Method and related apparatus for performing short and open circuit testing of ink jet printer head

1. Field of the Invention

The present invention relates to an ink jet printer, and more particularly, to a method and related apparatus for detecting short-circuit and open-circuit problems of inkjet units of the inkjet printer.

2. Description of the Prior Art

Having advantages of low-cost and excellent printing performance, ink jet printers have become one of the most popular electrical output devices.

Please refer to FIG. 1 , which is a schematic diagram of a circuit of a prior art printer 10 (U.S. Pat. No. 5,736,997). The printer 10 comprises a plurality of ink jet units A 11 to A 13 , A 21 to A 23 , and A 31 to A 33 disposed in matrix. The printer 10 further comprises a controller 12 for controlling the functionality of the printer 10 , a power circuit 14 for providing energy to the ink jet units, an address circuit 16 A for selectively controlling the ink jet units, a driving circuit 16 B for selectively driving the ink jet units, and a detecting circuit 18 . As each ink jet unit has the same structure, the ink jet unit A 13 is described as an example. The ink jet unit A 13 comprises a field effect transistor T, a heating element D, and a corresponding nozzle K. A gate electrode Tc of the transistor T serves as a control end of the ink jet unit A 13 . A source electrode of the ink jet unit A 13 and a drain electrode of the ink jet unit A 13 are respectively connected to ground and to one end of the heating element D. The other end of the heating element D is connected to a node Ti and serves as an input end of the ink jet unit A 13 . The heating element D is usually a resistor and is installed inside an ink container (not shown) of the ink jet unit A 13 for transforming electrical energy into heat energy to heat the ink stored in the ink container when currents are flowing through the heating element D. When the temperature of the ink exceeds a threshold, the ink will spray via the corresponding nozzle K.

The address circuit 16 A comprises three address lines Aa 1 , Aa 2 , and Aa 3 corresponding to the three-row disposition of the plurality of ink jet units A 11 to A 13 , A 21 to A 23 , and A 31 to A 33 . The address line Aa 1 is connected to the control ends of the ink jet units A 11 , A 12 , and A 13 . The address line Aa 2 is connected to the control ends of the ink jet units A 21 , A 22 , and A 23 . The address line Aa 3 is connected to the control ends of the ink jet units A 31 , A 32 , and A 33 . The driving circuit 16 B also comprises three driving lines Pa 1 , Pa 2 , and Pa 3 corresponding to the three-column disposition of the plurality of ink jet units A 11 to A 13 , A 21 to A 23 , and A 31 to A 33 . The driving line Pa 1 is connected to the input ends of the ink jet units A 11 , A 21 , and A 31 . The driving line Pa 2 is connected to the control ends of the ink jet units A 12 , A 22 , and A 32 . The driving line Pa 3 is connected to the control ends of the ink jet units A 13 , A 23 , and A 33 . The controller 12 of the printer 10 determines the functionality of the ink jet units by controlling the address circuit 16 A and by controlling the driving circuit 16 B. For example, when the controller 12 determines that the ink jet unit A 13 sprays ink, the address circuit 16 A raises the voltage level of the address Aa 1 by using the energy provided by the power circuit 14 . The high level voltage of the address line Aa 1 actuates the transistor T. At the same time, the driving circuit 16 B raises the voltage of the driving line Pa 3 also by using the energy provided by the power circuit 14 . Then driving currents Id generated by the power circuit 14 flow through the driving line Pa 3 , to node Ti, and finally into the ink jet unit A 13 . The heating element D of the ink jet unit A 13 transforms the electrical energy of the driving currents Id into heat energy, which will heat the ink stored in the ink jet unit A 13 and will make the nozzle K of the ink jet unit A 13 spray ink. On the contrary, when the controller 12 determines that the ink jet unit A 12 needs not spray ink, the controller 12 will controls the address circuit 16 A to keep the voltage of the ink jet unit A 12 at low level. Thus, when the voltage of the address line Aa 1 is at high level, the driving line Pa 2 will not convey any currents to the ink jet unit A 12 . When the controller 12 also determines that the ink jet unit A 23 needs not spray ink, the controller 12 will control the address circuit 16 B to keep the voltage of the address line Aa 2 at low level. The low level voltage of the address line Aa 2 is not capable of actuating the transistor T of the ink jet unit Aa 2 , so the ink jet unit A 23 still does not spray any ink even when the driving line Pa 3 has been kept at high level.

