Transmission system

A transmission system transmits a signal from a transmission terminal to a reception terminal via a pair of transmission lines. The transmission terminal side of the transmission lines is connected to a first resistor having a predetermined resistance value depending on the characteristic impedance of the transmission lines.

TECHNICAL FIELD

The present invention relates to a transmission system used for signal transmission for disaster prevention monitoring such as fire monitoring.

Priority is claimed on Japanese Patent Application No. 2008-182347 filed Jul. 14, 2008, the contents of which are incorporated herein by reference.

BACKGROUND ART

Conventionally, a disaster prevention monitoring system that monitors for abnormalities such as fire or gas leakage by connecting a sensor such as a fire detector or gas detector to a transmission line of a receiver has been put to practical use. In this disaster prevention monitoring system, a digital signal serving as a downward telegraphic message of for example, control information is transmitted from the receiver to a terminal (sensor) in a voltage mode. On the other hand, a digital signal serving as an upward telegraphic message of, for example, sensor information is transmitted from the terminal to the receiver in the current mode.

FIG. 11shows a transmission system100used for conventional disaster prevention monitoring (for example, refer to Patent Document 1).

A pair of transmission lines112aand112balso functioning as power supply lines are led out from a transmission output circuit116provided in a receiver110toward a monitoring terminal side. For example, sensors114(114a,114b,114c) having a digital transmission function are used as a plurality of monitoring terminals, and these sensors114are connected to the pair of transmission lines112aand112b.

Incidentally, in the transmission system shown inFIG. 11, if a digital signal comprising a rectangular pulse train is transmitted from the transmission output circuit116to the sensors114without performing impedance matching, the signal is reflected and travels back and forth between the transmission ends and the last ends of the pair of transmission lines112aand112b, that is, between the transmission output circuit116and the sensor114c. As a result, as shown in the waveform inFIG. 12, ringing occurs in the digital signal at the terminal end side of the pair of transmission lines112aand112b. Therefore, transmission and reception of the digital signal cannot be performed normally between the receiver110and the sensors114.

Conventionally as a countermeasure against this problem, as shown inFIG. 13, a resistor R3having a resistance value equal to a characteristic impedance Z of the pair of transmission lines112aand112bis inserted into the sensor114cconnected to the terminal ends of the pair of transmission lines112aand112b, to perform impedance matching. As a result, as shown inFIG. 14, waveform ringing in the digital signal can be suppressed on the terminal end side of the pair of transmission lines112aand112b.[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H09-91576

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

However, in the conventional transmission system for disaster prevention monitoring, to achieve impedance matching, an operation for checking the sensor114cconnected to the terminal ends of the pair of transmission lines112aand112bto insert the resistor R3into the sensor114cis complicated. Moreover, if the resistor R3is inserted by error into the sensor114aor114bother than the sensor114cconnected to the terminal ends of the pair of transmission lines112aand112b, transmission of the digital signal cannot be performed normally between these sensors114aand114band the transmitter-receiver110.

Furthermore, in the disaster prevention monitoring system aimed at fire monitoring and security, the transmission lines from the receiver are branched along the way and led out toward each terminal side. In the case of the transmission system in which the transmission lines are branched in this way, the terminal ends of the transmission lines are at a plurality of locations. Therefore, it may be difficult to apply such a method where a resistor is inserted into a sensor at terminal ends to achieve impedance matching.

The invention was made with respect to the above-described problems, it is an object of the present invention to provide a transmission system that can suppress ringing in a digital signal by achieving impedance matching without inserting a resistor into a sensor on the terminal end side of transmission lines.

Means for Solving the Problem

The present invention adopts the following measures in order to solve the above-described problems and achieve the object of the present invention.

(1) A transmission system of the present invention is for transmitting a signal from a transmission end to a reception end via a pair of transmission lines, wherein a first resistor having a predetermined resistance value depending on a characteristic impedance of the transmission lines is connected to the transmission end side of the transmission lines.

