Forwarding traffic from port ttyS3 to ttyUSB0 - input/output error

Question

I am attempting to set up a basic pipe that'll transfer all data written to ttyS3 to ttyUSB0. I found a few solutions to the problem such as this, but they don't seem to help much. The issue seems to be that anytime I do anything with ttyS3, I get this:

stty: /dev/ttyS3: Input/output error

Doing ls -l /dev/ttyS* and the same for /dev/ttyUSB* I get the following:

root@arm-64:~# ls -l /dev/ttyS*
crw-rw---- 1 root dialout 4, 64 Feb  9 13:08 /dev/ttyS0
crw-rw---- 1 root dialout 4, 65 Feb  9 13:08 /dev/ttyS1
crw--w---- 1 root tty     4, 66 Feb  9 13:08 /dev/ttyS2
crw-rw---- 1 root dialout 4, 67 Feb  9 13:08 /dev/ttyS3
crw-rw---- 1 root dialout 4, 68 Feb  9 13:08 /dev/ttyS4

root@arm-64:~# ls -l /dev/ttyUSB*
crw-rw---- 1 root dialout 188, 0 Feb  9 13:08 /dev/ttyUSB0

I've created the following script to do the job for me at startup. I changed the major/minor values to match that of USB0 after reading somewhere that this could work as a pipe. Although it does execute without throwing an Input/output error, it doesn't seem to work as intended.

#!/bin/bash
rm /dev/ttyS3
mknod -m 666 /dev/ttyS3 c 188 0
chown root.dialout /dev/ttyS3
chmod 666 /dev/ttyS3
stty -F /dev/ttyUSB0 speed 115200 cs8
stty -F /dev/ttyS3 speed 115200 cs8
cat /dev/ttyS3 > /dev/ttyUSB0 &

I just need to create a basic pipe that'll take all data written to ttyS3 and pass it on to ttyUSB0. Although I don't think it's relevant, I'm running Armbian bullseye on a TV box (Tx3 Mini)

Answer 1

I just need to create a basic pipe that'll take all data written to ttyS3 and pass it on to ttyUSB0

Don't see a problem so long as each serial terminal is properly setup and functional/operational. Before you create the "pipe", did you verify that each serial terminal is operating properly?

On a SBC I have the console on a serial terminal, and established two more serial terminals using a SoC USART and a USB adapter:

# ls -l /dev/tty*S*                                      
crw-rw---- 1 root dialout 246,  0 Jan  1  2012 /dev/ttyGS0                      
crw------- 1 root tty       4, 64 Jul 31 22:46 /dev/ttyS0                       
crw-rw---- 1 root dialout   4, 65 Jul 31 22:25 /dev/ttyS1                       
crw-rw---- 1 root dialout 188,  0 Jul 31 22:28 /dev/ttyUSB0                     
#

Note that the udev daemon created these device nodes, and no funny business (i.e. manual re-creating device nodes) was necessary to accomplish the "pipe".

To remove canonical processing, each serial terminal is put in raw mode and with matching baudrates:

# stty raw 115200 -F /dev/ttyUSB0
# stty raw 115200 -F /dev/ttyS1

A report of all termios settings:

# stty -aF /dev/ttyUSB0                                  
speed 115200 baud; rows 0; columns 0; 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; discard = ^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 -flusho -extproc                                                 
#

# stty -aF /dev/ttyS1                                    
speed 115200 baud; rows 0; columns 0; 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; discard = ^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 -flusho -extproc                                                 
#

Then when the command

# cat /dev/ttyS1 > /dev/ttyUSB0 &

is issued, whatever is typed on the remote terminal-emulator program connected to /dev/ttyS1 shows up on the remote terminal-emulator program connected to /dev/ttyUSB0.
This seems to behave like the desired "basic pipe that'll take all data written to ttyS? and pass it on to ttyUSB0".

Bottom line:
Unable to duplicate problems, and can create "pipe" of two serial links.

# uname -a                                               
Linux sama5d2-xplained 5.4.81-linux4sam-2020.10 #1 Thu Jan 14 12:54:56 UTC 2021 
armv7l armv7l armv7l GNU/Linux                                                  
#

---

The issue seems to be that anytime I do anything with ttyS3, I get this:

stty: /dev/ttyS3: Input/output error

... I'm running Armbian bullseye on a TV box (Tx3 Mini)

As previously mentioned, you need to verify that each serial terminal is operating properly.
Since a "TV box" doesn't really need five (!) serial terminals, you might be seeing/creating bogus device nodes that don't have any hardware to access.
Search the system log for the actual hardware that was initialized, e.g. 'dmesg | grep tty'. One of those UARTs might be used to interface to an IR receiver.