JPS63818B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Technical field to which the invention pertains This invention relates to a printed wiring board having a power supply unit and a signal supply unit, on which a microcomputer system or the like that does not have an independent power supply unit is mounted. The present invention relates to a circuit for protecting signal input elements of the other printed wiring board in a device for connecting the other printed wiring board with a connector.

(b) Background of the Invention In general electronic devices that require miniaturization and portability, a data processing section, a power supply section, and a signal interface section are mounted on separate printed wiring boards, and they can be connected as needed. I am trying to connect the connector.

In a device configured in this way, normally a first printed wiring board on which a power supply section and a signal interface section are mounted, and a second printed wiring board on which a data processing section is mounted.
After connecting the second printed wiring board with the connector, turn on the power, and turn off the power before removing the second printed wiring board. This is done in order to prevent a large reverse bias voltage from being applied to the signal input element of the second printed wiring board during the connector connection stage.

FIG. 1 is a diagram illustrating a phenomenon when both boards are not connected according to the above procedure. In the figure, the left side of the connector 1 is a power supply/signal interface section, and the right side is a data processing section. MOS transistor TR, which is a signal input element in the data processing section
1 has a protective diode D connected to the gate terminal. Normally, the MOS transistor TR1 and the diode D are constituted by an IC together with other elements (not shown). Further, the signal output element of the power supply/signal interface section is composed of an open collector transistor TR2.

A connector 1 that connects both boards includes power terminal sections 1a and 1b at both ends, and a signal terminal section 1i between them. The data processing section receives voltage supply through the power terminal sections 1a and 1b, and receives a signal through the signal terminal section 1i. Note that the power supply/signal interface section is provided with a power switch (not shown).

In the above configuration, the power switch is turned on before the connector 1 is connected, and the connector 1 is not connected normally, and at least the positive power terminal part 1a is connected and the negative power terminal part 1 is connected.
When a state in which b is not connected occurs, the current shown in the figure flows. That is, the power supply i flowing in from the positive power supply terminal section 1a passes through the IC and other circuit sections, and flows through the diode D, the signal terminal section 1i, and the output transistor TR. As a result, a forward voltage drop is generated across the diode D, and this voltage becomes a reverse bias and is applied to the gate of the MOS transistor TR1. Normally, the gate breakdown voltage (reverse bias breakdown voltage) of MOS transistor TR1 is
It is about 0.3V. Therefore, when current flows in as described above, the forward voltage drop of the diode D easily destroys the transistor TR1.

In this way, if you connect the connector with the power switch on on the power/signal interface side, even if the above condition occurs only momentarily, the signal input element of the data processing section can easily be destroyed. become.

(c) Purpose of the Invention The purpose of the present invention is to ensure that the signal input elements of the data processing section are An object of the present invention is to provide an input element protection circuit for a printed wiring board that is completely protected from breakdown voltage generated in an abnormal route.

(d) Structure and Effects of the Invention This invention provides that when a signal input element is destroyed when a connector is connected, that is, when a forward voltage drop occurs across the input protection diode, the voltage between the power supply terminals of the data processing unit is equal to or lower than the power supply voltage. This was done with a focus on the fact that they are becoming smaller.

This invention provides a printed wiring board on which a data processing section is formed, a voltage detection means for detecting a voltage between power terminals of the board, and a printed wiring board on which a power supply/signal interface section is formed. When the output voltage of the power supply terminal voltage detection means obtained through the connector is below a certain value, and when the connector is in a disconnected state, the signal output element of the power supply/signal interface section is forced to operate. A signal output element control means is provided which automatically turns off the signal output element when the connector is in an unconnected state.
And when the voltage between the power supply terminals of the data processing section is below a certain value, the signal output element is turned off to prevent forward current from flowing into the input protection diode.

According to this invention, even if the power switch on the power/signal interface side is turned on, if the connector is not completely connected, that is, the positive power terminal section and the negative power terminal section are connected to each other between both boards. If the connection is not complete, the signal output element is forced off. Therefore, even if the power switch is on and the connector is not connected properly, the signal input element is completely protected from excessive voltage, and the power supply/signal interface side This has the advantage that there is no need for a power switch, and there is no need to pay attention to board connections.

