SBUS over MOCD207M


I'm creating a control unit for a Sbus compatible gimbal based of these instructions ( I have it working but I want to isolate the control signal so I can power the gimbal and the arduino seperatley. I'm using a MOCD207M optocoupler
as I think it should switch fast enough for the SBUS signal (baud rate of 100000). I created a simple test circuit which seemed to work fine (see below) but when I cnnect my data signal instead of the 5v/ground connection I doesnlt work. I've never used optocouplers before so any advice would be greatly appreciated.

The LED resistor is 188Ohm and I've tried various values for the phototransitor side resistor, none of which seem to work (188, 282, 10k).

The connection labeled Tap is where my signal comes out to the gimbal


Not fast enough, the MOCD207M takes 10µs or more for each edge, many times too slow. No photo-transistor output opto coupler is very fast. There are fast logic opto-couplers (photo-diode plus buffer) and digital isolators (both capacitive and magnetic) for high speed signals.

Thanks for your reply. I'm trying to understand how baude rate relates to rise (or fall) time. I had assumed that a baude rate of 100000 would require a rise time of less than 10us (1/100000) and the data sheet states that the rise time is 7.5us so I had thought it would be okay. But thinking about it now I suppose the signal needs to be in a steady high/low state for a reasonable period before switching to the opposite state as well, is that correct? Is there a way to calculate the maximum rise time you can have for a given baude rate?

I have found this chip which is think is a digital isolator that has a rise time of 5ns, would that be sufficient?

Thanks again for you help/

Or in fact this chip as I can't find the other available anywhere.

At 100k baud the transitions come every 10µs, you want the switching to be on the order of a few µs only. MIDI was designed to use 31250 baud specifically to be able to work with standard phototransistor opto-couplers.

Phototransistors are typically slow because there's no base lead for stored charge to leak out of - though there are a few optocouplers that do bring out the base, so you can speed them up somewhat.

Yes 5ns is definitely fast enough for 100kbaud!

Thanks for your help. I'll post back here once I've been able to test it properly.

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