Hi Marco,
yesterday I've played with paper and simulator to find out how data are injected and basically how much voltage is really transferred.
On my side, I think that the RS485 solution shall be discarded, just because all the available IC doesn't allow to read back the values that are on the bus. This feature is important to build an easy collision detection. Relevant the push-pull, the main problem is the requirement of a dual-voltage +V,-V.
I'm using a capacitive coupling and a resistor to protect the output pin in case of collision. In fact, if two devices write at same time, the pins output pins may be damaged. Damage may happen if one pins apply H voltage and the other L, the resistor protect the pins and avoid the use of tri-state devices. The RS485 chip, use a tri-state enable to avoid this problems.
The open-drain will not create problems in discharging of the the bus, but require a non-floating bus. If fact, the bus will be forced at H voltage and discharged by the open drains MOSFET when a L shall be transmitted. Using an open drain, there is a common "load" for all the MOSFET on the bus an collision doesn't make longer electrical problems.
In case of MOSFET, on the line is required a "load" that shall be an inductor. Using a non-differential transmission, just a classic single inductor can be used. Problem on space and cost shall be addressed.
So I'm proposing two ways:
- Capacitive coupling with a protection resistor,
- Capacitve coupling with external open-drain MOSFET.
The first solution offer a less reliability to noise, because the voltage that is "injected" on the bus is splitted between the resistor that protect the pins, rather the open-drain solution offer the full voltage over it.
In my test I was using the internal resistor of the main power supply to transfer the FSK wave from the pins to the bus, but due to small resistance, the voltage drops mainly on the protection resistor. Using an inductor as load will solve the issue, if in series with the bus, the voltage will drop on it and so on the bus.
I will post an electric schematic to be more clear.
Let me know your opinion.
Regards,
Dario.