(e) Description of Embodiments FIG. 2 is a block diagram of an on-vehicle device for a bus equipped with an input element protection circuit according to an embodiment of the present invention, and FIG. 3 is a diagram showing a case configuration of the same device.

The above on-board device for buses uses a stabilized voltage of 24V supplied from an on-board battery (not shown).
A power supply/interface section 2 includes a power supply section 20 that converts to 5V, and a sensor interface section 21 that performs output processing of various sensors that detect boarding, alighting, and mileage at each bus stop, and the sensor interface section The data collector section 3 collects data from 21 and outputs necessary results. The data collector section 3 includes an I/O interface 30 that receives data from the sensor interface section 21, a microprocessor 31, and a battery backed up RAM 3.
2. The I/O interface 30 is composed of a microcomputer system including a ROM 33 for storing programs, and includes a voltage detection circuit between power supply terminals which will be described later. In addition, the sensor interface section 21
includes a signal output element control circuit that controls on/off of the signal output element based on a control signal from the power supply terminal voltage detection circuit. The data collector section 3 receives power supply voltage from the power supply terminal sections 1a and 1b of the connector 1, and receives signal data from the signal terminal sections 1c to 1h. Further, an output element control signal is sent from the voltage detection circuit between power supply terminals in the I/O interface 30 to the sensor interface section 21 via the output control terminal section 1m. In addition, the third
In the figure, the data collector section 3 is housed in a metal case 3a to which a handle 3b is attached.
Further, the power supply/interface section 2 is housed in a metal case 2a in which a data collector section 3 can be set.

Next, the power supply terminal voltage detection circuit in the I/O interface 30 and the signal output element control circuit in the sensor interface section 21 will be explained with reference to FIGS. 4 and 5.

FIG. 4 is a diagram for explaining the operation of the power supply terminal voltage detection circuit and the signal output element control circuit, and shows a case where the negative power supply terminal portion 1b of the connector 1 is in a non-contact state. Further, FIG. 5 is a specific circuit diagram of a voltage detection circuit between power supply terminals.

In FIG. 4, a power supply terminal voltage detection circuit 4 (hereinafter referred to as VCC' detection circuit) that detects the voltage between power supply terminals 1a and 1b of the data collector section has an input section connected to the power supply terminal section 1a (VCC' line). , 1b (earth line), and the output part is a transistor.
Connected to the base of TR3. As shown in FIG.
3, and a Zener diode ZD. As is clear from the above configuration, the comparator 4
The output of 0 becomes "1" when the following condition of equation (1) is satisfied.

V1 (Zener voltage) ≦R3・VCC′/(R2+R3)
...(1) Then, when the output of the comparator 40 is "0", the transistor TR3 is turned off and the terminal portion 1m of the connector 1 is made high. Further, when the output of the comparator 40 is "1", the transistor TR3 is turned on and the terminal portion 1m of the connector 1 is set to low. In this embodiment, the Zener voltage at which the comparator 40 becomes "1" is set to a value at which VCC'=4.5V. Therefore, when VCC' drops below 4.5V, the terminal 1m of the connector 1 is set high, and a signal is sent to the signal output element control circuit, which will be described below, to forcibly turn off the signal output element.

The transistor TR2, which is provided on the power supply/interface side and is a signal output element, is controlled by a signal output element control circuit 5. The signal output element control circuit 5 inputs the control signal from the terminal section 1m of the connector 1 and the signal from the sensor, takes the NOR condition (but positive logic), and outputs the above-mentioned transistor.
It is composed of a NOR gate 50 that outputs to TR2, and a pull-up resistor R4 that pulls up the control signal input side from the terminal section 1m to VCC.

Next, the operation when attempting to connect the connector 1 with the above configuration will be explained.

