Receiver particularly for a meter-bus

A receiver particularly suited for an M-BUS is described. During transmission, the receiver is disabled. After each transmission, nodes and states in the receiver are set to prepare the receiver to receive a signal. Once data is sensed, a feedback loop clips the input signal to the receiver to limit the swing of the input signal. The line of the power supply at the lower potential is modulated, rather than modulating the line at the higher potential, for the transmission of data.

FIELD OF THE INVENTION

The invention relates to the field of receivers for use with signals having a large dynamic range of amplitudes and time.

PRIOR ART AND RELATED ART

A standardized serial bus system for the remote reading of meters and various sensors, has been adopted in Europe. The so-called meter-bus or M-BUS permits, for instance, a power meter to act as a master/gateway for other household meters such as gas and water meters. The M-BUS interconnects the master (e.g. power meter) with the slaves (e.g. gas and water meters). Other sensors such as temperature sensors and actuators to, for example, shed electrical loads, may also be coupled to the M-BUS. The physical and link layer of the M-BUS is set forth in EN13757-2 and the application layer in EN13757-3.

The implementation of physical layer presents several challenges especially in the design of a fault tolerant power supply and a receiver. These are discussed in connection withFIGS. 1 and 2.

SUMMARY OF THE INVENTION

A receiver is disclosed for operating with a bus such as an M-BUS. The receiver senses current changes on the same bus on which a power supply potential is modulated to transmit data. The receiver includes a bias feedback loop which causes the input signal to be clipped once it has been sensed.

DETAILED DESCRIPTION

A receiver particularly suited for M-BUS is described. In the following description, numerous specific details are set forth. It will be appreciated that these details are used to provide a thorough understanding of the present invention, and other circuit configuration and values may be used within the scope of the present invention.

FIG. 1illustrates a typical arrangement in which the M-BUS is used. A power meter10, which is a master under the M-BUS standard, communicates with several slaves such as a gas meter12and a water meter14. The power meter10, for instance, may poll the gas and water meters to obtain readings over the M-BUS. Several power meters10may be coupled to a data concentrator15over an RF or IR link, or a hard-wired link. Data concentrators, then may communicate with a central station over, for example, the Internet. Alternatively, the power meter10may communicate directly over the Internet with a central station. The meter10may communicate not only its own readings but those of the other meters on the M-BUS, and may receive instructions which are communicated to the slaves on the M-BUS. In this manner, the power meter10acts as a gateway.

Referring toFIG. 2, the physical layer portion of the power meter is shown in block diagram form. The M-BUS may be ordinary conductors such as a twisted pair. In the M-BUS standard, one line is at +36 V with respect to the other line. Power for the slaves is drawn from the M-BUS. Data is transmitted from the master to the slaves by modulating the voltage on the power line from 36 volts to 24 volts. A logical “1” (referred to as a “mark”) is represented by the condition of the lines being at 36 volts. A logical “0” (referred to as a “space”) is represented by the reduction of the lines to 24 volts. For an embodiment of the present invention, the ground line is changed by 12 volts (0 for “space” or 12V for “mark”) to signal a space, while the other power line remains at 24 volts.

Information is sent from the slaves to the master by modulating the current consumed by the transmitting slave. When no slave is sending a space, a constant current is drawn from the master corresponding to the total quiescent current of all the slaves. A logical “1” or mark transmitted by a slave is represented by a constant current of up to 1.5 mA. A logical “0” or space is represented by the flow of an additional 11-20 mA. As currently implemented, the M-BUS standard utilizes only half duplex transmissions on the bus, and consequently, transmission is either from master-to-slave or slave-to-master, but not both at the same time.

