Hey Dale,
this is difficult to say, it could be any of those.
If I would encounter this, and it’s not an unsufficiently shielded and grounded spindle cable that puts its noise on the serial line, I would look whether the contacts on both sides of DB-25 connection are gold-plated. If they are not, I would apply a little Kontakt 60 on the male side, plug the breakout board adapter in and out a few times, and check for timeout again.
Then I would remove the serial cable, with a multimeter set to ohms check it for continuity of both wires between their ends, then check for short-circuit between the two wires, look at the state of the surface on the contacts (use of wire ferrules is strongly recommended, however not every wire ferrule crimp pliers are able to crimp wire ferrules smaller than of 0,5 mm² wires), and put the cable back into place. You could also replace the cable instead of measuring this.
Then, since I know of issues with the breakout board adapters, I would check if you have this issue (already mentioned above).
You would, as the author suggested, first apply pression on the breakout board adapter in direction to the controller box and observe the whether serial connection still reports timeout (you probably need to have stop/run command and speed control on the VFD back to “ModBus control” in order to make the VFD use the interface, and the correct VFD selected in the Onefinity Controller as “tool type”).
Next, as the author suggested, I would check: Does the timeout disappear after you removed the nuts (A) of the DB-25 Port, as shown in the author’s image, and connected the breakout board adapter without the nuts? This is a test where you don’t open the controller, just remove the two nuts on the outside of the DB-25 socket.
I don’t know how the traffic would show up with a multimeter, you can try and set it to AC and a low voltage. Note that the serial line is a balanced line, the signal is between the two wires. Sorry, I’m not at my lab, otherwise I would check how the traffic shows up with a multimeter. With an oscilloscope or a bus pirate it would be easy to check the signal. Note that RS-485 is bidirectional, so check on VFD’s serial port too.
Anyway next what I would do is open the controller case and check if there is continuity between the solder points of pins 13, 14 on the back of the DB-25 socket (on the AVR board) and the corresponding terminals of the breakout board adapter (while it is in place). Please ensure the Controller is powerless then. I would have a look at how the soldering on the back of the DB-25 connector looks like, if the soldering points have some peculiarities.
Another thing you can do instead is check with another VFD.