Using the above-mentioned controlling process, the printer 10 is capable of controlling individual ink jet unit to accurately spray ink according to an image. However, the printer 10 usually has some circuit problems. For example, the inkjet unit A 33 of the printer 10 has an open-circuit (OC) problem. No matter what voltage level the address line Aa 3 is at, the driving-energy provided by the power circuit 14 will not flow through the driving line Pa 3 into the ink jet unit A 33 . That is, the printer 10 is not capable of effectively controlling the functionality of the ink jet unit A 33 . As another example, the ink jet unit A 32 of the printer 10 has a short-circuit (SC) problem (possibly due to a breakdown of the transistor T or to a malfunction of the heating element D). No matter what voltage level the address line Aa 3 is at, whenever the driving circuit 16 B raises the voltage of the driving line Pa 2 , because the driving line Pa 2 is shorted to the ground, the currents flowing through the driving line Pa 2 become extremely high. The extremely high current may damage the control logic circuit of the driving circuit 16 B or may further damage the power circuit 14 . Thus far the printer 10 is useless.

To detect the above short-circuit problem, the prior art printer 10 relies on a detecting circuit 18 , as shown in FIG. 1 . The detecting circuit 18 of the printer 10 comprises three diodes D 1 to D 3 respectively connected to the three driving lines Pa 1 to Pa 3 . The detecting circuit 18 further comprises a comparator Vc 1 for generating a detecting signal 18 S. The comparator Vc 1 has a positive end and a negative end. The negative end of the comparator Vc 1 is connected to a resistor R 1 and to the anodes of the three diodes D 1 to D 3 . The positive end of the comparator Vc 1 is connected to a voltage divider composed of a voltage source Vcc and two resistors R 2 , R 3 by a contact point of the two resistors R 2 , R 3 . The functionalities of the detecting circuit 18 are described as follows. If the printer 10 functions normally (that is, no short-circuit problem), the three diodes D 1 to D 3 will be reverse biased and no currents will flow through the resistor R 1 . So the voltage of the negative end of the comparator Vc 1 equals Vcc. In the meantime, the voltage of the positive end of the comparator Vc 1 is always lower than Vcc due to the voltage dividing effect of the two resistors R 2 , R 3 . Therefore, the comparator Vc 1 is capable of determining whether the printer 10 has any short-circuit problems by comparing the voltage of the positive end with the voltage of the negative end. If all the ink jet units of the printer 10 function normally, the detecting circuit 18 generates a corresponding functioning-normally detecting signal 18 S. On the contrary, if the ink jet unit A 32 has a short-circuit problem, as described previously, the voltage of the driving line Pa 2 is decreasing, so the diode D 2 actuates and currents flow through the resistor R 1 . Thus the voltage of the negative end of the comparator Vc 1 is decreased. If the voltage of the negative end is lower than that of the positive end, the comparator Vc 1 then determines that the printer 10 has a short-circuit problem.

Although the detecting circuit 18 of the printer 10 is capable of detecting a short-circuit problem, the previously-mentioned extremely high currents could have possibly already destroyed the address circuit 16 A, the driving circuit 16 B, and the power circuit 14 before the detecting circuit generates the detecting signal 18 S (has detected the short-circuit problem). Furthermore, when the driving circuit 16 B is raising the voltage of the driving line Pa 3 , the open-circuit effect occurring in the ink jet unit A 33 will not affect the voltage of the driving line Pa 3 , so the detecting circuit 18 is not capable of detecting an open-circuit problem of any ink jet unit of the printer 10 .

It is therefore a primary objective of the claimed invention to provide methods for detecting short-circuit and open-circuit problems of an inkjet printer. The short-circuit problem will not further damage the corresponding circuits of the printer during a short-circuit problem detecting process even if a short-circuit has appeared in the printer.

Methods for detecting a short-circuit problem and an open-circuit problem are used for an ink jet printer. The ink jet printer includes at least an ink jet unit having an input end, a corresponding nozzle, and a control end. The ink jet printer further includes a driving circuit for providing energy to the ink jet unit via the input end. When the ink jet unit receives an ink jet signal via the control end, the ink jet unit is capable of spitting ink via the corresponding nozzle according to the energy received via the input end. The short-circuit problem detecting method includes the following steps: stopping transmitting the ink jet signal to the control end of the inkjet unit, stopping providing energy to the input end of the inkjet unit, and measuring currents flowing through the input end of the inkjet unit. The open-circuit problem detecting method includes the following steps: transmitting the ink jet signal to the control end of the ink jet unit and measuring currents flowing through the input end of the inkjet unit.

