Circuit for preventing burn-in spots on the picture screen of a visual display

Claim 2

A circuit arrangement for prevention of an spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19).

Claim 3

A circuit arrangement according to claim 1, wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerble one-shot multivibrator (50).

Claim 4

A circuit arrangement according to claim 1 or 2, characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16).

Claim 5

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein the logic circuit (42) has a third input which receives a second signal (CF, FC') which indicates the alignment of the electron beam (19) to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) transmits the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16) as a function of the occurrance of first and second indicating signals (LC, LC', FC, FC').

Claim 6

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein the logic circuit (42) has a third input which receives a second signal (CF, FC') which indicates the alignment of the electron beam (19) to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) transmits the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16) as a function of the occurrance of first and second indicating signals (LC, LC', FC, FC').

Claim 7

A circuit arrangement according to claim 3 wherein a scan generator 3 is connected to the second input of said logic circuit (42), and said scan generator (3) produces an output which is the first indicating signal (LC), said scan generator generating at least one deflecting voltage for the drive of at least one deflection system of said visual display; and a third input of said logic circuit (42) connected to an output (43a) of said scan generator (3) which outputs the second indicating signal (FC).

Claim 8

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21,22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a computer (33) supplies the second input of said logic circuit (42), said computer (33) producing outputs which are the first indicating signal (LC'), said computer emitting at least one deflecting voltage for the drive of at least one deflection system of the visual display e.g. via a digital-to-analog converter (34), and under certain conditions, the third input of said logic circuit connected to an output (44a) of the computer (33) which outputs the second indicating signal (FC').

Claim 9

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21,22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a computer (33) supplies the second input of said logic circuit (42), said computer (33) producing outputs which are the first indicating signal (LC'), said computer emitting at least one deflecting voltage for the drive of at least one deflection system of the visual display e.g. via a digital-to-analog converter (34), and under certain conditions, the third input of said logic circuit connected to an output (44a) of the computer (33) which outputs the second indicating signal (FC').

Claim 10

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a scan generator 3 is connected to the second input of said logic circuit (42), and said scan generator (3) produces an output which is the first indicating signal (LC), said scan generator generating at least one deflecting voltage for the drive of at least one deflection system of said visual display; and a third input of said logic circuit (42) connected to an output (43a) of said scan generator (3) which outputs the second indicating signal (FC), and characterized in that the second input of the logic circuit (42) is connected through a switch (S3) to the outputs (43, 44) of said scan generator (3) and said computer (33) which supply the first indicating signal (LC, LC'), and under certain conditions, the third input of the logic circuit (42) is connected through a switch (S3) to those outputs (43a, 44a) of the scan generator (3) and of the computer (33) which output the second indicating signal (FC, FC').

Claim 11

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a scan generator 3 is connected to the second input of said logic circuit (42), and said scan generator (3) produces an output which is the first indicating signal (LC), said scan generator generating at least one deflecting voltage for the drive of at least one deflection system of said visual display; and a third input of said logic circuit (42) connected to an output (43a) of said scan generator (3) which outputs the second indicating signal (FC), and characterized in that the second input of the logic circuit (42) is connected through a switch (S3) to the outputs (43, 44) of said scan generator (3) and said computer (33) which supply the first indicating signal (LC, LC'), and under certain conditions, the third input of the logic circuit (42) is connected through a switch (S3) to those outputs (43a, 44a) of the scan generator (3) and of the computer (33) which output the second indicating signal (FC, FC').

Claim 12

A circuit arrangement according to claim 3, characterized in that the logic circuit (42) is designed as an AND element.

Claim 13

A circuit arrangement according to claim 1, characterized in a control unit (16) receives a signal input (45a) which is a signal for the intensity control of the electron beam (19) and the output of the control unit (16) is supplied to the intensity control input (17) of the visual display, and a signal path between the signal input (45a) and the output of the control unit (16) is switched on depending on the occurrence of a control signal at the control input (45) of said control unit.

Claim 14

A circuit arrangement according to claim 1 characterized in that the signal serving for the intensity control of the electron beam (19) is taken at the detector output (12) of a scanning electron microscope (1).

Claim 15

A circuit arrangement according to claim 1 characterized in that the signal serving for the intensity control of the electron beam (19) is output by a digital-to-analog converter (37) by a computer (33) for controlling the position of the electron beam (19).

Claim 16

A circuit arrangement according to claim 1 characterized in that a deflection system (21) conducting the electron beam (19) across the picture screen (23) in a first direction receives a deflecting voltage; and a second deflection system (22) acting in a second direction receives a signal whose shape is to be displayed dependent on the respective deflection of the electron beam (19) on the picture screen (23) in the first direction, whereby the connection between the signal input (25) of the second deflection system (22) and a following differentiator (53) is interrupted.

