The present invention relates to methods of transmitting data and to data transmission systems. In particular, it refers to bi-directional radio frequency data transmission systems for use in building control and other low cost, low data rate applications.

Building control systems provide control and management of functions such as lighting, heating and security. To enable such control, individual items or groups of items within the building are assigned an identifier. The identifier needs to be capable of uniquely identifying each individual item or group of items within the building. Items may include switches, lamps, temperature sensors, heaters, pump, motorised valves, Passive Infra Red (PIR) sensors, motion detectors, alarm enunciators, door release mechanisms, door opening mechanisms, motors and motor drivers, level sensors, solenoid actuators and any such input or output devices as may enable or improve the management of a building or group of buildings.

In such systems, both the amount of data and the handling rate of that data are lower than in many other data systems. For instance, the operation of a light switch and the illumination of the light are two events that require a relatively small amount of data transfer and a response time of the order of tens or hundreds of milliseconds rather than microseconds. Similarly a thermostat requesting an alteration to the amount of heat input into a space does not require an extremely rapid response.

Benefits of building management are obtained from the enhanced control, security and effective management that can be exercised when all the functions are both visible to and controlled by an integrated system. The control of lighting within an area of a building can be optimised by taking account of factors such as time, occupancy, ambient light and external light as well as user requests. Similarly heating can be controlled more efficiently and with less damage to the environment if the whole building can be controlled as a set of related spaces. The control algorithm can be responsive to inputs similar to those in the case of the lighting example above.

A typical building control system comprises at least a central controller, a plurality of input devices, output devices or controlled device e.g. switches, actuators, prime movers, lights, heaters, security cameras, sensors, contacts, alarms etc. and means linking them together, hi a conventional wired system all input devices and controlled devices must be supplied with power and linked together in such a manner as to enable the control required. In a simple example, a switch is wired in series with a lamp across a power supply, hi a control system, the switch is connected to a central controller and the power supply for the lamp is driven either from a local controller connected to the central controller or directly from the central controller, hi such systems, separating each input and each load can lead to significant increase in wiring complexity and cost and also impose serious limitations on system flexibility.

One solution to the above problem is to use radio frequency (RF) communication between the various input devices and controlled devices to reduce the wiring requirement to the basic distribution of un-switched power feeds. Such a system allows input devices to be positioned anywhere in the building provided they remain in RF contact with their respective controlled devices and the central controller. Indeed in such embodiments, a user operated light switch may be battery powered and mounted on a wall, said user operated switch being operable to control by means of RF transmission to a central controller a powered switch provided in the power supply to the light.

A further benefit of RF systems is that the simpler input devices such as switches or thermostats can be battery powered and thus can be sited where convenient for a user rather than where it is convenient from a wiring point of view. If such a system is to be reliable, battery powered input devices must have sufficient battery life to be acceptable to the user. To achieve this aim an RF transmission system that allows data transmission be carried out with minimum power consumption is required.

In a typical building control system, data transmissions normally take place in a star formation between low power slave units, such as the battery powered light switch described above and a central master unit or controller. A slave unit will normally transmit a data message to the master in response to a change in status. The master unit will receive, decode and acknowledge the data transmission and initiate appropriate action, for instance transmit a signal to a powered switch to alter the power supply to a light. The slave unit may then power down once the message is acknowledged. A return channel to acknowledge receipt is a well-established technique, which helps to reduce the number of repeated transmissions. A return channel may similarly be used in communication between the master and actuators such as the powered switch above, however since such actuators are powered, low power signals are not such a requirement. To ensure accurate reception of messages sent by low power slave units and thus to reduce the number of repeated transmissions, there is a need for a 'fault intolerant' coding and transmission scheme which ensures correct reception and decoding. There is also a need to ensure that messages are sent in a format that allows the Phase Lock Loops (PLLs) typically used to decode such messages to operate as efficiently as possible.

It is therefore an object of the present invention to provide a data transmission method which, at least partially, overcomes or alleviates the above problems.

According to a first aspect of the present invention there is provided a method of transmitting binary data by transmitting alternate high and low pulses of defined durations wherein: a first binary data bit is encoded by a high or low pulse having a first defined duration; and wherein a second binary data bit is encoded by a high or low pulse having either a second defined duration or a third defined duration wherein the selection of the duration of pulse encoding a particular second data bit is determined as is necessary to maintain a substantially constant mean signal level in a transmitted data stream.

The above thus provides a method for achieving and maintaining a substantially mean signal level and thus provides a method by which fault free data transmission in a low cost low power RF transmission system with Phase Lock Loops can be achieved. The arrangement of the widths and sequences of the pulses are such that within any transmission any single bit decoding error caused by signal noise or timing inaccuracies results in a data bit or a data bit sequence that has a variation in mean signal level. Additionally, pulse sequences that do not correspond to valid data bits do not conform to the definitions of data bits and hence indicate a transmission error. This also allows errors caused by timing variations to be picked up, as variation in the duration of high and low pulses may effect the mean signal level.