(2) In the transmission system described in the above (1), the first resistor may be connected to the transmission end side of one transmission line of the pair of transmission lines.

(3) In the transmission system described in the above (2), the first resistor may have a resistance value equal to the characteristic impedance of the transmission lines.

(4) In the transmission system described in the above (2), the first resistor may have a resistance value of from 0.2 times to 0.8 times or from 1.5 times to 5.0 times of the characteristic impedance of the transmission lines.

(5) In the transmission system described in the above (1), a second resistor may be respectively connected to a transmission end side of both transmission lines of the pair of transmission lines, and the resistance values of the second resistors may be respectively half the resistance value of the first resistor.

(6) In the transmission system described in the above (5), the second resistor may respectively have a resistance value of half the characteristic impedance of the transmission lines.

(7) In the transmission system described in the above (5), the second resistor may respectively have a resistance value of half the resistance value of from 0.2 times to 0.8 times or from 1.5 times to 5.0 times of the characteristic impedance of the transmission lines.

(8) In the transmission system described in the above (1), an inductance that bypasses direct-current power may be further connected in parallel with the resistance by the transmission lines.

Effects of the Invention

According to the transmission system described in the above (1), the first resistor for achieving impedance matching needs only to be connected to the transmission line at the transmission end side such as a receiver. Therefore, it is not necessary to check the sensor connected to the terminal ends of the transmission lines to insert a resistor in the sensor. As a result, an operation for achieving impedance matching can be performed easily.

Moreover, even if the transmission lines are branched at a terminal side and the terminal ends are at a plurality of locations, impedance matching can be achieved reliably even in the branched transmission line, by only connecting one first resistor to the transmission end side of the transmission line for impedance matching.

Furthermore, conventionally, a sensor having a resistor connected to the terminal end of the transmission line and other sensors need to be discriminated and handled. However in the transmission system of the present invention, all the terminal devices such as sensors, which form reception ends, can be handled in the same manner. As a result, discrimination based on a difference of whether or not a resistor is to be inserted, is not required, and hence, workability in manufacturing of the terminal devices and installation thereof at a site can be improved.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1is a block diagram showing a transmission system1A (1) according to a first embodiment of the present invention. The transmission system1A of the first embodiment is schematically constituted by a receiver10having a transmission end (transmission output circuit16), a plurality of reception ends (sensors14), and a pair of transmission lines12aand12bthat electrically connect the transmission end to the reception ends. In the present embodiment, a first resistor R1is connected to the transmission end side of one transmission line12a.

As shown inFIG. 1, the transmission output circuit16that functions as a transmission end is provided in the receiver10. The pair of transmission lines12aand12bare led out from the transmission output circuit16toward a terminal side, and a plurality of sensors14serving as the reception ends, are connected between these transmission lines12aand12b. InFIG. 1, an example in which three sensors14are arranged is shown; however, the present invention is not limited to this number.

In the present embodiment, the first resistor R1for performing impedance matching is connected to the transmission end side of the transmission line12aof the pair of transmission lines12aand12bled out from the transmission output circuit16serving as the transmission end. For example, a resistance value r1of the first resistor R1is a resistance value equal to a characteristic impedance Z0of the pair of transmission lines12aand12b.

The characteristic impedance Z0of the pair of transmission lines12aand12bused in the present embodiment, that is, an equivalent impedance Z0as seen from one end side of the transmission lines12aand12bis, for example, Z0=100Ω, assuming that these transmission lines12aand12bare transmission lines having an unlimited length. In this case, the resistance value r1of the first resistor R1connected to an output end side of the transmission output circuit16(the transmission end side the transmission line12a) is r1=100Ω, corresponding to the value of impedance Z0.

FIG. 2is a block diagram showing a disaster prevention monitoring system to which the transmission system of the present embodiment is applied. As shown inFIG. 2, the receiver10includes a transmission circuit section22, a CPU20, a display section28, an operation section30, a storage section32, and a transfer section34. The transmission circuit section22, display section28, operation section30, storage section32, and transfer section34are connected to the CPU20.