Claim 17

A circuit arrangement according to claim 12, characterized in that the signal whose shape is to be displayed on the picture screen dependent on the respective deflection of the electron beam in the first direction is taken at the output (12) of the microscope column of a scanning electron microscope (1).

Claim 18

A circuit arrangement according to claim 12, characterized in that the signal whose shape is to be displayed on the picture screen dependent on the respective deflection of the electron beam in the first direction is output via a digital-to-analog converter (37) by a computer (33), which controls the position of the electron beam.

Claim 19

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuti connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), and wherein the logic circuit (42) has a third input which receives a second signal (CF, FC') which indicates the alignment of the electron beam (19) to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) transmits the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16) as a function of the occurrence of first and second indicating signals (LC, LC', FC, FC').

Claim 20

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuti connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), and wherein the logic circuit (42) has a third input which receives a second signal (CF, FC') which indicates the alignment of the electron beam (19) to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) transmits the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16) as a function of the occurrence of first and second indicating signals (LC, LC', FC, FC').

Claim 21

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems ,21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), and wherein a computer (33) supplies the second input of said logic circuit (42), said computer (33) producing outputs which are the first indicating signal (LC'), said computer emitting at least one deflecting voltage for the drive of at least one deflection system of the visual display e.g. via a digital-to-analog converter (34), and under certain conditions, the third input of said logic circuit connected to an output (44a) of the computer (33) which outputs the second indicating signal (FC').

Claim 22

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2, 3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems ,21, 22), particularly for a scanning electron microscope, comprising a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), and wherein a computer (33) supplies the second input of said logic circuit (42), said computer (33) producing outputs which are the first indicating signal (LC'), said computer emitting at least one deflecting voltage for the drive of at least one deflection system of the visual display e.g. via a digital-to-analog converter (34), and under certain conditions, the third input of said logic circuit connected to an output (44a) of the computer (33) which outputs the second indicating signal (FC').

Claim 23

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprisign a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a scan generator 3 is connected to the second input of said logic circuit (42), and said scan generator (3) produces an output which is the first indicating signal (LC), said scan generator generating at least one deflecting voltage for the drive of at least one deflection system of said visual display; and a third input of said logic cicuit (42) connected to an output (43a) of said scan generator (3) which outputs the second indicating signal (FC), and characterized in that the second input of the logic circuit (42) is connected through a switch (S3) to the outputs (43, 44) of said scan generator (3) and said computer (33) which supply the first indicating signal (LC, LC"), and under certain conditions, the third input of the logic circuit (42) is connected through a switch (S3) to those outputs (43a, 44a) of the scan generator (3) and of the computer (33) which output the second indicating signal (FC, FC').

Claim 24

A circuit arrangement for prevention of a spot burn-in phenomena on a picture screen (23) of a visual display (2,3) which has an electron beam (19) that is intensity-controlled by an intensity control and position-controlled by two deflection systems (21, 22), particularly for a scanning electron microscope, comprisign a first differentiator (47) electrically connected to the signal input (24) of at least one deflection system (21), a first comparator (48) with a first input connected to the output of said differentiator (47), a second input (48a) of said comparator receiving a reference voltage, a retriggerable one-shot multivibrator (50) connected to the output of said comparator (48), and a control unit (16) connected to the output (41) of said one shot multivibrator so as to influence the intensity control of the electron beam (19), wherein the signal inputs (24, 25) of said two deflection systems (21, 22) influencing the electron beam (19) are electrically connected to said first differentiator (47) and to a second differentiator (53) and to said first comparator (48) and a second comparator (54) whose second inputs (48a, 55) respectively receive first and second reference voltages, and an OR gate (56) connected to the outputs of said first and second comparators (48, 55) and the output of said OR gate connected to the input (49) of said retriggerable one-shot multivibrator (50), characterizing a logic circuit (42) connected to the output (41) of said one-shot multivibrator (50), said logic circuit connected with its output to the control input (45) of said control unit (16), said logic circuit (42) receives a second input which comprises a first signal (LC, LC') which indicates the alignment of the electron beam (19) relative to the area (28) of the picture screen (23) to be swept by said electron beam; and said logic circuit (42) sends the output signal of said one-shot multivibrator (50) to the control input (45) of said control unit (16), wherein a scan generator 3 is connected to the second input of said logic circuit (42), and said scan generator (3) produces an output which is the first indicating signal (LC), said scan generator generating at least one deflecting voltage for the drive of at least one deflection system of said visual display; and a third input of said logic cicuit (42) connected to an output (43a) of said scan generator (3) which outputs the second indicating signal (FC), and characterized in that the second input of the logic circuit (42) is connected through a switch (S3) to the outputs (43, 44) of said scan generator (3) and said computer (33) which supply the first indicating signal (LC, LC"), and under certain conditions, the third input of the logic circuit (42) is connected through a switch (S3) to those outputs (43a, 44a) of the scan generator (3) and of the computer (33) which output the second indicating signal (FC, FC').