InFIG. 2, the physical layer of the M-BUS is implemented with a 12 volt power supply20, an interrupt and retry circuit21and a DC-to-DC converter22. The 12 volt supply is stepped up to 24 volts for one line of the bus. The other line of the bus is modulated between 0 and 12 volts for the transmission of data from the master. These voltages are different from those specified in the standard, but it will be appreciated that the circuits described in this application are applicable to the specific voltages described in the standard, and that the selection of a particular voltage is somewhat arbitrary. The data transmitted from the power meter ofFIG. 2is represented by the block23, adjacent the power supply, to indicate that one line of the power supply is modulated to provide the mark and space for the master-slave direction.

The M-BUS standard requires that each of the slaves have a current limiting resistor at its input of approximately 430 ohms. This limits the current in the event of a short within a slave, to a maximum of 100 mA. A challenge in the receiver design is to sense data from one slave while another is experiencing a short. The receiver must be able to discern the mark and space both in the presence of a low current flow (no short) and a high current flow (slave experiences short). These modulation extremes span two orders of magnitude. Moreover, communications can take place at baud rates from 300-9600 Baud.

The circuit ofFIG. 3includes the transmitter circuit, although most of the components ofFIG. 3are used for receiving. The transmit data is coupled to the line39where it is connected to the base of an npn transistor36through a resistor. The emitter of this transistor is connected to ground through a shunt resistor35. The collector of transistor36, as well as the emitter of a pnp transistor32, are connected to a line30, one line (line2) of the bus. For the described embodiment, line2of the bus varies between 0-12 volts as data is transmitted. Line1of the bus remains at a constant 24 volts. The base of transistor32is coupled to a source of 12 volts through the resistor34to clamp the M-BUS in one of its states to 12 volts plus a diode junction drop.

The modulation of the lower potential of the power supply in M-BUS enables a more cost effective design since level shifting is not required to modulate the 24 volts. Referring briefly toFIG. 6, the waveform110illustrates the modulated power supply as set forth in the M-BUS specification. The modulation from 24 volts to a lower voltage is not easily dealt with when the circuitry operates at a lower potential, for instance, 0-5 volts. On the other hand, as shown by waveform111, when one line of the power supply (the line at the higher potential) is kept constant, for instance at 24 volts, and the other line is modulated between 0-12 volts, the modulated signal is adaptable for circuits operating with a power supply of, for example, 5 volts with respect to the 0 volts.

When the potential on line39is low, transistor36does not conduct and bus line2remains at 12 volts because transistor32is operating as a diode to 12 volts with the current flowing through the shunt resistor35. When the potential on line39is high, transistor36conducts and bus line2is brought close to ground potential (i.e. 1 volt) through the transistor36and resistor35. The transistor32acts as a diode to the 12 volt supply with current flowing through resistor35to minimize current swings during transmissions. The pnp transistor38, coupled to the base of transistor36, assures that transistor36has a fast shut-off time by removing base charges.

The receiver ofFIG. 3has a signal path for amplifying the signal represented by the current changes sensed on line2of the bus. This represents the data being sent in the case of the M-BUS from, for instance, the meters12and14ofFIG. 1to the power meter10. When data is being received, the transistor36is conducting and the changes in current in the bus are sensed by the shunt resistor35. These changes in current are converted to a voltage, representing the received input data, at node40. Node40is connected to the input of the operational amplifier50through the series capacitors41and42and the resistor45. The capacitors41and43remove the DC component from the signal on node40.

As will be described in more detail later, the capacitors41and43may be looked at as a single capacitor. In conjunction with the transistor86, the capacitors41and43operate as an analog switch which is open for the signal when node42is connected to ground, and closed when the node42floats. Moreover, as will be discussed, the voltage swing on the capacitors is limited by transistor88and89.

The operational amplifier50amplifies the signal with a negative gain and provides an output at node58. The positive input terminal to the amplifier50is biased at approximately +2.5 volts from the line70. The line70is maintained at 2.5 volts by the voltage divider comprising the resistors71and72. The amplifier50has a gain of minus 10.

The output of the amplifier50(node58) is coupled to an analog flip-flop realized with a comparator (operational amplifier60) with positive feedback. Data-out (node63) represents the received data.

