Thanks you all, adding the timings to the M569 commands as @jay_s_uk and @PCR suggested fixed it!

Strange, the SX1509B is in fact detected:

FIRMWARE_NAME: RepRapFirmware for Duet 2 WiFi/Ethernet FIRMWARE_VERSION: 3.3beta1 ELECTRONICS: Duet WiFi 1.02 or later + SX1509B expander FIRMWARE_DATE: 2021-02-14 16:00:49

however the pins don't get driven high (or low) using the M42 commands.
I am confident the I²C-Wiring is correct, since I tried using a MCP23017 based relay board that responds to I²C-Commands just fine.

Any suggestions or ideas are highly appreciated!

I upgraded to 3.3beta1 today, but it still doesn't work unfortunately

@dc42 thanks for your reply. I am using version 3.1.1, would a upgrade fix the issue?
yes, 20 would be sufficient, maybe a bit more would be even better still?

I may be wrong, but I think pull-up resistors can only be enabled on ports of the Duet board itself.
There are source-files specifically for this expansion chip (here), however I have no idea what the "DuetNG" denomination is about to be honest. Also, this branch seems to be of the 2.x firmware only?

Not that I was aware of..
However, I just checked the datasheet and according to it, all pins are configured as inputs upon power-on, so I think I would need to configure them as outputs. I don't really know what the duet firmware is doing internally here..

Good point!
Unfortunately no luck here either..

I2C nak errors 0, send timeouts 0, receive timeouts 0, finishTimeouts 0, resets 0

@alankilian true, but as far as I understand (see this note on the very same page this no longer works for RepRap Firmware 3.x

Hi everyone,

I have wired up a Sparkfun SX1509 (BOB-13601) to my Duet 2 WiFi.
The SX1509 is set to use the 0x71 I²C address (I double-checked it is indeed using 0x71 using a Arduino).
In my config.g I assign Ports to the Output pins like so:

; GPIO Ports									; Create GPIO Ports on SX1509B expander pins
M950 P5 C"sx1509b.0"			; Assign P5 to pin 1
M950 P6 C"sx1509b.1"			; Assign P6 to pin 2
M950 P7 C"sx1509b.2"			; Assign P7 to pin 3
M950 P8 C"sx1509b.3"			; Assign P8 to pin 4
M950 P9 C"sx1509b.4"			; Assign P9 to pin 5
M950 P10 C"sx1509b.5"			; Assign P10 to pin 6
M950 P11 C"sx1509b.6"			; Assign P11 to pin 7
M950 P12 C"sx1509b.7"			; Assign P12 to pin 8
M950 P13 C"sx1509b.8"			; Assign P13 to pin 9
M950 P14 C"sx1509b.9"			; Assign P14 to pin 10
M950 P15 C"sx1509b.10"			; Assign P15 to pin 11
M950 P16 C"sx1509b.11"			; Assign P16 to pin 12
M950 P17 C"sx1509b.12"			; Assign P17 to pin 13
M950 P18 C"sx1509b.13"			; Assign P18 to pin 14
M950 P19 C"sx1509b.14"			; Assign P19 to pin 15
M950 P20 C"sx1509b.15"			; Assign P20 to pin 16

Now the first issue is the number of pins, according to the Gcode dictionary (link) the Duet only supports up to 10 pins.
Is there another way, I could use those pins?

Secondly, by using

M42 P5 S1

or

M42 P5 S0

I would expect the first pin on the SX1509B Port Expander to toggle the state of said pin, however nothing happens.
Any ideas?
Thanks!

As far as I am aware, it can only do G-Code since it's basically just a list of G-Code commands executed in sequence.

Two more ideas which, however, are probably still not exactly what you are looking for:

1. Use a minimal Microcontroller powered off the Duet like an Arduino or even better a Bare Microcontroller with a WiFi Shield or something like that to monitor a Level-change on some pin and send out a notification to your Pi

2. Use a Serial/UART to WiFi Bridge and read the serial data from the Duet using the Pi.

Let me know if you find some other solution to your question, I'd be very interested in hearing about it..!

@Veti thanks.

I do understand the meaning of the error; but that's what I am saying - there is no way, there is over 2mm of height difference. And before I changed the probing positions, that error never showed up..