It is an advantage of the claimed invention that the claimed invention is capable of detecting the short-circuit problem and the open-circuit problem. When the printer is proceeding with the short-circuit detecting process, the driving circuit does not drive the driving lines even if the short-circuit problem already exists. Such a process will not further damage the driving circuit, power circuit, or any other important circuits of the printer. When the printer is proceeding with the open-circuit detecting process, the controller is capable of detecting what ink jet unit has open-circuit problems, what resistor is useless, or what switch of the printer has malfunctioned.

These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

FIG. 1 is a schematic diagram of a circuit of a prior art printer.

FIG. 2 is a schematic diagram of a circuit of a printer according to the present invention.

FIG. 3 is a schematic diagram of the printer when the printer is proceeding with a short-circuit problem detecting process.

FIG. 4 is a schematic diagram of the printer when the printer is proceeding with an open-circuit detecting process.

FIG. 5 and FIG. 6 are two schematic diagrams respectively corresponding to another two printers according to the present invention.

Please refer to FIG. 2 , which is a schematic diagram of a circuit of a printer 20 according to the present invention. The printer 20 comprises a plurality of ink jet units C 11 to C 13 , C 21 to C 23 , and C 31 to C 33 disposed in matrix. The printer 20 further comprises a controller 22 for controlling the functionality of the printer 20 , a power circuit 24 for providing energy to the ink jet units, an address circuit 26 A for selectively controlling the ink jet units, a driving circuit 26 B for selectively driving the ink jet units, and a detecting circuit 28 for detecting an open-circuit problem and a short-circuit problem. As each ink jet unit has the same structure, the ink jet unit C 13 is described as an example. The ink jet unit C 13 comprises a field effect transistor Q, a heating element H, and a corresponding nozzle Nz. A gate electrode Nc of the transistor Q serves as a control end of the ink jet unit C 13 . A source electrode of the ink jet unit C 13 and a drain electrode of the ink jet unit C 13 are respectively connected to ground and to one end of the heating element H. The other end of the heating element H is connected to a node N 1 and serves as an input end of the ink jet unit C 13 . The address circuit 26 A comprises three address lines A 1 , A 2 , and A 3 corresponding to the three-row disposition of the plurality of inkjet units C 11 to C 13 , C 21 to C 23 , and C 31 to C 33 . The address line A 1 is connected to the control ends of the ink jet units C 11 , C 12 , and C 13 . The address line A 2 is connected to the control ends of the ink jet units C 21 , C 22 , and C 23 . The address line A 3 is connected to the control ends of the ink jet units C 31 , C 32 , and C 33 . The driving circuit 26 B comprises three driving lines P 1 , P 2 , and P 3 corresponding to the three-column disposition of the plurality of ink jet units C 11 to C 13 , C 21 to C 23 , and C 31 to C 33 . The driving line P 1 is connected to the input ends of the ink jet units C 11 , C 21 , and C 31 . The driving line P 2 is connected to the control ends of the ink jet units C 12 , C 22 , and C 32 . The driving line P 3 is connected to the control ends of the ink jet units C 13 , C 23 , and C 33 . The controller 22 of the printer 20 determines the functionality of the inkjet units by controlling the address circuit 26 A and by controlling the driving circuit 26 B. For example, if the controller 22 determines that the ink jet unit C 13 is about to spray ink, the address circuit 26 A raises the voltage level of the address A 1 by using the energy provided by the power circuit 24 . The high level voltage of the address line Al actuates the transistor Q. In the meantime, the driving circuit 26 B raises the voltage of the driving line P 3 also by using the energy provided by the power circuit 24 . Driving currents generated by the power circuit 24 then flow through the driving line P 3 , to node N 1 , and finally into the ink jet unit C 13 . The heating element H of the ink jet unit C 13 transforms the electrical energy of the driving currents into heat energy, which will heat the ink stored in the ink jet unit C 13 and will make the nozzle Nz of the ink jet unit C 13 spray ink. On the contrary, if the controller 22 of the printer 20 determines that the ink jet unit C 12 need not spray any ink, the address circuit 26 A then keeps the voltage of the ink jet unit C 12 at low level. Thus, even if the voltage of the address line A 1 is at high level, the driving line P 2 still will not convey any currents to the ink jet unit C 12 . If the controller 22 also determines that the ink jet unit C 23 need not spray ink, the controller 22 controls the address circuit 26 A to keep the voltage of the address line A 2 at low level. The low level voltage of the address line A 2 is not capable of actuating the transistor Q, so the ink jet unit C 23 does not spray any ink even if the driving line P 3 has been kept at high level.