When data is being transmitted, the rise and fall of the potential on node40caused by transistor36cycling on and off, greatly disrupts the biasing points and the charges on the nodes in the receiver. It is necessary to effectively disable the receiver when data is being transmitted, and to override the bias points on transmission so that the receiver can be reset to a state needed to sense the first received bit. To this end, a gating circuit is used to trigger these events.

The transistor80provides gating to control various functions in the receiver, as will be described, as data is transmitted. When the potential on line39drops, indicating that data is being transmitted, transistor80conducts. The RC time constant associated with the resistor81and capacitor82provide delay on the rising edge of the signal on the base of transistor80and hastens the turning on of transistor80when the signal falls. Thus, when data is being transmitted, transistor80turns on more quickly than it turns off when a data bit ends. The resultant signal on line85effectively disables the receiver. Moreover, this signal controls three functions within the receiver that enable it to tolerate the relatively large voltage swings associated with the transmission of data and to prepare the receiver to receive data.

The signal on line85cause transistor86to conduct. This in turn clamps node42substantially to ground and substantially prevents signal transfer from the node42to the capacitor43.

The signal on line85also causes the transistor91to conduct. This conduction resets the flip-flop60. The operational amplifier60because of the positive feedback as mentioned, essentially operates as a bistable circuit or a flip-flop and needs resetting to be in the proper polarity for the first transition (start bit) of the received signal.

Finally, the gating signal on line85causes the transistor87to conduct, thereby connecting the biasing potential on line70(2.5 volts) to node90. This provides the necessary biasing at capacitor43for receipt of the first received bit.

Thus, to summarize, the gating signal on line85opens the analog switch, mentioned earlier, formed by capacitors41and43by grounding the common node between them. Secondly, the signal on line85resets the flip-flop60. And lastly, the gating signal on line85through the transistor87, provides biasing for the capacitor43.

The transistors88and89perform an important function when data is being sensed by the receiver. To understand the function, it first must be appreciated that the receiver operates over a wide baud rate. To enable this, particularly at the lower baud rate, the capacitors41and43are relatively large. At the higher baud rate, the transistors88and89are used to clip the signal level to limit the signal that would otherwise pass through these capacitors at the higher frequency.

When the flip-flop60changes state, indicating that data has been sensed, the input signal to the amplifier50can be effectively shut off. The change of state at line63is coupled through the capacitor62to the bases of the transistors88and89. These transistors conduct for a short time allowing the bias to be reset at the input to the amplifier50since sensing is no longer needed for the current bit.

InFIG. 4, waveforms are shown to demonstrate what is occurring in the receiver when data is transmitted. During the times100inFIG. 4, data is being transmitted from device to host onto the bus line2ofFIG. 3. The signal on nodes58and63during this time is shown.

Node58, which is the amplified signal on node90, quickly rises and falls with each data bit transmitted. The falling and rising (return from peak to bias point) is caused by the clipping performed by the transistors88and89.

The second waveform inFIG. 4is the potential on the output node63of the flip-flop60. As can be seen, there is a 50% duty cycle, that is, no time distortion.

InFIG. 5, the corresponding waveform to that shown on line58ofFIG. 4is again shown. This time however, without the operation of transistors88and89(capacitor62missing). As can be seen, the potential on the node58is not reset to the approximately 2.5 volts between each of the data bits. This wandering waveform prevents the data from being sensed at the output of the flip-flop60.

The spike120during the transmit period and the corresponding spike inFIG. 4demonstrate the signal on node58when a short occurs on the M-BUS during the transmission of data. Note the quick recovery to the 2.5 volt level after the short.

Thus, a receiver, particularly suited for an M-BUS has been disclosed.

The bias feedback principle can be extended to other applications such as for telecommunications where a wide range of amplitudes and/or a wide range of bandwidths (speed) are required.