Coroutines are excellent for encoding state machines. See http://eli.thegreenplace.net/2009/08/29/co-routines-as-an-alternative-to-state-machines/

coroutines_vs_state_machines.rb

# Encoding: UTF-8

require 'fiber'

HEADER = 0x61
FOOTER = 0x62
DLE    = 0xAB

def after_dle(x)
  x
end

# This accepts generated frames and prints them. 
target = Fiber.new do
  loop do
    frame = Fiber.yield
    puts "Frame received: #{frame}."
  end
end
target.resume

# This decodes frames and transfers control to the return value of the
# first yield, which should be a fiber that handles results. 
decoder = Fiber.new do |target|
  loop do
    byte = Fiber.yield
    frame = ''
    
    if byte == HEADER
      loop do
        byte = Fiber.yield
        
        case byte 
          when FOOTER
            target.resume(frame)
            break
          when DLE
            byte = Fiber.yield
            frame += after_dle(byte).to_s(16)
        else
          frame += byte.to_s(16)
        end
      end
    end
  end
end
decoder.resume(target)

[0x70, 0x24,
 0x61, 0x99, 0xAF, 0xD1, 0x62,
 0x56, 0x62,
 0x61, 0xAB, 0xAB, 0x14, 0x62,
 0x7].each do |byte|
  decoder.resume(byte)
end

Frame received: 99afd1.
Frame received: ab14.

Extremely elegantly decoupled. Like it a lot.