writing buffer to a file - how can I keep binary format?

I have this code in coffescript:

fs = require 'fs'

class PpmCanvas


  constructor: (@width, @height, @fileName) ->
        size = @width * @height * 3

        array = new Uint8ClampedArray size
        @buffer = new Buffer array

        for element, index in @buffer
            @buffer.writeUInt8 128, index


    plot: (x, y, r, g, b) ->
        true

    save: () -> 
        header = new Buffer "P6 #{@width} #{@height} 255\n"

        together = Buffer.concat([header, @buffer])

        fs.open @fileName, 'w', (err, fd) =>
            if err
                throw err

            fs.write fd, together.toString(), undefined, undefined, (err, written, buffer ) =>
                fs.close fd
canvas = new PpmCanvas 200, 200, 'image.ppm'
canvas.save()

I'm trying to make a ppm image class, and I have a problem with saving the image to disk. So I first create Uint8Clamped array to keep the pixels data, then wrap it with a Buffer to be able to write it to disk later. And I set all pixels to some value to have some initial color in a loop. And as long as the value is in range 0..127 everything is fine, every byte is written to file as one byte, but when the value is bigger then 127 - every byte is written as 2 bytes to disk and it breaks the image. I've been trying to set the buffer encoding to 'binary' and everything else but it still gets written as two bytes - so plz tell me what is the correct way to write binary data to disk using node.