JP2000028670A - Method for speeding up burn-out-type disconnection detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the speed of a burn-out response by adding a burn-out response acceleration circuit to a circuit, forming the sample-hold circuit configuration of an analog signal, and separating a sample-hold capacitor from an input circuit during a sample-hold period. SOLUTION: An analog input device 1 successively selects a plurality of analog input signals by a multiplexer 11, amplifies the signal by an operational amplifier 12, and then converts the signal into a digital data 16 by an A/D converter 13. The data 16 is outputted to an upper-level computer via a bus interface circuit 19. A hold circuit 3 sets a sampling state where a sample signal 17 and a switch element 5 are sampled, and periodically turns off the sample signal 17 and the switch element 5 for holding a sample. The capacity of a bypass capacitor CP6 is set to a value smaller than the capacity of a hold capacitor Ch4, and the capacitor Ch4 with a large capacity is separated from an input circuit during a sample-hold period, thus an input voltage 18 is rapidly increased, when disconnection occurs and a response is accelerated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting disconnection of an analog input signal line which is effective in a plant control apparatus using a computer, particularly in a system requiring high reliability.

[0002]

2. Description of the Related Art It is generally known that a burnout type disconnection detecting circuit applies a constant voltage to an input circuit via a high resistance and scales out A / D conversion data when a disconnection occurs. The reason why a high resistance is used as the voltage application resistor is that an error occurs due to the voltage division between the input signal source impedance and the voltage application resistor. In an actual circuit, a filter circuit is usually required as a countermeasure against noise in the input circuit, so the disconnection detection response according to the conventional technology is determined by the time constant of the burnout circuit by using a high-resistance voltage application resistor and the input filter circuit capacitor. And it takes several seconds to several minutes before the A / D conversion data scales out from the disconnection.

[0003]

## EQU1 ## Time constant of burnout circuit = capacitance of input filter (μF) × resistance for voltage application (MΩ)
Although the example described in Japanese Patent No. 40825 is considered to be a technology that has been widely and widely practiced for a long time in temperature measurement using a thermocouple, responsiveness is not considered.

In the example described in Japanese Patent Application Laid-Open No. 5-48153, a method of burning out with an AC signal is different from the present method. Although the responsiveness is not specified, it is not practical because there is a danger of a burnout malfunction due to external noise because the circuit is configured to take in an AC signal as disconnection information.

[0005]

In the prior art, since the burnout circuit time constant is large, the disconnection detection time (the time from the disconnection until the A / D conversion data is scaled out) is long. . If the burnout circuit time constant is reduced by reducing the resistance value of the burnout voltage application resistor, the disconnection detection response will be faster, but there will be a problem that an error occurs in the analog input signal due to the influence of the signal source impedance. The problem is to speed up the disconnection detection response without deteriorating the disconnection.

[0006]

SUMMARY OF THE INVENTION A burnout response acceleration circuit is added to a conventional burnout type disconnection detection circuit. The burnout response acceleration circuit has a sample-and-hold circuit configuration of an analog signal. During the sample-and-hold period, the burnout response is accelerated by disconnecting the sample-and-hold capacitor from the input circuit.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a circuit configuration of an analog input device 1 for increasing the speed of a burnout type disconnection detecting circuit.

The analog input device 1 sequentially selects a plurality of analog input signals by a multiplexer 11, amplifies the signals by an operational amplifier 12, and then converts the digital data 1 by an A / D converter 13.
Convert to 6. The digital data 16 is output to the host computer via the bus interface circuit 19.

The input circuit is a burnout voltage supply circuit 8,
The burnout response acceleration circuit 2 includes an input resistance (Rin) 9 and a burnout response acceleration circuit 2. The burnout response acceleration circuit 2 includes a sample and hold circuit 3, a bypass capacitor (Cp) 6, and a control circuit 10.

The sample-and-hold circuit 3 normally sets the sampling state in which the sample signal 17 and the switch element 5 are ON, and periodically sets the sample signal 17 and the switch element 5 to OFF to set the sample-hold state.
The capacity of the bypass capacitor (Cp) 6 is set to a value sufficiently smaller than the capacity of the sample-hold capacitor (Ch) 4. During the sample-hold period, the large-capacity sample-hold capacitor (Ch) 4 is disconnected from the input circuit. Input voltage (Vin) 1
8 rises sharply to accelerate the burnout response.

FIGS. 2A and 2B show a comparison between a conventional circuit and the present invention.

In the conventional burnout circuit, a burnout voltage Vb is supplied from a burnout voltage supply circuit via a high resistance.
(A voltage at which the A / D conversion data is scaled out to the input circuit) is applied, and when the analog signal line is connected, the impedance of the input signal source 20 is low. There is no influence of the burnout voltage Vb, and when a disconnection occurs, the input signal source impedance becomes infinite, and the burnout voltage Vb is directly applied to the input voltage (Vin) 18 to scale out the A / D conversion data. Generally known.

Here, a resistor 7 for applying a burnout voltage is used.
The reason why a high resistance is used is that when an analog signal line is connected, the voltage generated as a result of dividing the burnout voltage by the input signal source impedance and the voltage application resistor 7 becomes the input voltage (Vin) 18. This is for minimizing the influence of the error, and usually requires a resistance value of several MΩ. For this reason, the time constant of the burnout circuit becomes large, and there is a problem that it takes several seconds to several minutes for the disconnection detection time.

