We report preliminary results of a new time-dependent model of SiO maser emission in the extended atmosphere of an AGB star. We compare this model to time-series observations of SiO masers towards the Mira variable TX Cam. The model masers are predominantly radiatively pumped, at least in the second half of the stellar pulsation period. Predictions for the radius of peak maser brightness and the width of the maser ring are broadly in line with the observations.