The Industrial Raspberry Pi Forum

Hi all,

i am doing some tests and measuring the signal RS-485 on the pibridge. My devices are a revpi core 3 connected to an AIO and a DIO on the right side. I use the Picoscope series 5 to measure the signal RS-485 + (blue waveform), RS-485 - (red waveform) and difference of them (purple waveform). The picture below shows my results. As you can see, the signal from the core 3 to the device AIO or DIO looks very strange. It should be rectangular like the signal comes from AIO or DIO to Core 3, but it is not. So when I try to decode the signals with Picoscope, I get the wrong data. Can someone help me to explain it please.

Song

Dear Song, thank you for your detailed analysis and the evaluations based on the sum signal. This is normal as far as I can see (and also a quick test with Oszi confirms this with a Core 3). Signal levels in practice are often not perfect. As long as the thresholds are reached, it still works.

Hi Dirk,

thank you for the quick reply. Yes, I can see now. If I choose the appropriate threshold and hysteresis for decoding, I still get the correct data on the bus. But it is not clear why this not perfect signal occurs. Do the Core 3 and DIO/AIO use different chip for sending and receiving RS-485 data? And why doesn't the Core 3 use the same chip (if the chip is the reason) as the DIO/AIO if it can provide better performance or more correct signals on the bus?

Song

According to the schematics I can see that the Core 3 uses the CM GPIO14/15 pins. The DI/DO/DIO/AIO modules are using the STM32F205 processor for this job.