GB2174273A - Method and system of and apparatus for computer LED 220 communication along mains wiring - Google Patents

Abstract

A device is disclosed which enables remote computers to communicate via the mains wiring of a building. The device receives data to be transmitted via an R5232 interface, forms it into packets including an address to which the packet is to be sent, and sends it by impressing a frequency-shift-key modulated signal upon the neutral/earth wires of the mains, Several of the devices have the capability of being connected into a ring main and acting as a computer network. The units contain sufficient local intelligence for the user simply to specify the address to be sent to and then to transmit the data to be sent via an R5232 port. <IMAGE>

Description

SPECIFICATION Method and system of and apparatus for computer communication along mains wiring The invention relates to a method and system of and apparatus for computer communication along mains wiring. It relates in particular to communication by impressing a frequencymodulated packet signal upon the neutra I/earth wires.

In an office or business or home a need frequently exists to provide communication between computers or computer terminals situated in various parts of the building. One way of providing this communication is to provide dedicated cabling between all the required points. This is known as hardwiring. It has the advantage that communication may be at high speed, but is frequently unsuitable for reasons of cost. Hardwiring also has the disadvantage that it is very inflexible: computers or terminals need to be wired up in position and cannot then be moved without very considerable cost.

A second method is to provide a dedicated "ring network". This consists, generally of a ring of screened cable which is wired into the building and is provided with computer access points ("node points") where required. Communication between two computers attached to two of the nodes is as follows: the sending computer transmits the data to be sent to its corresponding node which then converts it to some suitable format and passes it along the ring. To each block of data the node affixes further information, such as the address of the node to which the data is being sent, and a checksum. When the data block reaches the next node along the ring, the address label is read to see whether it is directed to that particular node.If so, the data are accepted, transformed back into a suitable format and passed on to the recipient computer; the receiving node then sends a "data received" signal around the ring which will eventually reach the sending node, confirming that the data have been correctly received. If the data are not addressed to that node they are taken in by the node, regenerated, and passed on (in the same direction) along the ring; the process is then repeated at the next node. If no node accepts the data they are generally allowed to pass around the ring a second time after which, when they return to their sending node, they are stopped and a message is sent back to the user to say that the data, for some reason, have not reached the recipient.

In order to provide a cheaper and more flexible communication system it has been proposed to use the mains wiring of a building as the carrier medium. Such an idea has been in use for some time in the field of internal intercom systems. These transmit a signal by impressing an analogue voltage signal upon the live and neutral wires of the mains wiring which can be picked up anywhere in the building simply by plugging a suitable receiver into any mains socket. Recently a similar system has become available for the transmission of digital computer data in a similar way. It is believed that this system works by impressing a modulated signal onto the live/neutral wires using the method known as frequency-shift keying. This signal can be read by any suitable receiving device (tuned into the correct frequency) which is plugged into the ring main.True networking capability is thus lacking, since individual receivers cannot be independently addressed. However, through the use of several independent frequency-bands several groups of transmitters/receivers may be used at the same time, all those within one band being in communication with each other.

It is one aim of the present invention to provide a cheap and flexible computer communication system which combines the networking capabilities of dedicated ring networks with the ease of use of plug-in mainsborne communications systems. It is a further aim of the invention to provide a self-contained transceiver contained within a mains plug. It is yet a further aim to provide such a system having encryptation facilities.

According to a first aspect of the invention a method of transmitting data between computers comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires by frequency-modulating the impressed signal.

According to a second aspect of the present invention a method of transmitting data between computers comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires by frequency modulating the impressed signal, sensing packets received at each address location, and on so sensing, transmitting a packet received signal addressed to the transmitting location, the transmitting location sensing packets received and on not receiving a packet received signal, retransmitting the packet, after a time interval, and on still receiving no packet received signal retransmitting again, sensing retransmissions and providing an indication of such a condition.