The transmission circuit section22includes the transmission output circuit16and a transmission input circuit26. The pair of transmission lines12aand12bare led out from the transmission circuit section22, and the plurality of sensors14(for example, fire sensors and the like) are connected to these transmission lines12aand12b. InFIG. 2, one of the plurality of sensors14is representatively shown. The first resistor R1for performing impedance matching is connected to the one transmission line12aled out from the transmission circuit section22.

The transmission output circuit16provided in the receiver10of the disaster prevention monitoring system inFIG. 2, the transmission line12a, and the first resistor R1connected to the transmission end side of the transmission line12aconstitute a circuit on the transmission end side.

The sensor14includes a CPU36, a sensor section38, a transmission input circuit40, and a transmission output circuit42.

In the disaster prevention monitoring system shown inFIG. 2, the transmission circuit section22performs digital transmission of a downward telegraphic message and an upward telegraphic message at a data transmission rate of, for example, 19200 bps between the transmission circuit section22and the sensor14. For example, the CPU20in the receiver10regularly polls the sensor14and receives a normal response from the sensor14. When the normal response cannot be acquired, a fault warning of abnormality in the sensor14or the like is output to the transfer section34and the display section28.

The sensor14detects an increase in temperature (air temperature) or an increase in smoke density in a sensor section38thereof. When detecting an outbreak of fire, the sensor14transmits fire detection information to the receiver10. The receiver10side issues a fire warning to the transfer section34and the display section28in response to the fire detection information.

Digital transmission of the downward telegraphic message from the transmission output circuit16of the receiver10to the transmission input circuit40of the sensor14is performed in the voltage mode. The transmission output circuit16outputs a voltage pulse to between the pair of transmission lines12aand12bby a bit string corresponding to the downward telegraphic message from the CPU20of the receiver10. The voltage pulse output is detected as a change in voltage between the pair of transmission lines12aand12bby the transmission input circuit40of the sensor14, and is input to the CPU36of the sensor14. That is, the transmission output circuit16outputs a bit string as the downward telegraphic message to be received by the CPU36of the sensor14.

On the other hand, transmission of the upward telegraphic message from the transmission output circuit42of the sensor14to the transmission input circuit26of the receiver10is performed in a current mode. The transmission output circuit42of the sensor14inputs a bit string corresponding to the upward telegraphic message detected by the CPU36of the sensor14to between the pair of transmission lines12aand12b. At this time, when it is bit1, the transmission output circuit42short-circuits between the transmission lines12aand12bto a low impedance, passes a line current, and outputs a current pulse. The current pulse is input to the receiver10as an upward telegraphic message. The transmission input circuit26of the receiver10converts the current pulse output from the sensor14to a reception voltage at opposite ends of the first resistor R1to detect the reception voltage, and inputs the reception voltage to the CPU20of the receiver10as a bit string of the received upward telegraphic message.

In this digital transmission between the receiver10and the sensor14, impedance matching can be performed by connecting the first resistor R1to the transmission end side of the transmission line12aled out from the transmission output circuit16, when a digital signal, that is, a voltage pulse signal is transmitted from the receiver10to the sensor14in the voltage mode. As a result, at the time of transmission of the voltage pulse from the receiver10side to the sensor14, the occurrence of ringing due to the voltage pulse reflected at the terminal end side of the transmission lines12aand12bcaused by impedance mismatching between the transmission end side and the transmission terminal end side of the pair of transmission lines12aand12bcan be suppressed. As a result, digital transmission can be performed normally in the voltage mode.

FIG. 3shows a waveform of the voltage at the terminal end side of the transmission line12awith respect to a rise of the voltage pulse, when the resistance value r1of the first resistor R1is set to a value equal to the characteristic impedance Z0of the pair of transmission lines12aand12b(for example, r1=Z0=100Ω) in the first embodiment shown inFIG. 1.