The detecting circuit 28 of the printer 20 comprises a current-providing circuit 30 A (inside dotted lines shown in FIG. 2 ), a measuring circuit 30 B, and three switches S 1 to S 3 . The current-providing circuit 30 A is a current mirror, which is formed by two transistors M 1 , M 2 , and a DC power Vcc. A current end of the current-providing circuit 30 A is connected to a node N 1 and provides currents to the inkjet units. One electrode of the transistor M 1 is connected to the measuring circuit 30 B. The measuring circuit 30 B generates a detecting signal 28 S to report a short-circuit problem or to report an open-circuit problem according to currents flowing from the transistor M 1 . The three switches S 1 to S 3 are respectively connected between the corresponding driving lines Pa 1 to Pa 3 and the current end (at node N 1 ) of the current-providing circuit 30 A. The controller 22 controls the functionalities of the three switches S 1 to S 3 . For example, when the switch S 1 is closed, the driving line P 1 is electrically connected to node N 1 current end of the current-providing circuit 30 A; when the switch S 1 is opened, the driving line P 1 is not electrically connected to the current end. The three switches S 1 to S 3 are opened when the printer 20 is proceeding with printing processes.

Please refer to FIG. 3 , which is a schematic diagram of the printer 20 when the printer 20 is proceeding with a short-circuit problem detecting process. The voltages of all the address lines of the address circuit 26 A are kept at low level (represented by dotted lines shown in FIG. 2 ). At the same time, the controller 22 closes the three switches S 1 to S 3 . Thus the three address lines P 1 to P 3 are capable of connecting to the current-providing circuit 30 A via node N 1 . The driving circuit 26 B also stops driving the three address lines P 1 to P 3 (represented by dotted lines shown in FIG. 2 ). If all the ink jet units function normally, because the voltage of all the address lines are kept at low level, no transistors actuate. Therefore, no currents flow through the three driving lines P 1 to P 3 . No currents flowing through the three driving lines is equivalent to a large resistance being connected between node N 1 and the three driving lines. So, currents 12 , which flow through the transistor M 2 , are small. Currents 11 , which flow through the transistor M 1 , are also small due to the mirror effect. When measuring the small currents 11 , the measuring circuit 30 B determines that no short-circuit problems occur in any ink jet units and then generates a detecting signal 28 S to report that no ink jet unit has any short-circuit problem. On the contrary, if at least one ink jet unit has a short-circuit problem, such as ink jet unit C 32 and a short-circuit route SC 1 shown in FIG. 3 , an equivalent small resistance between node 1 and the ground is formed. The equivalent small resistance results in large currents flowing through the transistor M 2 and through the transistor M 1 due to the mirror effect. When measuring the large currents 11 , the measuring circuit 30 B then determines that short-circuit problems occur in some ink jet units and then generates a detecting signal 28 S to report the short-circuit problems of the printer 20 . As soon as the detecting circuit 28 has detected any short-circuit problems occurring in the ink jet units, the controller 22 controls the power circuit 24 and controls the driving circuit 26 B to stop functioning. Therefore, severe damage caused by the large currents 12 is prevented. In practice, the measuring circuit 30 B usually compares the currents 11 with a predetermined currents level. When the currents 11 are less than the predetermined currents level, the printer 20 does not have any short-circuit problem. When the currents 11 are greater than the predetermined current level, at least one inkjet unit of the printer 20 has a short-circuit problem.