According to a third aspect of the invention a data communication unit is provided with plug connectors for a mains socket and providing, within a housing of a size such as to allow direct plugging of the unit into a mains socket: a data input/output interface and a mains socket for receiving or transmitting data from or to a computer and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains; means for frequency-modulating the impressed signal; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet;; means for transmitting a signal to denote the reception of a packet of data; means for sensing the receipt of a message received signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval.

According to a fourth aspect of the invention a system for transmitting data between computers comprises an a.c. mains supply circuit, and a number of power input/output locations in the circuit, and N data communication units (where N is greater than one) each being directly connectable to the mains circuit at any of the power input/output locations, each data communication unit providing within a housing of a size such as to allow direct unsupported connection to a power input/output location; a data input/output interface and a mains socket for receiving or transmitting data from or to a computer adapted to communicate via the interface and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains;; means for frequency modulating the impressed signal; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet; means for transmitting a signal to denote the reception of a packet of data; means for sensing the receipt of a messagereceived signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval.

According to a fifth aspect of the invention a mains-socket-connectable communication device is arranged to communicate with another such device at another socket in a ring main circuit via the neutral and earth wires and provided with an RS232 interface by which data can be passed to or from a data producing or receiving unit, the device being arranged to produce packets of information and to transmit these via a frequency modulated carrier impressed on the neutral and earth wires.

According to a sixth aspect of the invention a method of transmitting data between computers on a network comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires to the selected address location for the packet by frequency-modulating the impressed signal, sensing all packets received at each address location, identifying any packets addressed to that location and on so sensing, transmitting a packet-received signal addressed to the transmitting location, the transmitting location sensing all packets received and on not receiving a packet received signal, retransmitting the packet after a time interval of random length, and on still receiving no packet received signal retransmitting again, sensing retransmissions and providing an indication of such a condition.

According to a seventh aspect of the invention a system for transmitting data between computers on a computer network comprising an a.c. mains supply circuit, and a number of power input/output locations in the circuit, and N data communication units (where N is greater than one) each being directly connectable to the mains circuit at any of the power input/output locations, each data communication unit providing within a housing of a size such as to allow direct unsupported connection to a power input/output location; a data input/output interface and a mains socket for receiving or transmitting data from or to a computer adapted to communicate via the interface and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains; ; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet; means for detecting whether the packet is addressed to the receiving unit and, if not, for rejecting the packet; means for transmitting a signal to denote the reception of a packet of data; means for sensing the receipt of a messagereceived signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval of random length.

The invention in the present case is intended to include any and all of the features herein disclosed, either alone or in any complatible combination.

The present invention has advantages over the prior art which include cost, flexibility and ease of use. None of the known prior art systems capable of communication along mains wiring are capable of acting as a full network without external programming of the host computers. The present invention, with suitable programming, may act as a full communications network in which the only information that has to be provided by the user on an external computer is the address of the unit to which the data are to be sent and, if desired, an encryptation key.

The invention may be carried into practice in various ways and one specific embodiment will now be described, by way of example, with reference to the accompanying diagrams, in which Figure 1 is a perspective view of a mains plug embodying the present invention; Figure 2 is a block diagram representing, in schematic form, the method of operation; Figure 3 is a circuit diagram of the power supply to the unit; Figure 4 is a circuit diagram of the carrier detector circuit; Figure 5 is a circuit diagram of the band pass filter; Figure 6 is a circuit diagram of the demodulator circuit; Figure 7 is a circuit diagram of the modulator circuit; Figure 8 is a circuit diagram of the line interface; Figure 9 is a circuit diagram of the LED circuit; Figure 10 is a circuit diagram showing the microprocessor included in the unit, and the connections to the R5232 interface.

Figure 11 is a block diagram of the main control loop; Figure 12 is a block diagram of data input on the RS232; Figure 13 is a block diagram of mains input and RS232 output; and Figure 14 is a block diagram of mains message output.

There are several advantages in using the neutral/earth wires to transmit data, over the live/neutral wires generally used at present.