Measurement of the waveform of the voltage shown inFIG. 3was performed, assuming that line lengths L of the pair of transmission lines12aand12binFIG. 1were respectively L=1 km. The time required for transmitting an electric signal for the line length L=1 km is about 5 microseconds. Therefore, the time required for the voltage pulse sent from the transmission output circuit16to be reflected at the terminal end of the transmission line12aand returned is about 10 microseconds.

FIG. 3(A) shows a waveform of the voltage at the terminal end side of the transmission line12a. When the voltage pulse is raised at time t=0, the voltage pulse rises to peak voltage after a predetermined delay time.FIG. 3(B) shows a voltage pulse transmitted at a transmission rate of 19200 bps. The pulse width T1of the voltage pulse to be transmitted at 19200 bps is about 52 microseconds, Bit determination under this condition is performed based on whether the voltage exceeds a threshold voltage Vth set to half the peak voltage at a timing of T2=26 microseconds, which is half the pulse width T1.

The waveform of the voltage at the terminal end side of the transmission line12ashown inFIG. 3(A) exceeds the threshold voltage Vth when T2=26 microseconds have passed since the rise of the voltage pulse. Therefore, even the sensor14arranged at the terminal end side of the pair of transmission lines12aand12bcan correctly perform bit determination (determination of bit1).

FIG. 4shows a waveform of the voltage measured at the transmission output circuit16side, that is, at the transmission end side of the transmission line12afor when r1=Z0=100Ω. In the waveform measured at the transmission end side, when the voltage pulse is raised at time t=0, the voltage pulse first rises to a voltage due to a partial voltage of the first resistor R1and the characteristic impedance Z0of the transmission line12a.

Here because r1=Z0=100Ω, the voltage first rises to a voltage half the peak voltage. Subsequently, when 10 microseconds have passed, which is the time required from the voltage rise for shuttling between on the transmission lines12aand12bhaving a length of L=1 km, a signal component reflected at the terminal ends of these transmission lines12aand12bcan be acquired. The voltage rises toward the peak voltage due to the signal component.

Here when the voltage at a point in time after T2=26 microseconds required for bit determination of the voltage pulse at 19200 bps have passed from t=0 is seen, the voltage exceeds the threshold voltage Vth and substantially reaches the peak voltage. Accordingly, correct bit determination (determination of bit1) can be performed even by the sensor14arranged at the transmission end side of the receiver10inFIG. 1.

When the resistance value r1of the first resistor R1inserted for impedance matching is set to a value equal to the value Z0of the characteristic impedance of the pair of transmission lines12aand12b, as shown inFIG. 4, the waveform of the voltage on the output side of the transmission line12afirst rises to half the peak voltage, and then rises to the peak voltage. Therefore, because the first voltage rise value of the sensor14connected to a position closest to the receiver10is around the threshold voltage Vth, a normal electric signal (voltage pulse) may not be received normally.

Therefore, in order to solve this problem, it is desired that the resistance value r1of the first resistor R1is set to 0.2 to 0.8 times the characteristic impedance Z0of the pair of transmission lines12aand12b. That is,

Here when Z0=100Ω, the resistance value r1is set to a range of r1=20Ω to 80Ω.

If the resistance value r1of the first resistor R1is less than 0.2 times the characteristic impedance Z0of the pair of transmission lines12aand12b, impedance matching cannot be expected. Therefore, 0.2 times the characteristic impedance Z0is set to a lower limit. Moreover, if the resistance value r1of the first resistor R1exceeds 0.8 times the characteristic impedance Z0of the pair of transmission lines12aand12b, it corresponds to a case of r1=Z0, and hence, 0.8 times the characteristic impedance Z0is set to an upper limit.

