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Does UART require a pullup resistor?

I am trying to understand how UART communication works.

I have not read anything stating that pullup resistors are required for UART, but conceptually some type of pullup is in fact required because otherwise there would be an unknown state.

Is it part of the UART standard that each device on the bus must have it's own pull-up resistor?  If so how can this be because if the pull-ups are of different voltage then the state is again unknown.

Thanks

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used an I2C translator for 1.8 to 3.3 V

works great.  thanks guys!

5 volt logic, 3.3 volt logic, 1.8 volt logic. There's a 2.something in there too, but each can talk to another with bidirectional level translation or level shifting chips. Sometimes it's just as simple as a couple of transistors connected between the devices.

There is no "UART standard". A UART is simply a device that can trasmit and receive serial data at the same time. Whether you connect it to 5 volts, 12 volts, +/- 100 volts, infra-red, radio, laser, that's up to you. How you design the interface will determine if you need "pullups". That's why you "have not read anything stating that pullup resistors are required for UART".

What 2 devices are you trying to connect.