The first of these is that the signal is less likely to leak from the building. The reason for this is that all recent mains-wiring includes a PME (protected multiple earth): a point at which the earth and neutral wires are physically connected. Clearly any signal sent into the ring main of a building will be able to pass this point and escape into the external supply only after great attenuation; no such attenuation is provided if one uses neutral/live wires. Secondly, the use of neutral/earth provides some measure of safety over neutral/live, and it will also, to some extent act as a confirmation of the integrity of the earth wiring. If the system is inoperative it is possible that a fault exists in the earth.

Should it be necessary to ensure absolute data security, any signal escaping from the ring main can be removed entirely by the provision of a suitable filter. Pairs of such filters could also, if desired, be used to isolate sections of the ring main, to allow a particular region to be dedicated to a particular use.

Referring to Figure 1 a plug 10 is shown embodying the invention. The plug 10 includes a housing 20, within which is housed all the electronics necessary to the invention. The plug includes the usual three pins 30 for connection to a standard mains socket, and sockets 40 to allow attachment of a standard 13 amp mains plug. In the front face of the housing is mounted an RS232 interface socket 50 and an LED 60.

In use, the unit is plugged into a mains socket, and the interface socket 50 is attached to the computer 1/0 port. If desired, the mains power to the computer may be provided via the sockets 40. A second unit is similarly connected to a second computer and plugged into a mains socket elsewhere in the building. Communication between the two computers is achieved simply by keying in at the terminal the address of the unit to which the data are to be sent and then routing the data to be sent to the 1/0 port. The unit then converts this into a packet format and transmits it into the ring main, via the earth and neutral wires. It may then be picked up by the second unit, where it is converted back by demodulator into a format understandable by the computer, and passed on via the corresponding RS232 interface.Suitable programming will allow the unit to read the address of any data on the mains line and to reject it if they are not addressed to the reading unit.

This can be done without fully decoding and reading the data, if necesary. The microproccessor and the computer are thus not kept busy reading irrelevant data, as may be the case with prior art units.

The data sent out on the ring main are formatted using frequency-modulation ("frequency-shift keying"). The signal to be sent is first formed by the unit into a "packet". This consists of a data-section of between one and eight characters, an address character (indicating the unit to which the packet is directed), a status character (specifying the number of data characters present in the packet, and whether the packet is a new message or is simply a reply to an earlier one containing no data and a check sum character. The packet is then set out by impressing an AC signal on the earth and neutral wires of the ring main.

Two frequencies are used: 129 kHz, which represents a "1"; and 121 kHz, which represents a "O". The power transmitted is less than 5mW which represents an imposed potential difference of typically less than half a volt. With this power output, communication can be effected over a range of at least several hundred metres. Speed of transmission between computer equipment is at 9600 baud. The units may be used between any two suitable computers having an RS232 interface. Naturally, other interfaces may be provided if desired, and the system may also be used for transmitting the receiving information to and from devices other than computers: for example it may be convenient to use a pair of units to transmit information from a computer to a printer in another part of the building.

When a receiving unit receives a packet directed to it (other than a simple reply), it responds by returning a "message received" signal to the sending node. If, after a certain "random" time interval, this signal has not been received by the sending unit, it is programmed to repeat the message. No further packets are transmitted by a unit if a response is still awaited from a previous message. This system allows virtually 100% data transfer security whatever the noise present on the ring main. Even if a large amount of noise is present the performance is not significantly degraded. If there is much traffic being sent, the effective transmission speed will naturally decrease somewhat due to the larger probability of two packets "colliding". However, this is unlikely to interfere significantly with the ease of use of the units or indeed to be apparent to the user.

The use of unit addresses and suitable programming may allow many units to be connected to the ring main and for communications to be possible between any two of them. It is also possible, by the use of different frequency-bands to provide several entirely independent sets of units. The advantage of using differeing frequencies is that units of one set can communicate with each other without being affected by traffic on the mains at another frequency: filters are provided within each unit to remove unwanted frequencies. For example four units could be connected to the same ring main, one pair operating at one frequency and one at another.

