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I am attempting to use the UART pins on the Raspberry Pi 2 to interface with the console port on a Cisco Access Point.

I've stripped an old RJ45 cable to connect the right TX, RX and GND pins from the console port to the UART ports on my Pi.

I have enabled UART through raspi-config and disabled the serial console.

I know from previous tutorials that the correct baud rate from this AP is 9600, and I'm opening a console using Minicom sudo minicom -D /dev/ttyAMA0 -b 9600

Unfortunately, I only receive indecipherable characters:

Indecipherable characters in a console

I know these symptoms usually relate to incorrect baud rate setting, so I've tried all of the following to no avail:

  • 300
  • 1200
  • 2400
  • 4800
  • 9600
  • 19200
  • 38400
  • 57600
  • 115200
  • 230400
  • 460800
  • 500000 They all produce different sets of characters but none are readable.

On one occasion I did get some partially correct text but this never occurred again:

partially readable text

I've tried the following tasks:

  • Adding core_freq=250 to /boot/config.txt and rebooting.
  • Adding force_turbo=1 to /boot/config.txt and rebooting.
  • Adding init_uart_baud=9600 to /boot/config.txt and rebooting.
  • Adding init_uart_clock=48000000 to /boot/config.txt and rebooting.
  • Using Raspi-OS instead of Ubuntu Core.
  • Various combinations of parity, data bits and flow control.
  • Using UART1 instead of UART0 on the same pins GPIO14 and 15 by adding into /boot/config.txt:
dtparam=uart0=off
dtparam=uart1=on
dtoverlay=uart1,txd1_pin=14
dtoverlay=uart1,rxd1_pin=15

I also had to add 8250.nr_uarts=1 into /boot/cmdline.txt

None of these made any difference. I am out of ideas and looking for advice on further troubleshooting my UART pins in case they may be broken, or anything else that might make this work.

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    I could not find out from a quick online search, and you should make it explicit: What is the logic level voltage of the console port on the Cisco device? I suspect it is higher than 3.3V. You may be risking damage to the Pi this way. If you do not know the voltage, do not connect them again until you find out. Commented Mar 6, 2022 at 16:13
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    If the serial interface is on a 9 or 25-way 'D' type connector, then it is probably incompatible with your 3.3V Pi I/O; it could be up to +/- 15 volts, and may well damage the Pi. You need to level-shift the signals, or just use a USB-to-serial adaptor that has a similar 9-way connector. Commented Mar 6, 2022 at 20:40
  • @goldilocks can I measure the voltage using a multimeter? Couldn't find a guide for it which wires do I connect to? Commented Mar 7, 2022 at 16:45
  • Generally UART buses have the TX pin held high when idle. I can't guarantee that is the case here, but you could try it. Make sure you have the wires spread out or attached to a breadboard so you can test on pins, because if you send that TX pin straight to ground it could wreck the port, amongst other things. Commented Mar 7, 2022 at 17:08
  • @goldilocks both multimeter cables connect to the TX pin? Commented Mar 7, 2022 at 17:25

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