In the present invention, the above problem is solved by adding a disconnection detection response acceleration circuit 2 to the above circuit. In the sample hold circuit 3 of the disconnection detection response acceleration circuit 2, the sample hold capacitor (Ch) 4 having a large capacity is disconnected from the input circuit by periodically setting the sample hold state. In this case, the input voltage (Vin) 18 rapidly rises, and the burnout response is accelerated.

The conventional burnout circuit time constant Tbo is

[0017]

Tbo = 2Rb × 2Rin × Ch In contrast, the burnout circuit time constant T according to the present invention is
bn

[0018]

## EQU3 ## Tbn = 2Rb.times.2Rin.times.Cp. Since Ch (.mu.F order) .fwdarw.Cp (pF order), Tbo.noteq.Tbn.

FIGS. 3A and 3B show a comparison between the timing charts of the prior art and the present invention.

The analog input device has a point address 1
4 while switching, a plurality of analog input signals are sequentially selected, and A / D conversion is performed by the A / D conversion signal 15. The A / D conversion data 16 has a timing to be taken into the internal circuit of the bus interface circuit every time.
In the prior art, since the burnout circuit time constant is large, when a disconnection occurs, the input voltage (Vin) 18 slowly rises toward the burnout voltage Vb, reaches the burnout voltage Vb, and A / D A / D conversion is performed many times before the conversion data 16 is scaled out. Therefore, the disconnection detection time td is delayed. During this time, since the system cannot detect an abnormality, there is a possibility that erroneous A / D conversion data 16 is taken in as control information. In the present invention, a sample / hold cycle of an analog input signal is periodically set to A / D with respect to the prior art.
Insertion between conversions accelerates the burnout response.

The period in which the sample signal 17 is OFF is a sampling period, and the analog input signal is charged in the sample and hold capacitor (Ch). The period in which the sample signal 17 is ON is a sample hold period, and the analog input signal is temporarily held in the sample hold capacitor (Ch) disconnected from the input circuit. In this state, since the capacitance component of the input circuit is only a bypass capacitor of the order of pF, even if the resistance value of the burnout voltage application resistor is increased, the burnout time constant is suppressed and the disconnection detection time td Can be speeded up.

Before the A / D conversion signal 15 is output, a sample hold cycle is always inserted. If a disconnection occurs, the input voltage (Vin) is applied during the sample hold period.
The length of the sample hold period is set so that 18 reaches the burnout voltage (several times the burnout time constant). Further, by adjusting the ratio of the sample hold period to the insertion period, it is possible to set the influence on the response of the analog input signal within a range where there is no problem in practical use (at a ratio of 1/10, the effect value is about 10%).

[0023]

According to the present invention, it is possible to speed up the disconnection detection time of the burnout type disconnection detection circuit without deteriorating the accuracy of taking in the analog input signal, and to improve the reliability of the system.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an analog input device according to an embodiment of the present invention.

FIGS. 2A and 2B are burnout voltage supply circuit diagrams comparing a conventional technique and the present invention.

FIGS. 3A and 3B are diagrams comparing a conventional technology and a time chart of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Analog input device, 2 ... Burnout response acceleration circuit, 3 ... Sample hold circuit, 4 ... Sample hold capacitor, 5 ... Switch element, 6 ... Bypass capacitor, 7 ... Burnout voltage application resistance, 8 ... Burnout voltage Supply circuit, 9: input resistance, 10: control circuit, 11
... Mux, 12 ... Op amp, 13 ... A / D converter, 14 ... Point address, 15 ... A / D conversion signal, 16 ... A / D conversion data, 17 ... Sample signal, 1
8: input voltage Vin, 19: bus interface circuit, 2
0: Input signal source.

Continued on the front page (72) Inventor Hideo Otani 5-2-1 Omikacho, Hitachi City, Ibaraki Prefecture Inside Hitachi Process Computer Engineering Co., Ltd. (72) Inventor Shuichi Nagayama 5-2-1 Omikacho, Hitachi City, Ibaraki Prefecture Hitachi Process Computer Engineering Co., Ltd. (72) Inventor Toshiro Genmoto 5-2-1 Omikacho, Hitachi City, Ibaraki Prefecture Hitachi Process Computer Engineering Co., Ltd. (72) Inventor Yofumi Kurisu Omikacho, Hitachi City, Ibaraki Prefecture 5-2-1 F-term in Hitachi, Ltd. Omika Plant F-term (reference) 2G014 AA02 AB34 AC18 5B083 BB01 EE16

Claims (2)

[Claims] A burnout type disconnection detection circuit for scaling out A / D conversion data of an analog input device, the burnout type disconnection detection circuit including a burnout response acceleration circuit for the purpose of shortening the disconnection detection time. How to speed up. 2. A burnout response accelerating circuit according to claim 1, wherein a sample and hold circuit for an analog signal is formed, and a capacitor of an input filter circuit is disconnected during a sample and hold period, so that a burnout response at the time of disconnection occurs. A method for increasing the speed of a burnout type disconnection detection circuit characterized by increasing the speed.