JP3152319B2 - Serial printer device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a serial printer, and more particularly to a serial printer having a synchronizing signal generating circuit for synchronizing the movement of a carriage having a print head and the printing operation of a print head.

[0002]

2. Description of the Related Art In a serial printer, printing (printing) is performed while scanning a carriage on which a printing head of a printing means is mounted in a horizontal direction with respect to a printing medium. Then, density unevenness of the recording result occurs. In particular, in a color printer, a shift in color registration occurs, which is a problem.

Conventionally, as one method for solving these problems, a configuration in which the amount of movement of a carriage equipped with recording means with respect to the apparatus main body is detected, and the recording operation is performed by the recording means while synchronizing with the detection result. It has been known.

That is, a scale section of a linear encoder is fixed on a main body side, and a detection section of a linear encoder is mounted on a carriage which moves relatively to the scale section, and an output signal from the detection section is amplified. Then, the recording signal is taken out of the carriage and a recording signal is generated in synchronization with the amplified signal, thereby preventing print density unevenness and color registration deviation.

A conventional example will be described with reference to the drawings.
Numeral 3 is a circuit diagram showing a configuration of a conventional synchronous signal generating circuit, which detects a scale portion of a linear encoder mounted on the carriage and fixed to the apparatus main body, and moves the carriage relative to the apparatus main body. A detection unit 101 of a linear encoder that detects a position is composed of an MR element head or the like that operates based on the magnetoresistance effect, and integrally includes a pair of magnetic detection elements 102 and 103. The detection unit 101 is connected to a substrate 5 mounted on a carriage indicated by a broken line in the figure. Amplifiers 104 and 105 for constituting a constant current circuit, an amplifier 106 for amplifying a detected signal, and a comparator 107 are connected to the substrate 5 in a known manner.
Is output. The comparator 107 is connected to a variable resistor 201 for determining a reference voltage.
It is mounted on a substrate 5 and is adjusted on a carriage.

The operation of the above circuit configuration will be described. The magnetic sensing elements 102 and 103 have constant current circuits 104 and 1 respectively.
A constant current is supplied through the switch 05. Detection unit 101
Moves with respect to the scale portion of the linear encoder fixed to the apparatus main body, on which the magnetic pattern is recorded in advance at regular intervals, the resistance values of the magnetic detection elements 102 and 103 change, and the amount of change in the resistance value Is detected as a voltage change and amplified by the amplifier 106, and the amplified signal is input to one input terminal of the comparator 107. In the comparator 107, the amplified signal is
It is compared with a reference voltage set in advance by the adjustment of the variable resistor 201 and input to the other input terminal of the comparator 107, and an output signal 303 is obtained as a synchronization signal.

Further, depending on the detection device and the circuit system, the temperature dependency is high, which may adversely affect the printing / recording result. More specifically, based on the drawings, FIG. 14A is a diagram illustrating a relationship between a signal input to the comparator 107 and a reference voltage, and FIG.
FIG. 6 is a pulse waveform diagram showing a relationship between (a) and an output signal 303 of a comparator, and FIG.
Numeral 01 indicates a waveform close to a sine waveform that changes at a constant period as shown in the figure.

On the other hand, the pulse-like output signal 303 of the comparator is obtained by using the reference voltage as a threshold value. As a result, as can be seen from the figure, the difference between the input signal 301 and the reference voltage 302 causes the duty fluctuation of the output signal. It appears as If the recording and printing operation is performed in synchronization with the output signal 303, a shift in ruled lines and density unevenness in an output image occur, and the recording quality is significantly reduced.

FIG. 15 is an explanatory diagram of the recording operation, and shows a state in which the driving of the recording means is executed on the recording medium in synchronization with the output signal 303 to record the dot D. As shown in the figure, the fluctuation in the pitch P between the dots D causes unevenness in the density of the printing result, which causes a color registration deviation especially in a color printer.

[0010]

As described above, in the conventional apparatus, since the recording and printing operation is performed in synchronization with the output signal, the duty fluctuation of the output signal pulse waveform is directly reduced in the quality of the printing result. Cause.

Further, in the conventional device, since the means for suppressing the duty fluctuation of the output signal pulse waveform depends on the stability of the circuit element itself, expensive parts must be used, and the cost increases. was there.

