Why can't I see RT-Thread message in Putty?

[DerrickLau]

I build the master branch without any changes. I created a Raspberry Pi OS image on my 16GB SD card, and copied rtthread.bin onto it. In that same folder, I edited my config.txt as such:

[config.txt] ===
# For more options and information see
# http://rpf.io/configtxt
# Some settings may impact device functionality. See link above for details

# uncomment if you get no picture on HDMI for a default "safe" mode
#hdmi_safe=1

# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=16
#overscan_right=16
#overscan_top=16
#overscan_bottom=16

# uncomment to force a console size. By default it will be display's size minus
# overscan.
#framebuffer_width=1280
#framebuffer_height=720

# uncomment if hdmi display is not detected and composite is being output
#hdmi_force_hotplug=1

# uncomment to force a specific HDMI mode (this will force VGA)
#hdmi_group=1
#hdmi_mode=1

# uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
#hdmi_drive=2

# uncomment to increase signal to HDMI, if you have interference, blanking, or
# no display
#config_hdmi_boost=4

# uncomment for composite PAL
#sdtv_mode=2

#uncomment to overclock the arm. 700 MHz is the default.
#arm_freq=800

# Uncomment some or all of these to enable the optional hardware interfaces
#dtparam=i2c_arm=on
#dtparam=i2s=on
#dtparam=spi=on

# Uncomment this to enable infrared communication.
#dtoverlay=gpio-ir,gpio_pin=17
#dtoverlay=gpio-ir-tx,gpio_pin=18

# Additional overlays and parameters are documented /boot/overlays/README

# Enable audio (loads snd_bcm2835)
dtparam=audio=on

# Automatically load overlays for detected cameras
camera_auto_detect=1

# Automatically load overlays for detected DSI displays
display_auto_detect=1

# Enable DRM VC4 V3D driver
dtoverlay=vc4-kms-v3d
max_framebuffers=2

# Run in 64-bit mode
# arm_64bit=1

# Disable compensation for displays with overscan
disable_overscan=1

#added by DLau - Jan 8, 2022 to boot rtthread:
enable_uart=1
arm_64bit=1
kernel=rtthread.bin
core_freq=250

[cm4]
# Enable host mode on the 2711 built-in XHCI USB controller.
# This line should be removed if the legacy DWC2 controller is required
# (e.g. for USB device mode) or if USB support is not required.
otg_mode=1

# [all]

[pi4]
# Run as fast as firmware / board allows
arm_boost=1

[/config.txt] ===

As you can see, I put the rt-thread specific stuff at the end of the file.

I then plugged in my USB cable, attaching ground to ground, then USB TX to Raspberry Pi RX, and USB RX to Raspberry Pi TX. I checked my Windows 10 device panel and it said the USB cable with prolific driver was on COM4, so I opened Putty to COM4, 8 bits, no parity, control bit 1, flow control off. I do not see the default rt-thread message in the terminal window.

What am I doing wrong?