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Situation: I'd like to use my age-old VT420 console on my Pi 3B. As the UART is already occupied, I plugged a USB-to-serial dongle with CH341 chip (lsusb told me that) into one of the USB ports on the Pi and attached the terminal there. But: No data are neither received, nor transmitted! I tried it out using minicom, but no use. Here's my config:

pi@autoradio:~ $ ls -al /dev/ttyUSB*
crw--w---- 1 root tty 188, 0 Dez 27 17:59 /dev/ttyUSB0

pi@autoradio:~ $ sudo stty -F /dev/ttyUSB0 -a
speed 9600 baud; rows 24; columns 80; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff
-iuclc -ixany -imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt
-echoctl -echoke

pi@autoradio:~ $ lsmod | grep ch341
ch341                   6086  1 
usbserial              29943  3 ch341

pi@autoradio:~ $ uname -a
Linux autoradio 4.9.35-v7+ #1014 SMP Fri Jun 30 14:47:43 BST 2017 armv7l GNU/Linux

What puzzles me, is the following: I set the baudrate to 19200, but my dongle seems to ignore that and keeps on pretending the baudrate was 9600! The terminal, on the other hand, is set to 19200.

BTW: On my Linux PC, the concole does work when attached to an RS232 port.

What shall I do here? Thank you!

  • Exactly what USB to serial cable are you using? It needs to have RS232 signal levels (+-12V) not TTL level signals. – Craig Dec 27 '18 at 20:20
  • I'm using a CSL USB dongle with DB9 connector on the other side. From there, a normal RS232 cable with handshake wires runs to the console. – Neppomuk Dec 27 '18 at 21:37
  • To what extent does the USB-to-serial adapter support the handshaking signals besides the RXD and TXD data signals. Given that you are talking to some real serial hardware it might actually be expecting CTS, DTR and other control lines to be in the right state before it will play ball! {BTW +1 i.e. kudos to you for trying to use a real terminal rather than a terminal emulation!} – SlySven Dec 28 '18 at 2:05
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    Does the cable work when used with your linux PC? Could there be power issues with the Raspberry Pi? – Craig Dec 28 '18 at 14:59
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    If you can connect to the VT420 using the same dongle from a PC, my first guess would be that dongle needs more power than the Pi is providing. – Craig Jan 7 at 21:34
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OK, I attached the dongle to a wedge cable, which itself runs from the UPS mounted on top of my Pi to a radio tuner module, and suddenly, it started working! Apparently, it was really a max current issue, as suggested by Craig. Thank you for the hint!

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