HY VFD DCM connection

**Neale** · 08-05-2022

I'm currently replacing the motion controller in my router with a UC300ETH+UCBB board. Previous motion controller had an on-board relay for the VFD control which used a n/o contact to connect "For" to "DCM". DCM is the common ground for the digital inputs on the VFD. However, the UC300/UCBB combination does not have a relay. The UCBB digital outputs use a MOSFET to short the output to board ground, which is fine for things like the stepper drivers. I don't really want to have to install a separate relay board, so wonder if I can connect DCM to the UCBB ground and "For" on the VFD to a UCBB digital output. The main issue here is whether it is safe to connect VFD DCM to ground in the control box.

The HYmanual I have is no help on this one!

Anyone have experience of switching an HY VFD like this? I'm sure that there are plenty of UC300/HY VFD systems out there...

Thanks in advance,

**Doddy** · 09-05-2022

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That's from the manual. The UCBB should be more than able to drive the inputs to the VFD. Connecting the DCM to the main controller ground (/UCBB ground input) is perhaps something to be avoided if possible, but it's not the end of the world to connect them (arguably, less noise immunity). If there is a problem then you know you have a solution that is to provide true galvanic isolation with a relay. It might be worth examining your system - you *might* find you have a lightly coupled 0v reference in any case through any EMI capacitors in any SMPS you have, negating any advantage and the connecting of 0V to DCM may actually be beneficial.

Exec summary: It'll work, go into this with eyes open for noise immunity.

**Neale** · 09-05-2022

Thanks for the comments. My main concern was about the isolation of DCM inside the VFD but overnight it occurred to me that if I just check volts between FOR and DCM (expecting 12-24v or so), then both to control box ground, I can get some idea whether or not they are isolated. And if all else fails, I stick a relay in there. I've just found that I need to get a 16-pin IDC connector to pick up the analogue output on the UC300 as they don't provide that as part of the kit so things are going to be a little delayed anyway.

I have a copy of the manual - it's good that they show a circuit diagram of the board's interfaces. Gives a bit more insight into what's going on under the covers!

**dazp1976** · 09-05-2022

Originally Posted by Neale

Thanks for the comments. My main concern was about the isolation of DCM inside the VFD but overnight it occurred to me that if I just check volts between FOR and DCM (expecting 12-24v or so), then both to control box ground, I can get some idea whether or not they are isolated. And if all else fails, I stick a relay in there. I've just found that I need to get a 16-pin IDC connector to pick up the analogue output on the UC300 as they don't provide that as part of the kit so things are going to be a little delayed anyway.

I have a copy of the manual - it's good that they show a circuit diagram of the board's interfaces. Gives a bit more insight into what's going on under the covers!

You can use Dupont 2.54mm wires/ connectors directly on the UC300eth IDC pins.
VFD FOR - DCM is just a ground sinking line.
External relay (or opto module) is needed to control it, COM/NC to make or break that single line (on/off).

See my posts of this cncdrive thread:
http://www.forum.cncdrive.com/viewto...=3780&start=30

There are some wiring diagrams on page3 and images on page4

Diagram here too:
http://www.forum.cncdrive.com/viewtopic.php?f=19&t=3829

Shows connecting relay via Bob and also dupont uc300 direct.
Has AO1 wires too.

(your vfd may differ slightly)

**EddyCurrent** · 09-05-2022

I would definitely use an interposing relay.

**dazp1976** · 09-05-2022

Originally Posted by EddyCurrent

I would definitely use an interposing relay.

You need to. Or any other kind of 'switch'.
You can't directly connect a breakout board / controller output to vfd FWD / DCOM.
It simply does not work (or will break them) .

Put a single wire link between fwd an dcom and fwd will come on. Break that link and it will go off.
Use the relay to control the make or break link.

**Doddy** · 10-05-2022

Originally Posted by dazp1976

You need to. Or any other kind of 'switch'.
You can't directly connect a breakout board / controller output to vfd FWD / DCOM.
It simply does not work (or will break them) .

Why?

**dazp1976** · 11-05-2022

Originally Posted by Doddy

Why?

It's been done on another forum a while back.
Directly connected it and broke the axbb-e. Had to replace it.
May have done it wrong in first place.
Relay is the way to go.
24v coil spst.
24v module type.
24v ssr.

Think of vfd like sticking a jumper between for and dcom.
In = on, removed = off. Simples.

**JAZZCNC** · 11-05-2022

Originally Posted by dazp1976

You need to. Or any other kind of 'switch'.
You can't directly connect a breakout board / controller output to vfd FWD / DCOM.
It simply does not work (or will break them) .

My favorite Quote that you maybe should think about.?

"Better to remain silent and be thought a fool, than to open one's mouth and remove all doubt.!"

**dazp1976** · 11-05-2022

Originally Posted by JAZZCNC

My favorite Quote that you maybe should think about.?

"Better to remain silent and be thought a fool, than to open one's mouth and remove all doubt.!"

Hey. I'm only going on what someone else did and broke their axbb-e.
The bbe is basically the 300eth and a smaller ucbb.
So

Edit: although 1 side of it is non-isoltated. Could that be why.