Lagunmatic 3516-SX Retrofit

Based on my tests I'm fairly certain the 7i77 5v is grounding to chassis through the PC power supply (and therefore to the 24v common). My computer PS is connected to the existing power connector used for the old control computer. The PC power supply L/N/G are shared with the L/N/G going to my 24v and 5v power supplies.

So based on all that, I am hoping if I follow John's suggestion to unplug the DB25 connector from the 7i77 then my 24v power supply will continue working when I connect V(-) to common. This has been the crux of the situation - the 24v circuit had to connect to common/ground but everytime I did the power supply stopped working.

If that test is successful, then the question becomes "How should I power the PC?" I would prefer to use only the existing 3ph incoming power so perhaps an additional isolation transformer for the 3 power supplies? It's not clear to me how I might create an isolated ground with that setup.

Thanks a ton for all your help I think we're getting there!

Umm... There should not be any issue with the 5V side being grounded to chassis ground nor any need to isolate the 7I77 5V side from ground unless you have a ground loop issue (noise)

If your 24V power supply stops working when you connect 24V common to ground you have some issue with the +24V side wiring you need to resolve.

Floating the 5V to remove a 24V short issue in your system is very likely to damage components all through your system

I had a quick look in your shema. The ac side of your first transformer is connected to ground. If this ac voltage is rectified in any graez both sides, plus or minus, will shortcut one have wave if connected to ground. You can not have grounding in both sides of a graez rectifier. You would need complete floating electronis ... With every shilding you risk a shortage.
m5c

PCW wrote: Umm... There should not be any issue with the 5V side being grounded to chassis ground nor any need to isolate the 7I77 5V side from ground unless you have a ground loop issue (noise)

If your 24V power supply stops working when you connect 24V common to ground you have some issue with the +24V side wiring you need to resolve.

Floating the 5V to remove a 24V short issue in your system is very likely to damage components all through your system

Sorry - I misinterpreted your last response to mean my wiring was wrong because my 24v and 5v circuits shared a ground. I've had no issues with the 5v the way it is - the sole issue now is that when I connect 24v (-) to common, as required by the way the relays are setup, the PS shuts down.

So John's original suggestion to disconnect all 7i77 outputs to start with seems on point. I'm not sure if it matters but most of the outputs, currently, are active as soon as the 7i77 is powered on.

Fixing this issue if obviously over my head so I really appreciate your patience and help.

_No_ 7I77 output should be on at power up

This either indicates a major wiring error of some kind or that the 7I77's 24V output section has been damaged

PCW wrote: _No_ 7I77 output should be on at power up

This either indicates a major wiring error of some kind or that the 7I77's 24V output section has been damaged

Let me double check tonight what the behavior is... when I tested in the past my LinuxCNC config was running which could have activated the outputs.