UART4/rs485 - incoming but not outgoing

Hi all!

I’ve got a few CBB-Serial-r01 capes and I need to get rs485 out on Kernel 4.x.

On kernel 3.8, default build, I can get rs485 tx/rx going with manual GPIO setting on the CBB-Serial-r01 cape.

With kernels 4.x I’m getting, issues. Currently, I’m trying it on 4.6-rc2.

So far: HDMI is disabled

UART4-RTSCTS device tree is loaded.

Transmitting data from external => beagle _ works_ and I see traffic on the wire with oscilloscope, and the beagle recieves data.

Transmitting data from the BBB shows RTS signal (which shows that the UART4 rts/cts works), but no traffic for the data transmit.

However, when I in desperation hooked the oscilloscope to the rs232 pins on the cape, I measure traffic when transmitting on those ports.


import serial
import struct, fcntl

ser = serial.Serial("/dev/ttyO4", 2400)
fd = ser.fileno()
serial_rs485 = struct.pack(‘hhhhhhhh’, 1, 0, 0, 0, 0, 0, 0, 0)
res = fcntl.ioctl(fd, 0x542F, serial_rs485)

while True:
data =

Can you pastebin that version of the file you are using?