i am trying to connect two devices via UART to a raspberry 4. one being a stepper driver (i dont need much throughput), one being an image sensor (the higher the baud rate is i can get, the better). both are currently connected via USB-to-serial dongles, which i want to get rid of to get a better fit with the box it's housed in. i would be very happy if i could these to the pi using the 40-pin-header instead.

i have gone through what little documentation i could find, my config.txt looks like this (only the last two added by me):

dtoverlay = disable-bt

and listing my serial devices looks like this:

root@raspberrypi:/home/pi# ls -la /dev/serial*
lrwxrwxrwx 1 root root 5 Jun  6 08:13 /dev/serial0 -> ttyS0
lrwxrwxrwx 1 root root 7 Jun  6 08:13 /dev/serial1 -> ttyAMA0

i have read this: Where are the uarts on the raspberry pi 4?

which states that there are more available PL011 UART ports - i take it that those could be used for my purpose, but they are not visible to the OS ... do i have to set them up from python with something like wiringpi? or is there another configuration that i missing?

  • 2
    You might like to read my old posts on how to use multiple UART ports on Rpi USB and on board: (1) penzu.com/journals/18951407/58119498, (2) raspberrypi.stackexchange.com/questions/96184/…. Cheers.
    – tlfong01
    Commented Jun 6, 2021 at 8:11
  • 2
    néih hóu. from the second post (and the links therein), i learned a lot of little things. thank you very much for that. however, the first link only points to a login page on penzu, and the second one (as far as i can see) does not contain anything about getting two serial ports on the 40pin. i am beginning to suspect that i am missing something important ...
    – rmalchow
    Commented Jun 6, 2021 at 8:30
  • 3
    Have you read through either the raspberrypi.org web site documentation for config.txt or /boot/overlays/README ?
    – joan
    Commented Jun 6, 2021 at 8:39
  • 2
    @joan such a polite way of saying "it's right there in the documentation, you idiot" :) but it did help me to figure it out: it is dtoverlay=uart[X] where "X" is the number of the UART. also, tlfong's link spells it out. thank you! :)
    – rmalchow
    Commented Jun 6, 2021 at 8:51
  • 2
    @tlfong01 would you care to write a proper answer, so i can upvote you? if you don't feel like doing this, i'll do it myself. just so this answer is easier to find for the next person.
    – rmalchow
    Commented Jun 6, 2021 at 8:52

1 Answer 1


so where was one big misunderstandings. thanks to @joan for pointing to the docs and @tlfong01 for their link to their article:


essentially, on the raspberry 4, there are 6 UARTs available, but two of them use the same pins by default (board pin 8 and 10). additional UARTs can be enabled one-by-one, doing so eats into the available GPIO pins.

uarts are enabled through separate "dtoverlay=" entries in


so, all on looks like this:


these are mostly off, in favour of having the corresponding pins configured as "regular" GPIO pins. once enabled, they will appear as

/dev/ttyAMA[0,1,2 etc]

this thread here with contributions by @joan and @milliways has some details on the default pins:

Where are the uarts on the raspberry pi 4?

and the README on overlays

here: https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README

or on the pi at: /boot/overlays/README

has some additional details on available configuration options, e.g.:

Name:   uart1
Info:   Change the pin usage of uart1
Load:   dtoverlay=uart1,<param>=<val>
Params: txd1_pin                GPIO pin for TXD1 (14, 32 or 40 - default 14)

        rxd1_pin                GPIO pin for RXD1 (15, 33 or 41 - default 15)

my setup is now like this:


and my loopback test:

import serial
import time
test_string = "[serial port test]".encode('utf-8')
port_list = ["/dev/ttyAMA0","/dev/ttyAMA1","/dev/ttyAMA2","/dev/ttyAMA3","/dev/ttyAMA4" ]
for port in port_list:
  ok = False
    buff = bytearray(len(test_string))
    serialPort = serial.Serial(port, 115200, timeout = 2, writeTimeout = 2)
    bytes_sent = serialPort.write(test_string)
    bytes_read = serialPort.readinto(buff)
    ok = bytes_read == bytes_sent
  except IOError:
  print("port %s is %s" % (port, "OK" if ok else "NOT OK"))

with the the RX/TX pairs for 4 and 5 bridged like this:

enter image description here

gives me the expected results.

Where are the uarts on the raspberry pi 4?

  • 2
    Many thanks for your answer. Your explanation is concise and clear, and your loopback test program is professional, comparing with my long winded newbie level demo programs. Cheers
    – tlfong01
    Commented Jun 6, 2021 at 10:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.