Question

I’m writing a program which reads and writes from and to the serial port. There are two threads; one thread reads and writes from/to the serial port every 500ms, and the other thread writes to the serial port every 3s.

The issue I’m having is, since 500ms is a multiple of 3s, at 3s, 6s, 9s… the 500ms loop fails to read/write from/to the serial port. Is there are way to check whether the serial port is in use?

    counter = 0
    switchRelays = DoEvery([3], 20) do
        sp.write "@00 WR #{relays[counter]}\r"
        counter = (counter + 1) % relays.length
    end

    # This thread samples every 500ms.
    sp.write "@00 AI 0\r"
    sleep 0.2
    sample500 = DoEvery([0.5], 1.0/0.0) do |timeStamp|
        if switchRelays.alive? == false
            csv.close
            puts 'Done'
            sample500.exit
        else
            sleep 0.1
            analogueStatus = sp.readline
            sp.write "@00 AI 0\r"
        end
    end

Answer 1

This is a classic example of a shared object where you need to synchronize concurrent access attempts.

You can use Mutex, for example.

require 'thread'
semaphore = Mutex.new

a = Thread.new {
  semaphore.synchronize {
    # write to the port
  }
}

b = Thread.new {
  semaphore.synchronize {
    # write to the port
  }
})