Since there can be no "collisions" of packets at different frequencies, each pair of units appears to have the ring to itself. The present invention will allow the use of up to 16 pairs of units operating on a single ring main in a single band, and probably more. Three separate bands may be provided, centred, for example at 115, 125 and 135 kHz.

Secure communication may be effected by encrypting the data to be sent before modulating it. Naturally the unit to which the data are sent will need corresponding decryptation facilities. Encryptation may be provided by suitably programming the microprocessor included in each unit: something that will be evident how to achieve to the man skilled in the art. In order to be able to read the encrypted data being sent, each unit will need to be provided with a key to the encryption. This could be done in a variety of ways, for example, allowing some limited programming of the node microprocessor from the keyboard. A daily key could be provided, for instance, which is to be typed in at each of the intercommunicating terminals each morning.

Alternatively, the microprocessors could be individually programmed with a different encryption procedure for each use who requests it.

Referring now to Figure 2 a schematic block diagram of the unit electronics is shown.

Power is provided by a power supply 70 which draws power from the mains (labelled L and N) and supplies it to the other components contained within the unit. The unit is thus completely self-contained and needs no other input other than a source of RS232 data. Data are supplied to the unit via an RS232 interface 80 and passed along lines 90 to a microprocessor 180, within which the data are split up into packets, and have added to them address, status and checksum characters. Encryption may also be provided, if required, by the microprocessor. The packets are then passed along a data-out line 110 to a modulator 120. This takes the raw datastream and provides an output alternating between and upper and a lower frequency to represent "1" 's and "O"'s respectively.The modulated output is passed to a line interface 130 which is essentially a high-frequency coupling transformer. This provides mains isolation for the rest of the components within the unit. The output signal is thus impressed on the neutral and earth wires (N and E) of the mains ring by the interface 130.

When the unit is acting to receive data from the ring main it functions as follows. The signal is picked up by the line interface 130 and passed to a band-bass filter 140. The interface circuit itself acts as an filter and removes much of the noise on the line. The active bandpass filter 140 is tuned to allow signals to pass only if they are of interest. Thus, noise and frequencies in other bands are eliminated. The output from the filter is passed along lines 150 and 160 to a demodulator 170 and a carrier dectect circuit 180, respectively. The carrier detect circuit acts rapidly to determine whether the output from the filter 140 is a signal or not. If it is, it signals this by sending a "carrier-present" signal to the microprocessor 100 along a line 190, warning it that a message has been received. The demodulator 170 decodes the signal from the filter 140 and passes it on along a line 200 to the microprocessor where any necessary de crypting may be done. The recovered data are then sent along lines 210 to the RS232 interface 80 and along to the receiving computer.

The microprocessor is programmed to provide a suitable signal to an LED circuit 220, enabling the user to ascertain the status of the unit by means of the LED 60.

Turning now to Figures 3 to 10, these show in detail some specific electronics which may be used to perform the operations set out in the block diagram of Figure 2. Each figure represents one of the blocks of Figure 2.

In the description of Figures 3 to 10, for ease of reference, the hundreds unit of the reference numerals will be equal to the figure number.

Figure 3 shows details of the power supply circuit. This draws power from the mains wiring 302 via a step-down transfer 304. The terminals of this are connected to a rectifier bridge which provides a rectified signal between the lines 308 and 310. A parallel capacitor 312 acts as a smoothing capacitor to produce a relatively ripple-free output at +V volts along a line 314. This output is also fed to a voltage regulator 316 (for example of 7805 type) which produces a further output at +5V along a line 318. Further smoothing is provided by capacitors 320, 322 and 324, connected between the +5V and OV lines.A signal at --V volts is also provided via a negative-pass diode 326 connected between the OV lines 310 and a -V volt line 328 and via a capacitor 330 connected between one output of the transformer 304 and the -V volt line 328. A further diode 332 is provided in the line 320, and smoothing is achieved by capacitors 334, 336.