FIG. 5shows a waveform of the voltage at the transmission end side of the transmission line12awhen r1=0.3Z0, for example, r1=0.3×100Ω=30Ω. In this case, the resistance value r1of the first resistor R1is substantially ⅓ the characteristic impedance Z0of the pair of transmission lines12aand12b. Therefore, as shown inFIG. 5, when the voltage pulse is raised at time t=0, the voltage at the transmission end side of the transmission line12arises up to about 70% of the peak voltage. The voltage value at the rising edge is larger than that of the voltage shown inFIG. 4, and sufficiently exceeds the threshold voltage Vth set to half the peak voltage. Therefore, when r1=0.3Z0, stable reception of the voltage pulse can be ensured even by the sensor14connected to the position closest to the receiver10.

Actually, bit determination is performed at a timing after T2=26 microseconds have passed since time t0. Because the voltage value at this time is near the peak voltage as shown inFIG. 5, bit determination (determination of bit1) can be reliably performed.

Moreover in the present embodiment, the resistance value r1of the first resistor R1can be set to a larger value than the characteristic impedance Z0of the pair of transmission lines12aand12b. In this case, it is desired that the resistance value r1of the first resistor R1is set to 1.5 times to 4 times the characteristic impedance Z0of the pair of transmission lines12aand12b. That is,

Here when Z0=100Ω, the resistance value r1is set to a range of r1=150Ω to 400Ω.

When the resistance value r1of the first resistor R1is less than 1.5 times the characteristic impedance Z0of the pair of transmission lines12aand12b, because it corresponds to a case of r1=Z, then 1.5 times the characteristic impedance Z0is set to the lower limit. Moreover when the resistance value r1of the first resistor R1exceeds 4 times the characteristic impedance Z0of the pair of transmission lines12aand12b, because the rise of the voltage pulse to the peak voltage is too late, then 4 times the characteristic impedance Z0is set to the upper limit.

FIG. 6shows a waveform of the voltage at the transmission end side of the transmission line12ain the case of r1=3Z0=300Ω, as the resistance value r1of the first resistor R1. Thus, when the resistance value r1of the first resistor R1is 3 times the characteristic impedance Z0of the pair of transmission lines12aand12b, the voltage at the transmission end side of the transmission line12afirst rises up to about 25% of the peak voltage with respect to the rise of the voltage pulse at time t=0. Thereafter, the voltage increases stepwise upon reception of a component reflected at the terminal end of the transmission line12a.

Also in this case, bit determination is performed based on the voltage value at a point in time after T2=26 microseconds have passed since the rise of the voltage pulse at time t=0. Because the voltage at this time sufficiently exceeds the threshold voltage Vth, correct bit determination (determination of bit1) can be performed even by the sensor14arranged at the position closest to the receiver10.

FIG. 7is a block diagram showing a transmission system1B (1) according to a second embodiment of the present invention, in which the transmission lines are branched. InFIG. 7, a pair of transmission lines12aand12bare led out from a transmission output circuit16provided in a receiver10. These transmission lines12aand12bare branched to two systems, and a plurality of sensors14are connected to the respective systems.

Even in this case in which the pair of transmission lines12aand12bare branched, a first resistor R1for impedance matching is connected to a transmission end side of one transmission line12aled out from the transmission output circuit16. As a resistance value of the first resistor R1, any value of:

can be taken, as in the embodiment described above.

FIG. 8shows the waveform of the voltage at the transmission end side of the transmission line12awhen r1=Z0=100Ω in the embodiment shown inFIG. 7. Measurement was performed, assuming that a line length from the receiver10to a branch point inFIG. 7was L1=400 m, and line lengths of the transmission lines12aand12bfrom the branch point are L2=400 m and L3=200 m, respectively.

Also in the present embodiment, as in the first embodiment, bit determination of a reception pulse by the sensors14in the respective systems provided at positions closest to the receiver10is performed at a timing after T2=26 microseconds have passed from the rise of a voltage pulse at time t=0. In this case, as shown inFIG. 8, because the voltage at T2sufficiently exceeds a threshold voltage Vth and is near a peak voltage, bit determination (determination of bit1) can be correctly performed.

