So... here's an odd one. I've been adding safety devices to some KRC2s and KRC4s that lacked them, before. All the safeties work properly, but there's one problem: whoever designed this add-on hardware included LEDs to indicate circuit statuses, and those don't work.
I've attached a basic circuit diagram of what they've done. Now, to my eye, this circuit should have created safety faults -- it introduces cross-talk between the A and B channels, which should be verboten. And if you look at the LEDs, the current flow could only pass from Test Output A, through the LED, and "ground out" through Enable Input B.
There's also the question of how the left LED (which is supposed to indicate the circuit has power) was ever expected to work when the safety contacts were open -- how would you expect current to flow from one Test Output to another?
Instead, there is 0V across either LED, there is zero current through either LED, and both sides of the LED measure 27V to earth ground. The safety contacts work exactly as they should, but neither LED illuminates.
These are industrial LEDs, rated to be wired directly across 24VDC (they have internal resistors in the housing). I removed the LED "bulb" and used a basic meter diode-check function, and they read out at 2V, so I'm reasonably certain they're not blown open or shorted. I also tried removing them from the circuit entirely, and reversing their wiring polarity (a-la Star Trek ), none of which had any effect.
My best guess, at the moment, is that the X11 safety signals simply can't source enough current to illuminate these LEDs (along with their current-limiting resistors), and since the LEDs can't conduct under these conditions, that's saving me from the channel cross-talk faults.
Personally, I think that wiring LEDs into X11 safety signals like this simply isn't allowed, but I don't have enough detailed information on the low-level electrical characteristics of these signals to say for certain. And if I'm going to tell the persons who designed this box that their circuit can't work, I need solid evidence to argue my case with.
So... am I right? Or is this circuit OK, and I'm just not looking at it correctly?