Hi Ganesh, I saw your email to me as well (I recommend using this forum rather than email for asking questions). I believe your model has two outputs and two measured inputs. To extract the transfer function for one I/O pair (i, j), you could do:
sys = arx221('m') sysij = sys(i, j)
Note that an identified model is of the general form: y=G u + H e, where G is the transfer function between the measured inputs and outputs and H is the transfer function between the unmeasured or noise inputs and outputs. When you convert a model to tf format of Control System Toolbox, by default, the noise channels are treated as measured ones and the overall transfer function has ny extra inputs, where ny is the number of model outputs (since there is one noise channel per output).
If you believe arx221 model should have had 1 input channel, please check your estimation data; is it SISO? What does size(data) return? The most likely scenario is that you are inadvertently using a 2 input, 2 output data set.
HTH. Rajiv
