I've already read dozens of manuals and stack topics, but I do not figure out if it even possible run both UARTs on RPi 3B. Some tutorials advice to map uart to other pins in /boot/config.txt I tried many combinantion of pins and none of them working.

My config.txt

# Disable Bluetooth

# UART0 /dev/ttyAMA0

# UART1 /dev/ttyS0

Using more than one GPIO based UART is not possible on any Pi other than the compute modules.

All other models of Pi do not bring out the needed GPIO to the expansion header.

  • Why everywhere they writes, that RPi 3 has two UARTs and RPi 4 has 4 UARTs?
    – Sahasrar
    Jul 14 '21 at 19:42
  • They write that because it is true. Unfortunately the additional UARTs can not be accessed because you can't connect to the needed GPIO. The needed GPIO are not accessible.
    – joan
    Jul 14 '21 at 21:20

In fact the Pi3 (or more precisely the SOC) has 2 UART.

See How do I make serial work on the Raspberry Pi3 or later

One is connected to the Bluetooth module the other can be used and is accessible on pins 8, 10. These can be switched but only one is accessible.

The Pi4 has 6, 4 in addition to the 2 on earlier models, but only 4 can be used due to conflicting pin assignments.

See Raspberry Pi4 UART

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