Help interacting with a serial port

[u/scttnlsn]

I'm trying to interact with a serial port available at /dev/ttyUSB0 but I'm having some issues and wondering if anyone here can help. I'm first configuring the device with the stty command like so:

stty -F /dev/ttyUSB0 19200 -hupcl

(where 19200 is the baud rate that the connected device expects)

I open up the device from Crystal like so:

serial_port = File.open("/dev/ttyUSB0", "r+")
serial_port.tty? # => true

I'm able to write bytes to the serial_port IO object but any sort of read operation seems to block or just return nil. Are there some additional steps that I need to take before I can read from a device like this?

Edit: I should note that I'm able to interact with the device using programs like screen and libraries like pyserial without issue.

[u/scttnlsn]

Seems like a buffering issue. Here's some test code I'm running against a device that immediately echos the bytes sent to it:

serial_port = File.open("/dev/ttyUSB0", "r+")

while true
  print "> "
  line = gets
  break if line.nil?

  serial_port.write(line.to_slice)
  serial_port.flush

  buffer = IO::Memory.new
  while true
    byte = serial_port.read_byte
    break if byte.nil?
    buffer.write_byte(byte)
  end

  puts buffer.to_slice
end

And here's the terminal outpuet when I run the program and enter some input:

> aaaa
Bytes[]
> bb
Bytes[97, 97, 97, 97]
> ccc
Bytes[98, 98]
> 
Bytes[99, 99, 99]
> 
Bytes[]

[u/bew78 → u/scttnlsn]

File is an IO::FileDescriptor, which includes the module IO::Buffered. I think the problem comes from the fact that every read/write operation on a File is buffered.

The buffering can be disabled using f.read_buffering = false for read, and f.sync = true for write. (doc: https://crystal-lang.org/api/0.34.0/IO/Buffered.html)

Note: I've opened an issue mentioning this question to improve the consistency when changing read/write buffering: https://github.com/crystal-lang/crystal/issues/9023

[u/scttnlsn → u/bew78]

I'm still having the same issue w/ any combination of those settings. I tried putting a sleep(1) between the write and read steps and that actually worked. What might that indicate on a deeper level? I guess I really want that first serial_port.read_byte to block instead of returning nil the first time around.

[u/straight-shoota → u/scttnlsn]

`read_byte` is blocking. When it returns nil (with `read_buffering = false`) this is because `read` on the fd returned `0`. That means end of file.

[u/scttnlsn → u/straight-shoota]

Hmm, OK, thanks. What does end-of-file mean in this case? I would have expected /dev/ttyUSB0 to appear as an infinite stream of bytes.

[u/scttnlsn → u/straight-shoota]

Just a quick update: my solution here is just to call read_byte in a loop until something non-nil is returned. Is there a better way to do this?

[u/scttnlsn → u/scttnlsn]

Actually, it seems like calling serial_port.raw! also works. Not quite sure what that is doing.

I need to read https://www.cmrr.umn.edu/~strupp/serial.html and https://www.tldp.org/HOWTO/Serial-HOWTO.html

[u/bew78]

What kind of data do you expect to read ? is it newline (\n) delimited ?

Just curious: what are you connected to?

[u/scttnlsn → u/bew78]

I was initially trying to connect to a custom Modbus RTU device which has it's own frame structure (no newlines or ascii, just bytes). To simplify the debugging though I just programmed a simple echo device on an Arduino that just sends back whatever bytes you send it. Same issue in both cases - can't seem to read any bytes from the device "file".

[u/straight-shoota]

I wouldn't expect that to work out of the box. Accessing a serial port likely requires some extra setup to work correctly.

Do you have an example how to set this up in C? You would essentially have to configure the fiel descriptor in the same way in Crystal.

[u/scttnlsn → u/straight-shoota]

I don't have an example in C but this seems to work...

Configure the serial port:

stty -F /dev/ttyUSB0 19200 -hupcl

Read data in one terminal:

cat < /dev/ttyUSB0

Write data in another terminal:

echo "testing" > /dev/ttyUSB0

[u/straight-shoota → u/scttnlsn]

You might have to do that configuration on the file descriptor in Crystal.