UARTs only unidirectional, black + robotic cape

I can read from UARTs but I can not write to any. This:

echo /boot/vmlinuz-4.14.71-ti-r80 > /dev/ttyS5

returns immediately for any UART, not even an error message. They can read fine, they can not write, any of them.

There is a thread https://groups.google.com/forum/#!category-topic/beagleboard/uart/0Yr9ed6PO2o about pinmuxing, cape overlays, device trees, bootloaders etc. which try to make BB UARTs work. Should I follow it to see, why it does not work also on my board?

$ dmesg | grep tty
[ 0.000000] Kernel command line: console=ttyO0,115200n8 root=/dev/mmcblk0p1 ro rootfstype=ext4 rootwait coherent_pool=1M
[ 0.001858] WARNING: Your ‘console=ttyO0’ has been replaced by ‘ttyS0’
[ 1.241161] 44e09000.serial: ttyS0 at MMIO 0x44e09000 (irq = 30, base_baud = 3000000) is a 8250
[ 2.089133] console [ttyS0] enabled
[ 2.093844] 48022000.serial: ttyS1 at MMIO 0x48022000 (irq = 31, base_baud = 3000000) is a 8250
[ 2.103698] 48024000.serial: ttyS2 at MMIO 0x48024000 (irq = 32, base_baud = 3000000) is a 8250
[ 2.113638] 481a6000.serial: ttyS3 at MMIO 0x481a6000 (irq = 33, base_baud = 3000000) is a 8250
[ 2.123518] 481a8000.serial: ttyS4 at MMIO 0x481a8000 (irq = 34, base_baud = 3000000) is a 8250
[ 2.133333] 481aa000.serial: ttyS5 at MMIO 0x481aa000 (irq = 35, base_baud = 3000000) is a 8250

I am sorry, I typed 'echo" instead of ‘cat’. This works as expected:

cat /boot/vmlinuz-4.14.71-ti-r80 > /dev/ttyS5