AndyUK's Build - 1.2x1.0m Gantry

[Doddy]

I suspect I've totally missed the point here though :) Please educate me further!

I'm trying not to mis-direct this thread (tangent?, what tangent...), but to explain my logic....

If I redraw your schematic (lesson #1, "Eagle" is not good for this)



and zoom into a bit of text that I torturously added



If you /assume/ that the opto-isolator on the UB1 OSSD output has a forward current through the LED (I1) of 10mA (not unreasonable, and likely a design goal). Also, /assume/ the CTR is 0.5 for that device. That means that the switching current (Iossd) would be I1 * CTR = 5mA.

Now, Kirchoff's law says basically that the sum of the currents at a node equals zero. So, if 5mA is flowing into the OSSD input, then the sum of currents through the 4 separate '806 drivers must also equal 5mA. Give them the benefit of the doubt and say that all '806's are created equal, then your forward current through each '806 LED is that 5mA, divided equally by 4, or 1.25mA. Not the 10mA that you believe you have. Then you're at the mercy of the circuit design in the '806 behind the internal opto-isolator. That 1.25mA is multiplied by the local CTR and further diminished. Also, the CTR is impacted by the forward current - it's less at 1.25mA than if driven by 10mA, and so the internal switched voltage swing is further diminished, and may, or may not impact on the performance of the ENA input.

The bit of maths around Vossd is easily verified by yourself - measure the voltage, wrt ground, at the OSSD output when you have a fault condition. If the OSSD output was an ideal switch you'd expect it to be 0V. I expect it to be rather higher (calculated around 20V), based on the less-than-ideal switching behaviour of the opto-isolator (and if I have to correct my auto-correct for opto/onto once more....).

I'm cautious of playing too much behind a calculator - measure the voltage yourself. Reality beats theory any day. The one thing that worries me about my theory is that I would expect the behaviour to be accurately described in the owner's manual of the UB1. And the manual doesn't describe my perceived behaviour - so I could be wrong here.

I've tried to look online for a high-res image of the UB1 for information on the opto-isolator chosen - to no avail. If you can check and advise? It could be an intelligent selection of a device with an unusually high CTR. Pretty sure they are not buffered devices as they look to be 4-pin devices.


EDIT:

Here's an idea. Short the OSSD output to ground (or just the wire from the OSSD output). Either that will work as expected (inhibit each driver) or not. That's a quick way to fault-find the circuit.