Preferably, before transmitting data a pair of synchronisation pulses is transmitted, said pair of synchronisation pulses preferably comprising a high or low pulse of a fourth duration followed by a low or high pulse of a fourth defined duration. Preferably, each synchronisation pulse comprises a pair of pulses each of 4 units duration, each first binary data bit comprises a pulse of duration of 2 units, and each second binary data bit comprises a pulse of a duration of either 1 unit or 3 units.

Preferably, the method is in a wireless data transmission system. Most preferably, the wireless data transmission system operates at radio frequencies (RP). Preferably, said RF signals are encoded and or decoded by Phase Lock Loops (PLL).

The method may be used to send data messages between a plurality of individual units forming part of a larger system. Preferably in such a system, an acknowledgement message is transmitted in response to any safely received data message.

According to a second aspect of the present invention there is provided a method of transmitting binary data by transmitting alternate high and low pulses of defined durations wherein: data is encoded as bit pairs, each bit pair comprising a unique sequence of high and low pulses, each pulse in each bit pair having a defined duration wherein each bit pair has an equal mean signal level.

The method of the second aspect of the present invention may incorporate any or all of the features described in relation to the first aspect of the invention as appropriate. In particular, bit pairs according to the second aspect of the present invention may be used in implementing the method of the first aspect of the present invention.

Preferably, the bit pair '00' is encoded either by a high pulse of duration 1 unit followed by a low pulse of duration 1 unit or by a low pulse of duration 1 unit followed by a high pulse of duration 1 unit. Preferably, the bit pair '11' is encoded either by a high pulse of duration 2 units followed by a low pulse of duration 2 units or by a low pulse of duration 2 units followed by a high pulse of duration 2 units. Preferably the bit pair ' 10' is encoded either by a low pulse of duration 2 units followed by a high pulse of duration 1 unit, a low pulse of duration 1 unit, and a high pulse of duration 2 units or by a high pulse of duration 2 units followed by a low pulse of duration 1 unit, a high pulse of duration 1 unit, and a low pulse of duration 2 units. Preferably, the bit pair Ol' is encoded either by a low pulse of duration 3 units followed by a high pulse of duration 2 units, a low pulse of duration 1 unit, and a high pulse of duration 2 units or by a high pulse of duration 3 units followed by a low pulse of duration 2 unit, a high pulse of duration 1 unit, and a low pulse of duration 2 units.

According to a third aspect of the present invention there is provided a data signal comprising alternate high and low pulses of defined durations wherein: a first binary data bit is encoded by a high or low pulse having a first defined duration; and wherein a second binary data bit is encoded by a high or low pulse having either a second defined duration or a third defined duration wherein the selection of the duration of pulse encoding a particular second data bit is determined as is necessary to maintain a substantially constant mean signal level in a transmitted data stream.

According to a fourth aspect of the present invention there is provided a data signal comprising alternate high and low pulses of defined durations wherein: data is encoded as bit pairs, each bit pair comprising a unique sequence of high and low pulses, each pulse in each bit pair having a defined duration wherein each bit pair has an equal mean signal level.

The signals of the third and fourth aspects of the present invention may incorporate any or all of the features described in relation to the method of the first or second aspects of the invention as appropriate.

According to a fifth aspect of the present invention there is provided a building control system comprising a master unit and a plurality of slave units wherein data is transmitted between the master unit and the plurality of slave units according to the method of the first or the second aspects of the invention or using signals according to the third or fourth aspects of the present invention.

The building control system according to the fifth aspect of the present invention may incorporate any or all of the features described in relation to the first, second, third or fourth aspects of the invention as appropriate. A proportion of the plurality of slave units may be battery powered slave units operable to transmit a message in response to a change in status and further operable to power down once an acknowledgement of their change of status message is received by said master unit. Additionally or alternatively, the slave units may be powered by energy conversion means such as solar panels, RF energy extractors, piezoelectric means etc. Said master unit may comprise or be connected to a controller or processing unit. Preferably said battery powered slave units are user operated switches or similar.

According to a sixth aspect of the present invention there is provided a security access system comprising: a control means and a data reading or data input means wherein data is transmitted between the control means and the data input or data reading means according to the method of the first or second aspects of the present invention or using signals according to the third or fourth aspects of the present invention.

The security access system according to the fifth aspect of the present invention may incorporate any or all of the features described in relation to the first, second, third or fourth aspects of the invention as appropriate.