Before connecting the connector 1, the terminal portion 1m is set to high (logic 1) by the pull-up resistor R4. Therefore, the signal output element transistor TR2 is not turned on. Connector 1 is connected, and terminal parts 1a, 1i and 1m are connected to data collector part 3.
When the power supply/interface section 2 and the power source/interface section 2 are connected, a current will flow through the diode D as shown in FIG. However, when current flows at this time, the voltage VCC′ increases as mentioned above.
Since it becomes lower than VCC, the voltage VCC′ becomes
It will be checked by the VCC' detection circuit 4. If the voltage VCC' is less than 4.5V, the transistor TR3 does not turn on but remains off, keeping the NOR gate 50 closed.
Therefore, as long as there is a possibility that a voltage dangerous enough to destroy the transistor TR1 is generated across the diode D, the signal output element control circuit 5 prohibits the conduction of the transistor TR2. On the other hand, if the voltage VCC' exceeds 4.5V, the VCC' detection circuit 4 turns on the transistor TR3. When the transistor TR3 is turned on, the open condition of the NOR gate 50 is eliminated, and the transistor TR2 is turned on and off in response to a signal from the sensor. In this way, excessive voltage is prevented from being applied to the gate of transistor TR1. It should be noted that the destructive voltage applied to the gate of the transistor TR1 is approximately 0.3V or more, but when the current i flows, there is a voltage drop of about 0.2V between the collector and emitter of the transistor TR2. Therefore, the voltage that switches transistor TR3 from on to off
If VCC is 5V, the value of VCC' needs to be 4.5V or higher as mentioned above.

FIG. 6 is a diagram showing the operating state of the transistor TR3 when the power is turned on and off with the connector 1 connected. In the figure, points p and q are when the transistor TR3 is inverted, and this timing is when VCC' reaches 4.5V.

In the above embodiment, a NOR gate was used as the signal output element control circuit, but if a 3-state buffer is used as the signal output element instead of the transistor TR2, its control terminal should be directly connected to the terminal section 1m of the connector 1. This allows the 3-state buffer itself to be used also as a signal output element control circuit. FIG. 7 shows the circuit diagram. If this 3-state buffer is used, the circuit configuration will be further simplified.

As described above, the VCC′ detection circuit detects the voltage between the power supply terminals on the data collector side, and by determining whether the voltage is at a level of 4.5V or less when the connector is connected, the signal output on the power supply/interface side I am trying to control the elements. Therefore, when the signal output element turns on, it is limited to when the voltage between the power supply terminals on the data collector side is at a level that exceeds 4.5V, so it is necessary to turn on the power switch on the power supply/interface side. Even if the connector is left in place and the connector is improperly connected, the transistor TR1 is completely protected.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating an operating state when a signal input element is destroyed. FIG. 2 is a block diagram of an on-vehicle device for a bus equipped with an input element protection circuit according to an embodiment of the present invention, and FIG. 3 is a diagram showing a case configuration of the same device. FIG. 4 is a diagram illustrating the operation of the power supply terminal voltage detection circuit and the signal output element control circuit, and shows the case where the negative power supply terminal portion 1b of the connector 1 is in a non-contact state. FIG. 5 is a specific circuit diagram of a voltage detection circuit between power supply terminals. FIG. 6 is a diagram showing the operating state of the transistor TR3 when the power is turned on and off with the connector 1 connected. FIG. 7 is a circuit diagram of another example of the signal output element control circuit. 1... Connector, 2... Power supply/interface section (first printed wiring board), 3... Data collector section (second printed wiring board), 4...
Voltage detection circuit between power supply terminals, 5...Signal output element control circuit.

Claims (1)

[Claims] 1. A MOS transistor with an input protection diode connected to a first printed wiring board having an open collector element or a 3-state buffer as a signal output element and a power supply terminal in the connector part is used as a signal input element, and a power supply input terminal In the apparatus for connecting a second printed wiring board with a connector, the second printed wiring board is provided with a power terminal voltage detecting means for detecting a voltage between the power terminals of the second printed wiring board; When the output voltage of the power supply terminal voltage detection means provided on the printed wiring board 1 and obtained through the connector is below a certain value, and when the connector is in a disconnected state, the signal output element An input element protection circuit for a printed wiring board, comprising signal output element control means for forcibly turning off the input element.