Patent ID: 7547087

Claim:
A fault detector apparatus for a printer having a first print head and a second print head with a plurality of print head elements in each print head, the apparatus comprising: a test circuit in signal communication with all the print heads to test the plurality of print head elements, the test circuit comprising: a test power supply for generating a test voltage; a first resistor in series connection between the test power supply and the plurality of elements in the first print head; a second resistor, the first resistor and the second resistor in series connection with the test power supply and the plurality of elements in the second print head; and an analog to digital converter in signal communication with the second resistor; wherein in response to a flow of current from the test power supply through the first resistor and one of the plurality of elements in the first print head, the analog to digital converter detects a first voltage; and wherein in response to a flow of current from the test power supply through the first and second resistors and one of the plurality of elements in the second print head, the analog to digital converter detects a second voltage; wherein in response to a flow of current through the one element in the first print head operating in accordance with an acceptable operational characteristic, the first voltage is described by: res ⁢ ⁢ 116 res ⁢ ⁢ 116 + element ⁢ ⁢ 1 ⁢ ( v ⁢ ⁢ 66 ) ; and wherein in response to a flow of current through the one element in the second print head operating in accordance with an acceptable operational characteristic, the second voltage is described by: res ⁢ ⁢ 116 + res ⁢ ⁢ 132 res ⁢ ⁢ 116 + res ⁢ ⁢ 132 + element ⁢ ⁢ 2 ⁢ ( v ⁢ ⁢ 66 ) where: v 66 represents the test voltage; res 116 represents a resistance of the first resistor; res 132 represents a resistance of the second resistor; element 1 represents a resistance of the properly operating element in the first print head; and element 2 represents a resistance of the properly operating element in the second print head.