In the above-described short-circuit problem detecting process of the printer 20 , because the driving circuit 26 B will not raise the voltages of the three driving lines P 1 to P 3 during the short-circuit detecting process, the short-circuit problem is not capable of damaging the driving circuit 26 B or of damaging the power circuit 24 . Since the currents 12 are provided by the voltage source Vcc, the level of the voltage source Vcc can be set lower than that of the working voltage of the printer 20 so as to further protect the driving lines from damaging during the short-circuit problem detecting process. Furthermore, the short-circuit problem detecting process of the present invention is also capable of detecting the short-circuit problems occurring on the driving lines. For example, to detect if the driving line P 1 has any short-circuit problems, the switch S 1 is closed and the switches S 2 , S 3 are opened. Then only those ink jet units connected to the driving line P 1 will affect the equivalent resistance at node N 1 . If those ink jet units connected to the driving line P 1 have any short-circuit problems, the low equivalent resistance at node N 1 makes the currents 12 , 11 increase. In such a circumstance, because the switches S 2 . S 3 are opened, the short-circuit problems of the inkjet units connected to the remaining driving lines P 2 , P 3 will not affect results of the short-circuit problem detecting process performed on the driving line P 1 .

In addition to the short-circuit detecting process, the printer 20 is capable of proceeding with an open-circuit detecting process on the inkjet units of the printer 20 . Please refer to FIG. 4 , which is a schematic diagram of the printer 20 when the printer 20 is proceeding with an open-circuit detecting process. The controller 22 of the printer 20 opens the switches S 1 , S 2 and closes the switch S 3 , which is connected to the driving line P 3 and is connected to the ink jet unit C 33 . At the same time, the controller 22 controls the address circuit 26 A to raise the voltage of the address line A 3 , which is connected to the inkjet unit C 33 . The voltage-raised address line A 3 actuates the transistor Q of the ink jet unit C 33 . The address circuit 26 A keeps the remaining address lines A 1 , A 2 at low voltage level. The low voltage level address lines A 1 , A 2 and the driving lines P 1 , P 2 , which are not connected to node N 1 due to the opened switches S 1 , S 2 , are illustrated with dotted lines and are shown in FIG. 4 . For the time now only the ink jet unit C 33 is actuated by the address line A 3 and is electrically connected to node N 1 via the closed switch S 3 . If the ink jet unit C 33 functions normally, the transistor Q of the ink jet unit C 33 will actuate due to the high voltage of the address line A 3 . The actuated transistor Q is equivalent to a resistor with a small resistance connecting between the ground and node N 1 . So, the transistor M 2 actuates large currents 12 , which correspondingly induce another large current 11 to flow through the transistor M 1 due to the mirror effect. As the measuring circuit 30 B measures the large current 11 , the measuring circuit 30 B then generates a detecting signal to report that the ink jet unit C 33 does not have any open-circuit problem. On the contrary, if the ink jet unit C 33 indeed has an open-circuit problem and has an opened route OC 1 , the opened route OC 1 is equivalent to a resistor with an extremely large resistance connecting between the ground and node N 1 . So the transistor M 2 only actuates small currents 12 , which correspondingly induce another small current 11 flowing through the transistor M 1 due to the mirror effect. As the measuring circuit 30 B measures the small current 11 , the measuring circuit 30 B generates a detecting signal to report that the ink jet unit C 33 has an open-circuit problem. The controller 22 is capable of detecting the open-circuit problem for each ink jet unit by sequentially raising the voltage of the address line corresponding to the ink jet unit and by sequentially closing the switch corresponding to the ink jet unit. Because the controller 22 of the printer 20 is not capable of controlling an ink jet unit having an open-circuit problem to spray ink, if an ink jet unit has an open-circuit problem, the controller 22 will control another ink jet unit, neighboring to the malfunctioned ink jet unit, to spray ink when the malfunctioned ink jet unit is determined to spray ink.

In summary, no matter what problem detecting process the printer 20 is proceeding with, the driving circuit 26 B neither raises the voltage of the three driving lines P 1 to P 3 nor transmits any energy to the three driving lines P 1 to P 3 . When the printer 20 is proceeding with the short-circuit detecting process, the controller 22 keeps the voltages of the three address lines A 1 to A 3 at low level and closes the three switches S 1 to S 3 to respectively electrically connect the three driving lines P 1 through P 3 to node N 1 . If an ink jet unit of the printer 20 has a short-circuit problem, an extremely small equivalent resistance appears at node N 1 . The small equivalent resistance at node N 1 results that the transistor M 2 generates extremely large currents, which induce another extremely large current to flow through the transistor M 1 . If the ink jet units of the printer 20 all function normally, an extremely large equivalent resistance appears at node N 1 . The large equivalent resistance at node N 1 results that the transistor M 2 generates extremely small currents, which induce another extremely small current to flow through the transistor M 1 . When the printer 20 is proceeding with the open-circuit detecting process, the controller 22 keeps the voltage of the address line, which is connected to a specific ink jet unit, at low level and closes the corresponding switch electrically connected to the specific ink jet unit. If the specific ink jet unit functions normally, an extremely small equivalent resistance appears at node N 1 . The small equivalent resistance at node N 1 results that the transistor M 2 generates large currents, which induce another large current to flow through the transistor M 1 . If the specific inkjet unit has an open-circuit problem, an extremely large equivalent resistance appears at node N 1 . The large equivalent resistance results that the transistor M 2 generates small currents, which induce another small current to flow through the transistor M 1 . The detecting circuit 28 generates a corresponding detecting signal 28 S by determining the currents flowing from the transistor M 1 .

