08-27-2025, 02:23 AM
So I finally got around to spending an hour on this. My test setup is the AB and of the RS485 are connected to DMX+ and - respectively of an Eldoled Linear 100 driver, terminated with 120ohm at the driver. The board ground is connected to the driver DMX ground. The scope is a cheapy Hantek so I don't trust it too much in terms of absolute levels.
The link is probed with two floating probes connected to the busses and in the scope the math is A-B. A is clearly the original and B the inverted, however the waveform that comes out deserves no prizes, the inverted side is significantly different in voltage level than the positive, resulting in a skewed signal (A is 1.1v p2p, B is 2.5 p2p). The final combined output signal is 2.8v p2p.
Long story short, I got this working a few times but the driver simply doesn't like the signal coming out at all. It worked a few times with a really short wire and no termination resistor, though I can't see the difference between with or without on the scope.
Time to get myself some better measurement tooling and a genuine MAX13487 and try again.
If I could upload images I would throw in a screenshot.
The link is probed with two floating probes connected to the busses and in the scope the math is A-B. A is clearly the original and B the inverted, however the waveform that comes out deserves no prizes, the inverted side is significantly different in voltage level than the positive, resulting in a skewed signal (A is 1.1v p2p, B is 2.5 p2p). The final combined output signal is 2.8v p2p.
Long story short, I got this working a few times but the driver simply doesn't like the signal coming out at all. It worked a few times with a really short wire and no termination resistor, though I can't see the difference between with or without on the scope.
Time to get myself some better measurement tooling and a genuine MAX13487 and try again.
If I could upload images I would throw in a screenshot.