Alarm Terminal Device

The present invention relates to an alarm terminal device such as a fire alarm terminal device or the like. More particularly, the present invention relates to an alarm terminal device for converting to a digital signal an analog signal indicating a smoke concentration, a gas concentration, a temperature, or other parameter.

It is known to arrange an alarm terminal device such as a fire alarm terminal device such that a detected signal (analog signal) from a temperature sensor or the like is converted to a digital signal which is received by a receiver. Therefore, information other than the detected signal indicating a temperature or the like cannot be obtained. For example, information corresponding to a disconnection or short circuit of a platinum resistor as a major component of a temperature sensor cannot be transmitted through the same transmission line. In order to transmit breakdown data such as data indicating the disconnection or short circuit of the platinum resistor, another transmission line must be provided, or different types of data must be transmitted in accordance with time division multiplexing, resulting in a complex configuration and high cost. Furthermore, according to the conventional alarm terminal device, a sensor output which is less than a predetermined value is transmitted as digital data, e.g., "000", and another sensor output which exceeds the predetermined value is transmitted as digital data, e.g., "111". As a result, a disconnection or short circuit of the platinum resistor cannot be detected upon reception of such data. Since a platinum wire is very thin, the platinum resistor has a tendency to become disconnected or to short-circuit. Therefore, a disconnection or short circuit of the platinum resistor must be constantly monitored.

The present invention eliminates the conventional drawback described above, and has for its object to provide an alarm terminal device wherein a sensor output and any other data can be separately obtained from output signals from a single analog-to-digital converter.

In order to achieve the above objective, there is provided an alarm terminal device having a first sensor for generating an analog signal indicating one of the parameters smoke concentration, gas concentration or temperature, and an analog-to-digital converter for converting the analog signal to digital data. A discriminator is provided for discriminating the analog signal in accordance with at least one reference value. At least one second sensor is provided for detecting an analog data signal, indicating information other than that presented by the analog signal from the first sensor. The analog data signal from the at least one second sensor is supplied in parallel with the analog signal from the first sensor to the analog-to-digital converter.

FIG. 1 is a circuit diagram of an alarm terminal device according to a preferred embodiment of the present invention; and

FIG. 2 is a graph for explaining the relation between quantization steps and code assignment thereto in the embodiment shown in FIG. 1.

An alarm terminal device according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram of an alarm terminal device to which the present invention is applied. The alarm terminal device serves to convert to a digital signal an analog signal produced from a temperature sensor having a platinum resistor. Referring to FIG. 1, a series circuit of a platinum resistor R.sub.1 and a reference resistor R.sub.2 is connected between a power supply and ground. A voltage divided by the platinum resistor R.sub.1 and the reference resistor R.sub.2 is supplied to the noninverting input terminal of an operational amplifier OP. A reference voltage is supplied to the inverting input terminal of the operational amplifier OP. When a resistance of the platinum resistor R.sub.1 changes in accordance with a change in temperature, the divided voltage changes. A difference between the divided voltage and the reference voltage is amplified by the operational amplifier OP, so that the operational amplifier OP generates an analog signal corresponding to a temperature detected by the temperature sensor. The analog signal is supplied to an A/D converter AD through resistors R.sub.3 and R.sub.4. The output terminal of the resistor R.sub.3 is connected to the power supply through a Zener diode Z.sub.1, and to ground through a Zener diode Z.sub.2. In this embodiment, the temperature sensor comprises the platinum resistor R.sub.1, the reference resistor R.sub.2, the operational amplifier OP, the resistors R.sub.3 and R.sub.4, and the like. The analog detected signal from the temperature sensor is converted by the A/D converter AD to a digital signal.

When the analog voltage signal from the temperature sensor is less than a predetermined voltage E.sub.1, the Zener diode Z.sub.1 is turned on, and a current flows in the resistor R.sub.3. As a result, the sensor output is set at the predetermined voltage E.sub.1. The predetermined voltage E.sub.1 is referred to as a lower limit voltage E.sub.1. The Zener diode Z.sub.1 comprises a lower limit generator.

However, when the analog voltage signal from the temperature sensor exceeds a predetermined voltage E.sub.2, the Zener diode Z.sub.2 is turned on, so that the sensor output voltage is set at the predetermined voltage E.sub.2. The predetermined voltage E.sub.2 is referred to as an upper limit voltage E.sub.2. The Zener diode Z.sub.2 comprises an upper limit generator. An analog voltage of a sensor output which falls within the range between the lower limit voltage E.sub.1 and the upper limit voltage E.sub.2 is converted by the A/D converter AD to a digital voltage signal without modification.

Now assume that the A/D converter AD comprises a 5-bit converter, and that a potential difference between the power supply voltage and the ground voltage is quantized in accordance with 32 steps as shown in FIG. 2. In this embodiment, voltages between the lower limit voltage E.sub.1 and the upper limit voltage E.sub.2 are quantized into the range between, e.g., the 5th step and 28th step. For example, the lower limit voltage E.sub.1 corresponding to the 5th step is converted to the 5th digital data "00100" (=4 in decimal notation), and the upper limit voltage E.sub.2 corresponding to the 28th step is converted to the 28th digital data "11011" (=27 in decimal notation). Voltages corresponding to the steps not exceeding the 4th step ("00011") and not below the 29th step ("11100") are used to transmit any other data excluding the sensor output data. As is apparent from the above description, the reference voltage and the gain of the operational amplifier OP are preset so that the sensor output falls within the range between the lower limit voltage E.sub.1 and the upper limit voltage E.sub.2 when the detected temperatures fall within a normal temperature range between T.sub.min and T.sub.max.