Accordingly, it is an object of the present invention to provide a serial printing apparatus capable of suppressing a duty fluctuation.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a recording apparatus which performs reciprocating motion on an apparatus main body and performs recording by a recording means while synchronizing the movement of a carriage on which the recording means is mounted. In the serial printer device to be performed, a scale section of a linear encoder provided on the apparatus main body, a detection section of a linear encoder mounted on the carriage to detect the position of the scale section,
Synchronizing signal generating circuit means for receiving a signal output from the detecting section as an input signal and comparing with a reference voltage to generate an output signal having a pulse waveform as a synchronizing signal; When the duty of the pulse waveform fluctuates, the output signal is processed to control the synchronization signal generating circuit means to suppress the duty fluctuation. The duty fluctuation suppressing means comprises: A differentiating circuit for detecting switching of the signal level, a counter for counting a time between the switching of the level of the output signal and calculating a high level width and a low level width of the pulse of the output signal, A buffer for temporarily storing count values, a memory for storing at least a table of control algorithms and threshold values, It is to employ a serial printer apparatus characterized by having a MPU to control the counter and the memory based on the control algorithm.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view of a main part of a serial printer device shown together with a recording medium. In FIG. 1, a carriage 1 indicated by a dashed line has a recording portion 1h of an ink jet recording system or the like mounted thereon, and is guided by a guide shaft 11 having a spiral groove formed on an outer peripheral surface, and locked by rotation of the guide shaft 11. The recording sheet (recording medium) is driven along a spiral groove to reciprocate the recording sheet 13 wound on the outer peripheral surface of the platen 12 in a direction indicated by an arrow in FIG. A so-called serial printer which forms images and characters by recording dots D at a pitch P on the device 13 is constructed.

An encoder for obtaining a synchronization signal is built in the carriage 1 constructed as described above. This encoder is a magnetic linear encoder, which records a magnetic pattern on a magnetic material formed on the surface of a wire at a print pitch density corresponding to, for example, 180 dots / inch (dpi) or 360 dpi, or a scale portion of a linear encoder. 501 is fixed to the apparatus main body 100, while a magnetic head main body 502 composed of an MR element or the like is fixed inside the carriage 1, and enables position detection by movement of the carriage 1.

A flexible printed circuit board 503 for extracting an output signal from an MR element in the magnetic head to the outside is connected to the magnetic head body 502, and a contact portion 504 is connected to a connector (not shown). In this way, it is connected to the substrate 5 mounted on the carriage 1 and shown by the broken line in the figure.

(First Embodiment) Next, FIG. 2 shows a basic block diagram of a first embodiment of the present invention. In the figure, reference numeral 215 indicates a duty fluctuation suppressing means of the present invention, and this duty fluctuation suppressing means includes a duty observing means 215a, a detecting means 215b, and a control means 215c. The control output from the control unit 215c of the duty fluctuation suppressing unit is transferred to the control input of the reference voltage 302 of the DC voltage source 211. Thereby, the detection unit 101
Further, the reference voltage 302 as an output generation threshold for the input signal 301 input to the comparison unit 107 via the amplification unit 106 becomes stable with respect to the duty of the output signal.

FIGS. 3 and 4 show circuit configuration examples based on the block diagram (FIG. 2). In FIG. 3, a detection unit 101 includes magnetic detection elements (MR elements) 102 and 103. By scanning the magnetized portion of the scale portion of the linear encoder by the magnetic detection elements 102 and 103, a change in magnetic resistance in the magnetic detection element is detected by the circuit configuration of the substrate 5. In the circuit of the substrate 5, the constant current sources 104 and 105 are used for detecting a negative signal by the MR element.
Appropriate bias is given to the lever as a whole. The result of scanning the magnetization characteristic of the scale part of the linear encoder by the detection unit 101 is close to a sine wave,
The signal is transmitted to the amplifier 212. The sine wave is converted into a pulse output using the reference voltage 302 as a threshold. A comparator for this purpose is a comparator 107, which outputs an output signal 303. The pulse duty of the output signal 303 is compared and examined,
An input data signal 110 is generated by a duty fluctuation suppressing means (described later) configured in the control circuit board 4 and transferred to the D / A (digital / analog) converter 109 so that the duty ratio becomes 50%. . The D / A converter 109 outputs the input data signal 1 which is a digital value.
10 is converted into a control voltage signal 111 of an analog value, is input to a DC voltage source formed by transistors Q _111, Q _112. The DC voltage source 211 generates a reference voltage 302 having an appropriate voltage value based on the control voltage signal 111, and
7 to one input terminal.

