The invention provides a mechanism for allowing a transaction callback processing to be synchronized or to take effect prior to completion of a transaction or message. Resistance, and hence logic state, is determined by discharging a first charged capacitor through an unknown cell resistive element to be sensed at a fixed voltage, and a pair of reference capacitors. The device is characterized in that the bus-lines are formed from a material having a substantially low optical reflectivity. Embodiments of the squeeze-off apparatus may be locked in place once the tube is crimped, thereby permitting the flow to be reduced or shut off without continued application of force to the crimping jaws of the apparatus. According to the invention, one of the lateral edges of the base comprises a rib which corresponds to a longitudinal recess on the inner part of one of the sides of the guide, while the other side of the base comprises a crimp which can be engaged with an inner step on the other side of the guide.