Please refer to FIG. 5 and FIG. 6 . FIG. 5 and FIG. 6 are two schematic diagrams respectively corresponding to another two printers 30 , 40 according to the present invention. Printers 30 , 40 and the printer 20 respectively comprise the same elements except the detecting circuit. The detecting circuit 38 of the printer 30 , shown in FIG. 5 , comprises not only three switches S 1 to S 3 but also a current-providing circuit 40 A and a measuring circuit 40 B for generating a detecting signal 38 S. The current-providing circuit 40 A comprises a resistor R 0 , a diode D 0 , and a bipolar junction transistor M 3 . A base electrode of the transistor M 3 is connected to a cathode electrode of the diode D 0 . An anode electrode of the diode D 0 is connected to node N 1 and serves as a current end of the current-providing circuit 40 A. One end of the resistor R 0 is connected to the anode electrode of the diode D 0 and the other end of the resistor R 0 is connected to the voltage source Vcc. An emitter electrode of the transistor M 3 is connected to the ground. A collector electrode of the transistor M 3 is connected to the measuring circuit 40 B. When node N 1 is connected to an equivalent resistor with a large resistance, currents flowing through the resistor R 0 will be small. The small currents keep the voltage of the anode electrode of the diode D 0 at high level and consequently actuate the diode D 0 . The actuated diode D 0 keeps the voltage of the base electrode of the transistor M 3 at high level. The transistor M 3 actuates too. The actuated transistor M 3 actuates large currents 13 . On the contrary, when node is connected to an equivalent resistor with a small resistance, currents flowing through the resistor R 0 is large. The large currents keep the voltage of the anode electrode of the diode D 0 at low level and consequently do not actuate the diode D 0 . The transistor M 3 does not actuate either, so the currents 13 are very small.

The detecting circuit 48 of the printer 40 , shown in FIG. 6 , also comprises a current-providing circuit 50 A and a measuring circuit 50 B. The current-providing circuit 50 A comprises a bipolar junction transistor M 4 and a resistor R 4 . A base electrode of the transistor M 4 is connected to node N 1 and serves as a current end of the current-providing circuit 50 A. An emitter electrode of the transistor M 4 is connected to one end of the resistor R 4 . The other end of the resistor R 4 is connected to the voltage source Vcc. A collector electrode of the transistor M 4 is connected to the measuring circuit 50 B. When node N 1 is connected to an equivalent resistor with a large resistance, the transistor M 4 does not actuate and consequently currents 14 , flowing through the transistor M 4 , are very small. When node N 1 is connected to an equivalent resistor with a small resistance, the voltage of the base electrode of the transistor M 4 is reduced and then the transistor M 4 is actuated. The actuated transistor M 4 actuates currents 14 to flow through the transistor M 4 . Therefore, the measure circuit 50 B is capable of generating a corresponding detecting signal 48 S by determining the level of the currents 14 . No matter what process (the short-circuit detecting process or the open-circuit detecting process) the printer 30 or the printer 40 is proceeding with, the functionalities of the corresponding address lines, driving lines, and switches are the same as those of the printer 20 illustrated in the FIG. 2 through FIG. 4 , so further discussion is omitted.

In contrast to the prior art printer, which is only capable of proceeding with the short-circuit detecting process, the present invention printer is capable of detecting the short-circuit problem and of detecting the open-circuit problem. When the printer is proceeding with the short-circuit detecting process, the driving circuit does not drive the driving lines even if the short-circuit problem already exists. Such a process will not further damage the driving circuit, power circuit, or any other important circuits of the printer. When the printer is proceeding with the open-circuit detecting process, the controller is capable of detecting what ink jet unit has open-circuit problems, what resistor is useless, or what switch of the printer is malfunctioned.

Following the detailed description of the present invention above, those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.