FIG. 4 is a circuit block diagram of the above-described duty fluctuation suppressing means. In the figure, reference numeral 705 denotes a differentiating circuit which receives an output signal 303 from the converter 107 and detects switching of the level of the output signal 303, 701 denotes an MPU for controlling the operation of each functional element, and 702 Indicates a counter for measuring the width (in other words, duty) of the output pulse, and 703 indicates a counter 7
Reference numeral 704 denotes a buffer for temporarily storing the count value of 02, and 704 denotes a memory for storing a control algorithm, a table of threshold values, and the like.

FIG. 5 shows an operation flowchart of the observation means of the duty fluctuation suppression means. First, when the level of the output signal 303 switches, the differentiating circuit 705
A trigger output is issued to the MPU 701 (step S
1). After receiving the trigger output, the MPU 701 resets the counter 702 (step S2), and then starts the counting operation of the counter 702 to measure the pulse width of the output signal (step S3). After a period of time,
The level of the output signal 303 is inverted. At the time of level inversion, the differentiating circuit 705 outputs a trigger output (step S
4). At this time, the MPU 701 outputs the count value of the counter 702 to the buffer 703 (Step S5). At this time, the MPU recognizes that the data of the count value has been input to the buffer 703. Hereinafter, step S
Returning to step 2, the same steps described above are sequentially repeated to observe the duty of the output pulse.

FIG. 6 is a flowchart showing the operation of the detecting means of the duty fluctuation suppressing means. First, the contents of the buffer 703 are cleared when the system is set (step S11). Next, the output signal 303 accompanying the printing operation
Differentiating circuit 705 at the rise (or fall) of
Output a trigger (step S12). The MPU 701 detects the trigger, and transfers the count value of the counter 702 to the buffer 703 (Step S13). MPU
701 detects that the value of the buffer 703 has been updated, and fetches the data from the buffer 703 to detect the duty in the MPU 701 (step S1).
4). In these data transfer controls, the MPU
If 701 is always open to the same control, data may be directly transferred from a counter to a register in the MPU without using the buffer 703. Next, M
The PU 701 completes the measurement of one cycle of the pulse, and two values to be compared for calculating the duty, that is, the width on the high level side (Th in FIG. 8) and the width on the low level side (Th in FIG. 8) (Tl) is detected (step S15). If not, the process returns to step S12. If yes, the process proceeds to the next step, S16. Then, when the two values to be compared are obtained, the two values are subtracted from the MPU 701 and the widths are compared (step S1).
6).

FIG. 7 shows an operation flowchart of the control means of the duty fluctuation suppressing means. For the comparison value obtained through the above-mentioned observation means and detection means, MP
The U 701 accesses the memory 704 (step S2).
1), refer to the threshold value (step S22). The width of Th and Tl of the signal 303 in FIG. 8 is fetched by the MPU 701 as count value data, and compared by the MPU 701 as a magnitude relationship. If the duty ratio fluctuates and Th> Tl, the reference voltage 302 is lowered. Then Th
If <Tl, it is necessary to increase the reference voltage. That is, the duty ratio converges to 50% (that is, Th = T
1). The magnitude relationship between Th and Tl is compared with a threshold value determined by the system, and it is determined whether the data 110 needs to be corrected (step S23). As a result, when the necessity of the correction is recognized, the optimum reference voltage control table is extracted from the memory 704 (step S24) and output to the D / A converter 109 as the correction value data 110 (step S26). On the other hand, if the threshold value is within the allowable range and there is no need for correction, the correction value data 110 maintains the current value (step S25).

According to the above control, the output signal 303 has substantially equal pulse widths Th and Tl (ie, a duty ratio of 50) as shown in the pulse 401 of the output voltage waveform in FIG.
%), And the pitch P of the print output (dots) D becomes uniform as shown in FIG.

