Hi,
it is certainly possible to use an opamp to invert the signal but it shouldn't be necessary. Finding out what the problem is will probably
stand you in good stead down the road.
If you probe the input to the BoB, that is pin 14 of the DB25 connector. If you flip the board upside down you should be able to probe it without
shorting to any adjacent terminal. Be sure to read up on the pin numbering scheme for a DB25, it is different to most connectors.
I would expect to see a normal PWM, that is to say a short logic high followed by a longer logic low for low rpm. MDI a high rpm, s=20000 say,
and I would expect a long logic high followed by a shorter logic low. Given the description of your problem I think you will find that what you
see will not be what is expected indicating a fault or incorrect setting in the ETH400 which drives the BoB.
I use an ESS and so can't comment on the plugin of any of the CNCDrive products. I would have thought there must be some sort of plugin and settings
that can be made.
One reason I suggested that you disconnect the VFD is so that should the internal impedance of the VFD somehow feed back into the BoB its contribution
can be eliminated. If you experiment and can be 100% sure that there is no conflict between the VFD and the Bob then hook it back up. I don't know
how many times when diagnosing electronic faults I have overlooked or assumed a certain thing only to find later, usually much later, that the fault was in
fact very simple but I precluded myself from finding by virtue of my mindset. I try to eliminate faults like that but realise that as long as my anus points
downwards its not likely to happen!
What are you using for a power supply for the analogue control signal? The BoB itself is 5V so how is it supposed to produce 0-10V? I would have expected the
output buffer to be between 0 and 5V. Here again if the VFD is disconnected it would preclude any confusion as to which device is sourcing voltage.
Craig