FIG. 9is a block diagram of a transmission system1C (1) according to a third embodiment of the present invention. In the present embodiment, a second resistor R2is respectively connected at both transmission end sides of a pair of transmission lines12aand12bled out from a receiver10. These second resistors R2respectively have a resistance value r2half the resistance value r1of the first resistor R1connected to one transmission line12ain the first embodiment. The resistance value r2of the second resistors R2in the present embodiment can take values described below, corresponding to three types of resistance values r1of the first resistor R1in the first embodiment.

As shown inFIG. 1toFIG. 7, when the first resistor R1is connected to one transmission line12a, the pair of transmission lines12aand12bbecome unbalanced transmission lines. On the other hand, in the present embodiment, because the second resistor R2is respectively connected to both of the pair of transmission lines12aand12b, balanced transmission lines are formed. As a result, noise components in these transmission lines12aand12bare balanced out, thereby enabling to realize noise-resistant digital transmission.

FIG. 10Ais a block diagram of a transmission system1D (1) according to a fourth embodiment of the present invention. In the transmission system1D of the present embodiment, an inductance L (coil) is connected in parallel with a first resistor R1connected to a transmission end side of one transmission line12afor impedance matching. Direct-current power is bypassed to a terminal (sensor14) side by the inductance L.

As the inductance L, for example, an inductance of 30 μH to 20 mH can be used. Because the inductance L is connected in this way in parallel with the first resistor R1, a situation where the power-supply voltage supplied to the sensor14side decreases due to the insertion of the first resistor R1can be prevented.

FIG. 10Bis a block diagram of a transmission system1E (1) according to a fifth embodiment of the present invention. In the present embodiment, in the transmission system1C of the third embodiment shown inFIG. 9, an inductance L is respectively connected in parallel with the second resistor R2respectively connected to both transmission end sides of the pair of transmission lines12aand12b. In the present embodiment also, as in the transmission system1D of the fourth embodiment, a situation where the power-supply voltage supplied to the sensor14side decreases due to the insertion of the second resistor R2can be prevented.

A waveform of the current at the terminal end side of the transmission line12ain the embodiments shown inFIG. 10AandFIG. 10Bbecomes as shown inFIG. 3(A). In this case, supply of power is performed by using direct current. Direct-current resistance of the inductance L is set to a sufficiently small value. Accordingly, a loss of supply power for impedance matching is minimal.

In this manner, in the fourth and fifth embodiments, the inductance L is respectively connected in parallel with the first resistor R1connected to the transmission end side of one transmission line12aor the first and second resistors R2connected to the transmission end side of both of the transmission lines12aand12bfor impedance matching, so that the direct-current power is bypassed to the sensor14side. As a result, an embodiment preferable to the disaster prevention monitoring system such as fire monitoring that performs transmission of digital data at the same time as supplying power to the sensor14from the receiver10via these transmission lines12aand12bcan be obtained.

In the above-described embodiments, transfer of the digital data in the disaster prevention monitoring system that monitors for fire, gas leakage, and the like is taken as an example. However, the present invention is not limited thereto, and the present invention can be directly applied to an appropriate transmission system with a transmission end and a reception end having a configuration in which transmission lines are branched to a terminal side.

Preferred embodiments of the present invention are explained above, however, the present invention is not limited to these embodiments, and addition, omission, replacement, and other changes of the configuration can be made without departing from the scope of the present invention. The present invention is not limited by the above-described explanation, and is limited only by the scope of the appended claims.

INDUSTRIAL APPLICABILITY

According to the transmission system of the present invention, a first resistor needs only to be connected to a transmission line on a transmission end side such as a receiver for impedance matching. Therefore, it is not necessary to check a sensor connected to a terminal end of a transmission line to insert the resistor in the sensor. Accordingly, an operation for achieving impedance matching is facilitated.

Moreover, even if the transmission lines are branched to the terminal side and the terminal ends are at a plurality of locations, impedance matching can be reliably achieved even with the branched transmission lines, only by connecting one first resistor to the transmission end side of the transmission line for impedance matching.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS