W.r.t. "The issue is that there are only three wires on the receiver: VBus, Gnd, Signal (according to the Spektrum docs)." I don't know whether this applies to the Spektrum device but some devices only communicate by serial methods one way (and thus only use two wires/connection for that ie. "Signal" & ground"). For example I have a RS232 serial enabled DMM that can only send data to a computer (continuously, when activated):allaboutmike wrote: I have a Spektrum AR7700 receiver which apparently speaks SRXL over a serial bus at 115200. On another thread, aBUGSworstnightmare has commented that you can hook these up. I would love to ask directly about it, but I'm apparently too new to PM anyone .
It seems like this should be a fairly easy connect to the RPI3 UART given they are both at 3.3v levels. The issue is that there are only three wires on the receiver: VBus, Gnd, Signal (according to the Spektrum docs). The RPI3 has 3v3, Gnd, RX, TX. I assume this means that I need to combine the signals onto the one wire, but I don't know if there are any electronics involved to stop the uart from melting (or otherwise not working/failing/destroying something).
I have seen somewhere (can't find it right now) a simple circuit with a couple of resisters and a diode that seemed like it might work. Can anyone comment or hopefully provide a definitive answer. I'd really like to get bi-directional comms here to send telemetry back to the receiver.
Any help appreciated.
so connect to RX only. Assuming the receiver is the bus master, the protocol appears not to require an ACK from the slave, so no apparent need to connect up transmit for the basic stuff.Spektrum satellite receivers have a UART interface at 115kbaud --> connect them to the RPI UART-RX
Seeing as you can't easily switch the tx line to high impedance you're going to need something external that can tri-state. (You may be able to do it by altering the pin muxing for the tx pin between uart and input but timing that is non-trivial). Hooking rx up via a similar gate off the inverse enable signal will mean you don't receive your own transmissions which makes life easier - this is how most 2-wire RS485 transceivers are configured (they're also half duplex, but a differential pair). Typically RS485 will use the RTS line for direction control.Signal is standard 3.3V logic with normal UART levels (3.3V = idle line), however the bus is generally in high-impedance state using the micro’s internal pull-up. This is necessary for idle line detection, although it could leave us open to noise
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