(Second Embodiment) In the first embodiment, the pulse duty ratio of the output signal 303 is observed using a system clock and a counter, and "H (high)" and "L (low)" of the output pulse are output. Is measured, but the duty ratio is set to 5
From the viewpoint of keeping the duty fluctuation at 0%,
In the second embodiment, a comparison between the pulse widths of the high level width and the low level width is grasped as a power ratio, and the duty observation unit is formed of discrete components.

FIG. 9 is a circuit block diagram of the duty fluctuation suppressing means according to the second embodiment. In the figure, reference numeral 801 denotes the pulse widths Th and T of the output signal 303.
1 shows a power integrator for storing power proportional to the length of each of the power integrators 802a and 802b, respectively.
01 indicates a low-pass filter for extracting each charging power as a DC component, 803a and 803b indicate voltage-controlled oscillators that change the oscillation frequency with respect to output voltages from the low-pass filters 802a and 802b, respectively, and 805 indicates a voltage. A phase comparator which compares phases of output frequencies of the control oscillators 803a and 803b and outputs a phase variation in the form of a voltage change is shown. With the above configuration, the duty fluctuation of the output signal 303 is output as the voltage fluctuation of the phase comparator 805. That is, “T” of the output signal 303
The pulse widths of “h” and “Tl” are observed as a power product, and when a change in duty is recognized, the change is detected as a change in output voltage. The reference voltage 302 is adjusted by the voltage control constant voltage source 806 as a control signal of the voltage control constant voltage source 806 by matching the voltage with the next stage control signal voltage.

FIG. 10 is a circuit diagram from the power integrator 801 to the phase comparator 805 of the duty fluctuation suppressing means of FIG. Hereinafter, the configuration and operation of this circuit will be described. Due to the configuration including the transistors Q1 to Q3 and the capacitors C1 and C2, electric charge is accumulated in the capacitor C1 during the period when the output pulse 303 is high (that is, during the period of Th), and during the period when the output pulse 303 is low (that is, during the period of Th). , Tl), a charge is stored in the capacitor C2. The charge stored in the capacitors C1 and C2 is equal to the capacitance of the capacitor C11, the resistors R12 and R13, and the resistors C12 and R1.
1, R14) low-pass filter LPF802
a, 802b, and is transmitted as a DC potential to VCOs (voltage controlled oscillators) 803a, 803b. In this embodiment, the VCO 803a includes a transistor Q22,
It comprises a constant current source composed of Q23, Q27, and Q28, and a Schmitt trigger circuit composed of transistors Q41, Q42, Q43, and Q44.
803b includes transistors Q20, Q21, Q24, Q
25, and transistors Q31 and Q3
2, a Schmitt trigger circuit composed of Q33 and Q34. The two outputs from these VOCs are coupled to a phase comparator 80 comprising transistors Q51-Q58.
5 is input. The output from the phase comparator 805 is R
The signal is transferred to a transmission filter 804 (see FIG. 9, not shown in FIG. 10) through an LPF composed of R61, R62, and C61, and is then transferred to a voltage controlled constant voltage source 806 (see FIG.
(Not shown in FIG. 10).

FIG. 11 shows a waveform when the duty fluctuation observing means circuit of the duty fluctuation suppressing means is opened when the transmission block of the duty fluctuation suppressing means is opened. When the reference voltage 302 having the pulse generation threshold level fluctuates with respect to the waveform of the input signal 301, the high level width “Th, low level width” of the waveform of the output signal 303 is obtained.
Tl ″ changes. This pulse width information is stored in the power integrator 8
01 and the LPFs 802a and 802b, the potentials Vd1 and Vd2
And are input to the VCOs 803a and 803b, respectively. In this figure, Vd1 corresponds to Tl and Vd2 is Th
It corresponds to. When the reference voltage 302 is not bisected by the peak-to-peak value of the waveform of the input signal 302, a difference occurs between the potentials Vd1 and Vd2. Here, assuming that the characteristics of the VOCs 803a and 803b are equal, a deviation occurs in the oscillation frequencies generated and output by these VCOs, so that a phase difference φ occurs at the input to the phase comparator 805. In this figure, since the control loop is open, no phase tracking is performed. In addition, the output 30 which is quite different from the duty 50% when open
At 3, the output of each VCO will have a completely different frequency, so that the output of the phase comparator 805 will not show a correct value. The control loop is closed and the phase comparator 805 is closed.
Of the output signal 303 by controlling the voltage source 806 so that the output of the
Can be kept. Voltage source 806 is connected to phase comparator 805
In order to accurately control the transfer function, the transfer function of the transfer filter 804 is given an arbitrary function such as linear or non-linear according to the output characteristic of the phase comparator 805.

