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The raspberry pi 4 has at least two uarts (as shown here), I know where I can connect to one of them. Pins 8, 10 and 12 have a uart as an option.

Source:

This site describes accessing uart1, where is it? Do I have access to it?

The preliminary Pi4 datasheet shows there are 6 UARTs.

UART GPIO
0 14/15
1 14/15
2 0/1
3 4/5
4 8/9
5 12/13

Given that UART 0/1 are both connected to the same GPIO you can only use one of them at any one time. So potentially there are 5 accessible UARTs at any one time.

You can enable each of the UARTs on the Pi4B by making appropriate entries in /boot/config.txt.

See /boot/overlays/README for details.

At the simplest add the following entry to enable UARTx.

dtoverlay=uartx

As Joan has stated only 1 of UART0/1 is accessible (the other /dev/ttyS0 is connected to Bluetooth on later model Pi).

There are additional UART on the Pi4.

I haven't actually tried these.

I doubt UART2 would be usable as GPIO0/1 are used for HATs (and shown as reserved).

I suspect that the SDA0/SCL0 connected to these pins are used for other internal functions.

I have extracted the GPIO Alternate Functions from the data sheet, as I have been unable to find any existing list.

This shows the mode and function of each.

 ALT0 ALT1 ALT2 ALT3 ALT4 ALT5 
 0 SDA0 SA5 PCLK SPI3_CE0_N TXD2 SDA6 
 1 SCL0 SA4 DE SPI3_MISO RXD2 SCL6 
 2 SDA1 SA3 LCD_VSYNC SPI3_MOSI CTS2 SDA3 
 3 SCL1 SA2 LCD_HSYNC SPI3_SCLK RTS2 SCL3 
 4 GPCLK0 SA1 DPI_D0 SPI4_CE0_N TXD3 SDA3 
 5 GPCLK1 SAO DPI_D1 SPI4_MISO RXD3 SCL3 
 6 GPCLK2 SOE_N DPI_D2 SPI4_MOSI CTS3 SDA4 
 7 SPI0_CE1_N SWE_N DPI_D3 SPI4_SCLK RTS3 SCL4 
 8 SPI0_CE0_N SDO DPI_D4 _ TXD4 SDA4 
 9 SPI0_MISO SD1 DPI_D5 _ RXD4 SCL4 
10 SPI0_MOSI SD2 DPI_D6 _ CTS4 SDA5 
11 SPI0_SCLK SD3 DPI_D7 _ RTS4 SCL5 
12 PWM0 SD4 DPI_D8 SPI5_CE0_N TXD5 SDA5 
13 PWM1 SD5 DPI_D9 SPI5_MISO RXD5 SCL5 
14 TXD0 SD6 DPI_D10 SPI5_MOSI CTS5 TXD1 
15 RXD0 SD7 DPI_D11 SPI5_SCLK RTS5 RXD1 
16 FL0 SD8 DPI_D12 CTS0 SPI1_CE2_N CTS1 
17 FL1 SD9 DPI_D13 RTS0 SPI1_CE1_N RTS1 
18 PCM_CLK SD10 DPI_D14 SPI6_CE0_N SPI1_CE0_N PWM0 
19 PCM_FS SD11 DPI_D15 SPI6_MISO SPI1_MISO PWM1 
20 PCM_DIN SD12 DPI_D16 SPI6_MOSI SPIl_MOSI GPCLK0 
21 PCM_DOUT SD13 DPI_D17 SPI6_SCLK SPI1_SCLK GPCLK1 
22 SD0_CLK SD14 DPI_D18 SD1_CLK ARM_TRST SDA6 
23 SD0_XMD SD15 DPI_D19 SD1_CMD ARM_RTCK SCL6 
24 SD0_DATO SD16 DPI_D20 SD1_DAT0 ARM_TDO SPI3_CE1_N
25 SD0_DAT1 SD17 DPI_D21 SD1_DAT1 ARM_TCK SPI4_CE1_N
26 SD0_DAT2 TE0 DPI_D22 SD1_DAT2 ARM_TDI SPI5_CE1_N
27 SD0_DAT3 TE1 DPI_D23 SD1_DAT3 ARM_TMS SPI6_CE1_N