TB6600 connections

Hi
I have a Denford minimill but stupidly fried the driver board, so I decided to replace it with cheap TB6600 driver boards from Ebay, specifically ones like this
www.ebay.co.uk/itm/352981992806
I'm also using a generic Chinese BOB which always used to work.
I am using the same stepconf settings as before just to see if I can get any response from one axis
I am powering it with 24V. I am sending the STEP output to PUL+ and the DIR output to DIR+
Nothing on the ENA pins
When I run Test Axis, I measure a 3kHz square wave pulse going to PUL+ which looks about right
Step is going from 5V to 0V every few seconds
However, whether or not I have the stepper connected, all I get on all 4 A/B terminals is about 18V unchanging.
Something is not right but I don't have any idea what

300KHz ? According to the datasheet for the tb6600 chip max clk (step) frequency is 200kHz

Are you sure about that ?

Are your timing settings correct ?

Sorry that was a typo which I corrected, but not before you saw the original. It is about 3kHz

You have a quick schematic of your connections ?

drive.google.com/file/d/1Re5qHdVmKsiJkiP...o_P/view?usp=sharing

This is just showing how I have the 12 pins of the module connected.

Yeah ok.

The DIR- & PUL- need to be connected to the GND of your BOB. As these are the cathodes of the LED of the opto coupler.

Do not connect the GND of the BOB to the 24v GND of the driver.

Usually it’s not a good idea to supply power to the stepper driver and not connect the stepper motors.

Ok thanks - I'm confused about this

"Do not connect the GND of the BOB to the 24v GND of the driver."

They are the same, I think. I found the 24V by connecting one side of the multimeter to the GND of the BOB and probing various contacts with the other lead until I found 24V. Should I have looked for GND that isn't connected to the BOB?

I've also seen it suggested that I should put PUL+ and DIR+ to 5v+ and connect PUL- and DIR- to the BOB outputs (i.e. the other way round) - does that make sense?

Sorry, I'm still confused.

The AC output of the mains transformer goes to a (what is I presume a) rectifier unit which has a 24V DC output. The negative side of this is connected to the 0V of the BoB and all the other 0V terminals on the original drive board. So I don't see how I can avoid having the 0V of the BOB the same as the 0V of the 24V input.

If there was a -12V and +12V then I could see how this would be possible. But the wiring doesn't seem to arranged like this.

For what it's worth I doubt they are actually using the tb6600 chip. From what I understand all these really cheap TB6600 drivers we can find on ebay, amazon, etc are actually running a slightly cheaper chip. I can't remember off the top of my head the chip number. I pulled the aluminum case off mine to check and it also wasn't a tb6600 as advertised. But I knew this going into it and still use them.