When Do I Need and Inverter and or Converter?
I recently found this simple converter cable for sale on the web for about $9. It is small and neat, with flexible silicon insulated cables and works well for me.
I use a high-speed shottky diode 1n5711, but almost any high-speed signal diode will work. Available here 10 for 2$.
For a typical old-style INVERTED S.Port, the diode would be the other way round.
Shown on the Heltec WiFi Kit 32 board - F.Port Pin 13 = RX and pin 14 = TX
When the Mav2PT protocol translator talks to FrSky equipment it requires signal inversion, and sometimes it requires bi-directional two-wire to single-wire conversion. Yes, connecting the Mav2PT to a FrSky receiver or Taranis/Horus can be a little tricky! In summary, it works like this:
In Air Mode and Relay Mode, the Mav2PT translator talks to a FrSky receiver (say XSR), and interleaves sensor packets into the sensor stream going down to the Horus/Taranis. In order to interleave a packet at exactly the right moment, it has to read from the XSR and send into a suitable gap in the stream. The data thus read from the XSR must get all the way back into the Mav2PT. For convenience, from version 2.33, ESP32 S.Ports are not inverted for Relay & Air Modes, as the recommended converter both inverts and converts to single wire.
However, in Ground Mode, when the Mav2PT talks to the Taranis/Horus and not a receiver like the XSR, it does not need to interleave packets. It therefore does not need to read anything, and therefore only requires a Mav2PT-txPin to Tarnis/Horus S.Port (rx) pin link. The OpenTX simply reads the sensor packets, and combines them with any sensor packets received from the receiver on the 'craft. So, in Ground Mode, one wire (plus ground of course) is always enough. This is true of the ESP32, the Teensy 3.x and the STM32 variants.
All FrSky telemetry is inverted with reference to mainstream serial telemetry. FrSky pulses rise up from (nominal) zero to 3.3v, whereas conventional serial pulses drop down from 3.3v. Furthermore, FrSky encode both rx and tx signals on the same wire. So this means when we talk to an XSR, or a Horus or Taranis, we need one bi-directional wire.
The Teensy 3.x has the capability of inverting and converting to bi-directional single wire internally. No inverter needed. No bi-directional converter needed.
The ESP32 variants have the capability of inverting the signal, but not converting to single wire (as far as I know at this point). So we must still do the two-wire to single-wire conversion when talking to an XSR, because we need to slot in our packets
The STM32F103C variants have no capability of inverting the signal, and no capability of converting to single wire. So we must invert the pulses and also do the two-wire to single-wire conversion when talking to an XSR, because we need to slot in our packets.
In Ground Mode, the STM32F103C variants must naturally still invert the signal with an external inverter because they are talking to a FrSky device.