This example don't work with Cog 4.0.2701 in a iMac OSX 10.8.3:

|p s| 
p := SerialPort new.
p baudRate: 115200.
p openPort: '/dev/cu.usbmodem1a21'.

1 to: 5 do:[:i|
	s := p readString. 
	Transcript show: 'i=', i asString, '=>', s, '<=';cr.
	(Delay forSeconds: 1) wait.
	i=1 ifTrue:[p nextPutAll: 'M105', String lf.]
].
p close.

the same code works ok with the Squeak 4.2.5beta1U, this is connected to an Arduino Mega using only a USB cable.