The security access system may further comprise access enabling means wherein data is transmitted between the control means and the access enabling means according to the method of the first or second aspects of the present invention or using signals according to the third or fourth aspects of the present invention. Preferably, individuals, vehicles or articles desiring access are identified by unique codes which must be validated before access is granted. Said unique identifying codes may be contained within or on a data bearing means presented to a data reading means.

According to a seventh aspect of the present invention there is provided a stock control system wherein data is transmitted between point of sale apparatus, inventory control means and stock picking means according to the method of the first or second aspects of the present invention or using signals according to the third or fourth aspects of the present invention.

The stock control system according to the seventh aspect of the present invention may incorporate any or all of the features described in relation to the first, second, third or fourth aspects of the invention as appropriate.

In order that the invention is more clearly understood, it will now be described further herein, by way of example only and with reference to the following drawings in which:

Figure Ia shows a synchronisation or command pulse;

Figure Ib shows a pulse encoding a first binary digit;

Figure Ic shows a pulse encoding a second binary digit;

Figure Id shows an alternative pulse encoding a second binary digit;

Figure Ie shows a series of pulses encoding a series of first binary digits;

Figure If shows a series of pulses encoding a series of second binary digits; Figure Ig shows a series of pulses encoding a series of binary digits alternating between a first and a second binary digit;

Figure Ih shows the mean DC value of the series of pulses in figure Ig;

Figure Ii shows an alternative series of pulses encoding a series of binary digits alternating between a first and a second binary digit;

Figure Ij shows the mean DC value of the series of pulses in figure Ii;

Figure 2a shows a series of pulses encoding a OO' bit pair;

Figure 2b shows a series of pulses encoding a ' 11 ' bit pair;

Figure 2c shows a series of pulses encoding a ' 10' bit pair; and

Figure 2d shows a series of pulses encoding a '01 ' bit pair.

In a building control system, a master unit or central controller is provided which monitors and or controls the operation of a plurality of slave units connected to or associated with individual items or groups of items. To reduce the complexity of wiring the building, the slave units are connected to the master by wireless links. Some slave units are connected to a mains power supply for an item or group of items and are operable to vary the power supply to the item or items. Other slave units are battery powered but may provide, for example, user switches for controlling the operation of one or more items. Typically, the master and the slaves incorporate Phase Lock Loops for encoding and decoding data transmissions. In an example of a typical implementation, a user may operate a user switch to vary the operation of a light. The user switch is battery powered, and powers up when operated by the user and then transmits a data signal to the master unit. The data signal to the master unit identifies the slave unit and contains information about the change in status occasioned by the user operation of the switch. The master unit transmits an acknowledgement to the slave upon receipt of the message from the slave. When the slave receives the acknowledgement, it powers down. After receiving the message from the battery powered slave, the master unit also transmits a message to a powered slave unit connected to the power supply for the light in question, this message commanding the mains powered slave to vary the power supply to the lamp in the manner requested by the user.

There may be an additional command or synchronisation signal sent by the transmitting unit before the main signal to ensure that both transmitting and receiving units are synchronised or that both units are ready to communicate.

In order to save power, transmissions between at least the battery powered slaves and the master are encoded using a 'fault intolerant' coding and transmission scheme. This reduces the possibility that messages will need to be repeated. Referring now to figure 1, a 'fault intolerant' data transmission method according to the present invention is illustrated.

In figure 1, pulse lengths defining particular binary digits have been defined in terms of a time period T, and pulses are defined as the time period between two transitions between the high and low states. A pulse itself may be either high or low since in the present invention, it is the period of the pulse quantised to integer number of T periods that provides the coding not the polarity

A command or synchronisation pulse Ia has a duration of 4T. A command or synchronisation pulse is therefore a period between successive transitions that measures between 3.5T and 4.5T. The figure shows the two possible equivalent command pulses, first a 'low' command pulse and then a 'high' command pulse. A command or synchronisation pulse is normally sent as a command or synchronisation bit pair or dibit, said command or synchronisation dibit comprising a first command or synchronisation pulse of a first polarity followed by a second command or synchronisation pulse of a second opposite polarity.

A binary digit ' 1 ', is encoded as a pulse of duration 2T as is shown in Fig Ib. as above there are two possible pluses, a 'high' pulse and a 'low' pulse.

Figure Ic shows two possible pulses with a duration of IT which encode the binary digit '0'. Figure Id shows two possible extended pulses with a duration of 3 T, which also encode the binary digit O'. They are labelled here as 'Ox' or 'extended 0'.

Figure Ie shows a waveform encoding a succession of 'O's. Figure If shows a waveform encoding a succession of ' l's.

