Lua script read from serial port in OpenWRT

Question

I have Openwrt router with Arduino connected via USB FTDI adapter. Serial port is /dev/ttyUSB0

Arduino code prints some data:

First part of data printed with delay via command print(), for example:

Serial.begin(9600);
Serial.print(var1);
delay(1000);
Serial.print(var2);
delay(1000);
Serial.print(var3);
delay(1000);

And second part printed with println() command:

Serial.println("");
Serial.println(var4);
Serial.println(var5);
Serial.println(var6);

So when I open Serial port in terminal I can see something like this:

then timeout in 1 sec, then

1 2

next timeout. and then

1 2 3

last timeout and

It works in Terminal program and in console in OpenWRT, for example screen /dev/ttyUSB0

I need make a Lua script that will read Serial port and print the data in the same way. I have a simple script, but it doesn't work as expected.

rserial=io.open("/dev/ttyUSB0","r")
while true do
chain = nil
  while chain==nil do
    chain=rserial:read();
    print(chain)
  end
end

it shows all data at once. it doesn't show first 3 vars one by one with delays. Seems it is because of rserial:read() - it will read until it receives a newline character. It stated in similar question: How to read from a serial port in lua

I tried to run this command as was advised there:

stty -F /dev/ttyUSB0 -icanon

but it doesn't help and I don't understand why. Is it the way to fix this behavior via stty? or I definitely need to use another Serial libs for Lua script? All of these libs seems pretty outdated for now and I don't want to use outdated stuff..

Piglet Piglet · Accepted Answer · 2018-03-19T19:53:09

From the Lua Reference Manual:

When called without formats, it uses a default format that reads the next line (see below).

A new line is anything in the buffer until the next newline character.

So as long as you don't send a newline character Lua will wait one as it has been told by calling read()

Once a newline character is received you will be prompted any other character in that line.

Terminal programs usually update every byte to show what they receive in "real-time".

So if you want to have the same behaviour you may not use read() without any arguments.

Use read(1) to read every single byte without waiting for anything else.

Lua script read from serial port in OpenWRT

1 Answers