In the printer print control using the output of the linear encoder as used in the present invention, even when printing is performed during acceleration / deceleration of the carriage, the frequency of the pulse 303 of the output signal is set at the acceleration / deceleration stage. However, if the pulse duty ratios of “Th” and “Tl” do not change, the differential output is taken out at the next stage of the power integrator 801 and input to the VCO, so that the phase comparator 8
05 has no effect.

(Third Embodiment) In the second embodiment, the output signal pulse widths "Th" and "Tl" are used.
, And the duty fluctuation thereof is observed using a VCO and a phase comparator. As shown in FIG. 12, the output voltage of the power integrator 801 is subtracted by a subtractor (voltage comparator) 90.
Even when the reference voltage 302 is controlled so that the difference between them is made to be zero by comparison with 1, the duty of the output pulse 303 can be obtained at 50%.

[0030]

As described above, according to the present invention,
Observe the duty ratio of the print synchronous output signal pulse waveform,
By providing a configuration for controlling the synchronous output generation reference voltage based on the result, a serial printer device capable of performing high-quality printing with a stable synchronous output at any time can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view of a main part of a serial printer of the present invention.

FIG. 2 is a basic block diagram of an embodiment of the present invention.

FIG. 3 is a circuit diagram of a first embodiment of the present invention.

FIG. 4 is a circuit block diagram of a duty fluctuation suppressing unit according to the first embodiment of the present invention.

FIG. 5 is a flowchart of the observation means of the duty fluctuation suppressing means used in the first embodiment of the present invention.

FIG. 6 is a flowchart of a detecting unit of the duty fluctuation suppressing unit used in the first embodiment of the present invention.

FIG. 7 is a flowchart of a control unit of the duty fluctuation suppressing unit used in the first embodiment of the present invention.

FIG. 8 is a diagram for explaining a printing operation after duty control.

FIG. 9 is a circuit block diagram of duty fluctuation suppressing means according to a second embodiment of the present invention.

FIG. 10 is a circuit diagram of a main part of a duty fluctuation suppressing unit according to a second embodiment of the present invention.

FIG. 11 is a waveform diagram at the time of a circuit open loop of the observation means of the duty fluctuation suppression means according to the second embodiment of the present invention.

FIG. 12 is a circuit block diagram of a duty fluctuation suppressing unit according to a third embodiment of the present invention.

FIG. 13 is a diagram of a conventional synchronous signal generation circuit.

FIG. 14 is a waveform diagram for explaining an input signal waveform and an output signal waveform of a conventional synchronous signal generation circuit.

FIG. 15 is a diagram for explaining a recording operation based on a conventional synchronous signal generation circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Carriage 2 Flexible cable 4 Printer control circuit board 5 Board 11 Guide shaft 102, 103 Magnetic detection element 107 Amplifier 108 Synchronous output signal line 501 Linear encoder scale section 211 DC voltage source 215 Duty fluctuation suppression means 301 Comparison section input signal 302 Reference voltage 303 Synchronous output signal 109 D / A converter 110 Control signal 111 Control voltage 701 MPU 702 Counter 705 Differentiator 801 Power integrator 802a, 802b LPF 803a, 803b VCO 804 Transmission filter 805 Phase comparator 806 Voltage control constant voltage source 901 Subtractor 109 Compensation circuit unit 110 Temperature measurement unit

Continuation of the front page (56) References JP-A-63-7961 (JP, A) JP-A-59-43312 (JP, A) JP-A-59-79678 (JP, A) JP-A-4-235313 (JP) , A) JP-A-2-236416 (JP, A) JP-A 63-25539 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) B41J 19/18

Claims (9)