Figure Ig shows a waveform encoding a succession of '01' pairs. Figure Ih shows short term (solid line) and long term (broken line) mean signal level or DC mean of the waveform Ig. It can be seen that the long term DC mean is gradually rising. The performance of PLLs used to encode and decode the signals is optimised for a relatively constant DC mean and a long term variation in the DC mean can have a detrimental effect on the performance. To improve performance of the system it is thus necessary to maintain a more constant DC mean

This can be achieved by using the 'extended 0' Id. Figure Ii shows a waveform encoding a succession of '01' pairs wherein alternate 'O's being replaced by 'Ox's. The long term DC mean (broken line) of such a waveform, shown in figure Ij is constant. The performance of PLL in encoding or decoding such a waveform is significantly more reliable than that of a PLL encoding or decoding a waveform such as Ig.. hi addition if a DC level error over a message or part of a message is detected, this indicates that there has been a transmission error.

In a second preferred implementation of the invention illustrated by figure 2, the data is grouped into bit pairs, before being encoded and transmitted. Each bit pair encodes two successive binary digits '00', Ol', '10' or '11' and is a combination of alternate high and low pulse having a unique duration. The pulse sequence of each bit pair is adapted such that the overall DC mean of the bit pair is constant and is equal to the overall DC mean of every other bit pair.

Figure 2a shows a '00' bit pair. In the figure, OO' is encoded as a low pulse of duration 1 unit followed by a high pulse of duration 1 unit but may just as well be implemented by a high pulse of duration 1 unit followed by a low pulse of duration 1 unit. The OO' bit pair has an overall duration of 2T.

Figure 2b shows a ' 11 ' bit pair. In the figure, ' 11' is encoded by a low pulse of duration 2 units followed by a high pulse of duration 2 units but may alternatively be encoded by a high pulse of duration 2 units followed by a low pulse of duration 2 units. The ' 11 ' bit pair has an overall duration of 4T.

Figure 2c shows a '10' bit pair. The wave form comprises a pulses encoding ' 1001' in order to make a composite bit pair with a constant DC mean equal to the DC mean of OO' and '11'. In the figure, the Ol' bit pair is encoded as a low pulse of duration 2 units followed by a high pulse of duration 1 unit, a low pulse of duration 1 unit, and a high pulse of duration 2 units, alternatively of course, the '01' bit pair may be encoded as high pulse of duration 2 units followed by a low pulse of duration 1 unit, a high pulse of duration 1 unit, and a low pulse of duration 2 units. The '01 ' bit pair has a total duration of 6T.

Fig 2d shows a Ol' bit pair. The waveform comprises pulse encoding 'OxIOl ' in order to make a composite bit pair with a constant DC mean equal to the DC mean of OO' and '11'. In the figure, the '10' bit pair is encoded as a low pulse of duration 3 units followed by a high pulse of duration 2 units, a low pulse of duration 1 unit, and a high pulse of duration 2 units, alternatively, of course, the ' 10' bit pair may be encoded as a high pulse of duration 3 units followed by a low pulse of duration 2 unit, a high pulse of duration 1 unit, and a low pulse of duration 2 units. The '10' bit pair has a total duration of 8T.

The configuration of these bit pairs, with their unique patterns and lengths ensures that any binary message having an even number of digits may be expressed as a sequence of bit pairs and that such messages can be received and decoded with high accuracy. This is because no single edge error can be misinterpreted as an alternative valid message.

In use, a confirmation 'Acknowledge' signal, also coded in accordance with the teachings of the invention, may be sent from the receiving unit back to the transmitting unit, which thus ensures the message was received and decoded correctly. As this system provides such confidence in the accuracy of decoding, the number of repeat transmissions a transmitting unit makes can be reduced hence reducing the average energy per transmission. This allows battery powered transmitters units to have a longer operational lifespan before battery replacement is required. Additionally or alternatively, it allows the use of transmitter units powered by energy conversion means such as solar panels, RF energy extractors, piezoelectric means etc.

Since any bit error in transmission is detectable and further since no message can be sent, received and acted upon until all transmissions are declared correct a transmission system in accordance with the present invention can be considered a Fault Free Transmission System. Although the present invention has been described with particular respect to its application to building controls, it can of course be applied to any other applications requiring low power high reliability data transmission.

A Fault Free Transmission System of the type described above can also be used in security access systems wherein individuals, vehicles or articles desiring access are identified by unique codes which can be validated before access is granted. Such unique identifying codes can be contained within or on a data bearing means presented to a data reading means. Such data bearing and data reading means include, but are not limited to those using badge readers, transponders, biometric data readers and the like.

A Fault Free Transmission System of the type described above can also be used to communicate with transaction processing apparatus including but not limited to point of sale or stock control apparatus in commercial or retail premises.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiments which have been described by way of example only.