One end of the platinum resistor R.sub.1 is connected to the noninverting input terminals of comparators CP.sub.1 and CP.sub.2 in parallel therewith. The comparator CP.sub.1 serves to detect a short circuit of the platinum resistor R.sub.1. The comparator CP.sub.2 serves to detect a disconnection of the platinum resistor R.sub.1. A voltage slightly higher than the ground potential is applied to the inverting input terminal of the comparator CP.sub.1, so that the comparator CP.sub.1 normally generates a signal of high level. The output from the comparator CP.sub.1 is supplied to the A/D converter AD through a diode D.sub.1 and a Zener diode Z.sub.3. The diode D.sub.1 is reverse-biased by the high level output from the comparator CP.sub.1. Therefore, the output from the temperature sensor is normally supplied to the A/D converter AD.

However, when the platinum resistor R.sub.1 is short-circuited, the output from the comparator CP.sub.1 goes low. A current then flows through the resistor R.sub.4, the Zener diode Z.sub.3, and the diode D.sub.1. As a result, the temperature sensor output (i.e., the input voltage applied to the A/D converter AD) becomes a Zener voltage (forward bias voltage of the diode D.sub.1) of the Zener diode Z.sub.3. The Zener voltage of the Zener diode Z.sub.3 is preset to correspond to the 3rd step of the quantization steps. Therefore, when the platinum resistor R.sub.1 is short-circuited, the A/D converter AD generates digital data "00010" (2 in decimal notation). When the receiver receives this digital data, it detects that a short circuit of the platinum resistor has occurred.

A voltage slightly lower than the power supply voltage is supplied to the inverting input terminal of the comparator CP.sub.2, so that the comparator CP.sub.2 normally generates a signal of low level. The output from the comparator CP.sub.2 is coupled in parallel with the sensor output through a series circuit of a diode D.sub.2 and a Zener diode Z.sub.4. The anode of the diode D.sub.2 is connected to the output terminal of the comparator CP.sub.2, so that the low level output from the comparator CP.sub.2 is blocked by the diode D.sub.2.

However, when the platinum resistor R.sub.1 is disconnected, the voltage to be applied to the noninverting input terminal of the comparator CP.sub.2 is increased to the power supply voltage, and the output from the comparator CP.sub.2 goes high. This high level signal is supplied to the A/D converter AD through the diode D.sub.2 and the Zener diode Z.sub.4. The input voltage applied to the A/D converter AD is lower than the high-level voltage (power supply voltage) from the comparator CP.sub.2 by the Zener voltage (forward bias voltage of the diode D.sub.2) of the Zener diode Z.sub.4. The Zener voltage of the Zener diode Z.sub.4 is preset such that the voltage applied to the A/D converter AD corresponds to the 30th step of the quantization steps. In this case, the A/D converter AD generates the digital data "11101" (29 in decimal notation). When the receiver receives this digital data, it detects that the platinum resistor R.sub.1 is disconnected. It is possible to transmit any other information by using the digital data " 00000" (1st step) to "00011" (4th step) and the digital data "11100" (29th step) to "11111" (32nd step).

In this embodiment, the comparator CP.sub.1, the diode D.sub.1, and the Zener diode Z.sub.3 comprise a second sensor for generating an analog signal of a voltage lower than the lower limit voltage E.sub.1 so as to detect a short circuit of the platinum resistor R.sub.1. Similarly, the comparator CP.sub.2, the diode D.sub.2, and the Zener diode Z.sub.4 comprise another second sensor for generating an analog signal of a voltage higher than the upper limit voltage E.sub.2 so as to detect a disconnection of the platinum resistor R.sub.1. In this embodiment, only two second sensors are used. However, three or more second sensors may be used as needed.

According to the embodiment described above, the sensor output analog signal is preset to fall within the upper and lower limit voltages E.sub.2 and E.sub.1.

Furthermore, one second sensor is arranged to generate a voltage which is lower than the lower limit voltage E.sub.1, and the other second sensor is arranged to generate a voltage which is higher than the upper limit voltage E.sub.2. The voltage lower than the lower limit voltage E.sub.1 is applied to the A/D converter which then produces corresponding digital data indicating a piece of information excluding the sensor output. The voltage higher than the upper limit voltage E.sub.2 is applied to the A/D converter which then produces corresponding digital data indicating another piece of information excluding the sensor output. Therefore, erroneous operation and breakdown conditions such as a short circuit and a disconnection can be properly detected. Any desired information can be obtained by using the steps excluding those in the range between the upper and lower limit voltage steps. As a result, the conditions of the terminal device can be properly monitored, thereby improving the reliability of the detected data.

Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that we wish to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within our contribution to the art.