(57) [Claims] 1. A serial printer device which reciprocates on an apparatus main body and performs recording by a recording unit while synchronizing with movement of a carriage on which the recording unit is mounted, wherein a scale of a linear encoder provided on the apparatus main body is provided. And a detection unit of a linear encoder mounted on the carriage for detecting the position of the scale unit. A signal output from the detection unit is received as an input signal, compared with a reference voltage, and used as a synchronization signal. Synchronizing signal generating circuit means for generating an output signal of a pulse waveform; and synchronizing so as to suppress the duty fluctuation by processing the output signal when the duty of the pulse waveform of the output signal from the synchronizing signal generating circuit fluctuates. And a duty fluctuation suppressing means for controlling the signal generation circuit means. A differentiating circuit for detecting the level switching of the output signal, a counter for counting the time between the level switching of the output signal and calculating the high level width and the low level width of the pulse of the output signal, and a counter A serial memory comprising: a buffer for temporarily storing a numerical value; a memory for storing at least a table of a control algorithm and a threshold; and an MPU for controlling the counter and the memory based on the control algorithm. Printer device. 2. The serial printer device according to claim 1, wherein the duty fluctuation suppressing unit counts a high level width and a low level width of the pulse of the output signal based on the control of the MPU; Detecting means for detecting a difference between the count value of the high level width and the count value of the low level width of the pulse obtained by the observation means and outputting a difference value signal; and the reference based on the difference value signal from the detection means. And a control means for controlling a voltage. 3. The serial printer according to claim 2, wherein said control means determines whether or not the difference value of said detection means requires correction of a reference voltage, and said control means determines whether or not the difference value requires correction of the reference voltage. A serial printer device comprising: means for outputting a correction value stored in a corresponding memory; and means for outputting an instruction to maintain a current value when no correction is required. 4. A serial printer device which reciprocates on an apparatus main body and performs recording by a recording means while synchronizing with movement of a carriage on which the recording means is mounted, wherein a scale of a linear encoder provided on the apparatus main body is provided. And a detection unit of a linear encoder mounted on the carriage for detecting the position of the scale unit. A signal output from the detection unit is received as an input signal, compared with a reference voltage, and used as a synchronization signal. Synchronizing signal generating circuit means for generating an output signal of a pulse waveform; and synchronizing so as to suppress the duty fluctuation by processing the output signal when the duty of the pulse waveform of the output signal from the synchronizing signal generating circuit fluctuates. And a duty fluctuation suppressing means for controlling the signal generation circuit means. A power integrator that integrates power during periods of a high level width and a low level width of a pulse, two voltage controlled oscillators that receive respective voltages obtained by the power integrator and oscillate a frequency based on the voltage value; A phase comparator that compares phases of frequencies generated by the two voltage-controlled oscillators and outputs a difference,
A serial printer device that controls the reference voltage based on a difference output from the phase comparator. 5. The serial printer according to claim 4, further comprising a voltage controlled constant voltage source for applying said reference voltage. 6. The serial printer according to claim 5, further comprising a transmission filter between said phase comparator and said voltage controlled constant voltage source. 7. A scale of a linear encoder provided on an apparatus main body in a serial printer apparatus which performs reciprocating motion on the apparatus main body and performs recording by a recording means while synchronizing with movement of a carriage on which the recording means is mounted. And a detection unit of a linear encoder mounted on the carriage for detecting the position of the scale unit. A signal output from the detection unit is received as an input signal, compared with a reference voltage, and used as a synchronization signal. Synchronizing signal generating circuit means for generating an output signal of a pulse waveform; and synchronizing so as to suppress the duty fluctuation by processing the output signal when the duty of the pulse waveform of the output signal from the synchronizing signal generating circuit fluctuates. And a duty fluctuation suppressing means for controlling the signal generation circuit means. A power integrator that integrates the power during the high level width and the low level width of the pulse, and a voltage comparator that compares the respective voltages obtained by the power integrator and outputs a difference, wherein the voltage A serial printer device that controls the reference voltage based on a difference output from a comparator. 8. The serial printer according to claim 7, further comprising a voltage control constant voltage source for applying said reference voltage. 9. The serial printer device according to claim 8, further comprising a transmission filter between said voltage comparator and said voltage controlled constant voltage source.