Fluid detection circuit for fluid ejection head

1. A method for detecting the presence or absence of fluid in a fluid chamber of a fluid ejection head, the method comprising:

providing a fluid ejection head and an ejection head controller therefor, the fluid ejection head having:

a semiconductor substrate having at least one elongate fluid supply via etched therethrough;

an array of fluid ejectors disposed adjacent to the at least one elongate fluid supply via, wherein the elongate fluid supply via provides fluid to the array of fluid ejectors for ejection of fluid from the ejection head;

a first fluid sense cell for the array of fluid ejectors disposed at a proximal end of the at least one elongate fluid supply via, wherein the first fluid sense cell has a first fluid ejector, a first electrode disposed in a first fluid chamber for the first fluid ejector, and a second electrode disposed in a first fluid channel associated with the first fluid chamber, and a second fluid sense cell disposed at a distal end of the at least one elongate fluid supply via, the second fluid sense cell having a third electrode disposed in a second fluid channel and a fourth electrode attached to a second fluid ejector for a second fluid chamber associated with the second fluid channel, and wherein the fluid ejection head contains no more than two fluid sense cells for each elongate fluid supply via, and wherein the first electrode and the third electrode are electrically isolated from one another;

a first fluid detection circuit in electrical communication with the first fluid sense cell and a second fluid detection circuit in electrical communication with the second fluid sense cell for detecting the presence or absence of fluid in the first or second fluid chambers;

applying a bias voltage to each of the first and second electrodes of the first fluid sense cell; removing the bias voltage from the second electrode;

capturing a current pulse in the first fluid detection circuit upon removal of the bias voltage from the second electrode; and

comparing the current pulse to a reference current in the first fluid detection circuit;

applying a bias voltage to each of the third and fourth electrodes of the second fluid sense cell; removing the bias voltage from the fourth electrode;

capturing a current pulse in the second fluid detection circuit upon removal of the bias voltage from the fourth electrode; and

comparing the current pulse to a reference current in the second fluid detection circuit, wherein each of the first and second fluid detection circuits outputs a digital signal in parallel to a shift register for output in series to an ejection head controller.

2. The method of claim 1 , wherein each of the first and second fluid sense cells comprises a conductivity sense circuit, and each of the first and second fluid detection circuits provides a digital high output bit when fluid is detected in the first or second fluid channel and the first or second fluid chamber and provides a digital low output bit when fluid is absent from the first or second fluid chamber.

3. The method of claim 2 , further comprising receiving the digital high output bit or digital low output bit in a digital circuit having multiple stages for receiving bits in parallel and shifting bits out in series to the ejection head controller, wherein each of the multiple stages receives a bit generated by the first or second fluid detection circuit.

4. The method of claim 1 , wherein the ejection head controller terminates fluid ejection from each fluid ejector array when the first or second fluid detection circuit signals the absence of fluid.

5. The method of claim 1 , wherein the fluid ejection head comprises a latch circuit for each of the first and second fluid detection circuits for holding a digital bit from each of the first and second fluid detection circuits for transfer to the shift register.