ground screws.
As is your Router spindle or VFD motor by way of the grounded neutral.
I always back this up with a conductor from the PC chassis to the Earth star point.
There are two philosophy's out there whether to adhere to system isolation or common systems up to Earth Ground.
My personal one is as a general rule where possible is to make all systems common reference to earth ground, Bond all metallic parts of the machine and motor frames to the system star point, so far this has stood me in good stead now for more than a few decades, so I guess I will stick with it.
Where Opto isolation is used, it is usually used to transit from one logic or voltage level to another only.
Before embarking on this route, investigation and knowledge of the system you intend bonding to ground is needed in order to make an informed decision.
Probably one of the main reasons that BOB manuf. and others stress the importance of isolation is to promote a method which is the more simplistic in someways to reduce the chance of endangering their equipment by hooking up to unknown systems.
There have been a few previous posts where after applying the common grounding technique, has cured spurious nuisance tripping of inputs etc.
You could take the "prescribed" way and if you have problems go the common ground route.
I can only say what I do as a matter of course.
Hope this helps.
The PDF shows grounding of the "local" neutral (X2).
Al.