Figure 4 shows the carrier detection circuit.

The filtered output from the bandpass filter (shown in Figure 5) passes through a peak detector circuit comprising a voltage follower 402, a series resister 404, and a parallel capacitor 406 and resister 408 connected between the output of the voltage follower 402 and a OV line 410. The output of this circuit passes along a line 412: it will be low when no carrier is present, and high when one is. In order to provide a very sharp on/off signal, the output along line 412 is fed into the inverting input of an amplifier 414. The noninverting input is clamped at a fixed level between 0 and +5V by means of a voltage divider 416, 418. The switched output from the amplifier 414 passes through a resistor 420 and along an output line 422; the swing is limited by means of diodes 424, 426.

Thus, a signal entering the carrier-detector circuit is marked by a low voltage on the line 422; otherwise this line remains high.

Figure 5 shows the bandpass filter. This receives a raw signal picked up from the mains wiring along an input line 502 and outputs a filtered signal, to the carrier detection circuit (Figure 4) and to the demodulator (Figure 6), along an output line 504. The circuit comprises two identical standard second-order active bandpass filters connected in series, along with additional feedback from the output line 504 by means of a resister 506. The feedback resister and other components are chosen to provide a high 0, narrow band response, but with sufficient stability.

Figure 6 shows the demodulation circuitry.

This comprises a demodulator, 602 acting as a phase locked loop, and supporting circuitry.

The demodulator is an XR-2211, and is made by Exar; the operation, connection and supporting circuitry of the XR-2211 are all standard, and are described elsewhere.

Figure 6 shows the modulation circuitry.

This comprises a modulator 702, acting as a voltage controlled asullator, and supporting circuitry. The modulator is an XR-2207; the operation, connections and supporting circuitry are all standard and are described elsewhere.

Figure 8 shows the line interface between the unit and the mains wiring 802. Within the unit the mains wiring is connected in series with a transformer 804 and a capacitor-resistor combination 806. The purpose of the capacitor (which is of large value) is to present a high impedance to the mains wiring to prevent any 50Hz mains signal from reaching the transformer. It is of the type which, if it blows, produces an open circuit. The resistor prevents the capacitor from retaining any charge, and so acts to prevent a user from being shocked if he touches the neutral and earth pins 30 (see Figure 1). The other winding of the transformer 804 is held at +5V, and any signal picked up from it is passed along a line 806 to the carrier-detector circuit.

A capacitor 808 connected across the transformer windings produces a tuned circuit, causing the transformer to act like a high impedance, when the unit is transmitting, a modulated signal is passed along a line 810 via a resistor 812 to the base of the transister 814. This causes a corresponding voltage to be applied to the tuned circuit via a resister 816, thus impressing a signal upon the mains. The transistor 814 emitter is connected to the OV rail.

Figure 9 shows the LED circuitry used to illuminate the LED 60 (see also Figure 1). Control signals are passed down a line 904 from the microprocessor and are applied to the base of a transistor 902, causing current to flow from the +5V line, via the LED 60 and a resistor 906 to the OV line. A resistor 908 is also provided between the line 908 and the +5V rail. Figure 10 shows the microprocessor and the connections to the RS232 interface.

The microprocessor 1020 is of type 8035 and it takes its instructions from an EPROM 1040 (of type 2716) via an 8-bit latch 1060 (of type 74LS373N). The connections between these components and the means whereby the microprocessor retrieves information from the EPROM are entirely standard. Connected to the microprocessor 1020 is a 6MHz crystal 1080 which provides the necessary timing information. The reset connection of the microprocessor is connected in between a diode 1160 and a capacitor 1170 in series between the +5V and OV rails. Thise ensures that the microprocessor receives a reset signal on power-up. The input and output connections with the rest of the unit are those five at the bottom right of the diagram. The input lines are 1100 (which receives demodulated data) and 1110 (which receives a "carrier present" signal).The output lines are 1114 and 1116; these are on/off and data-out lines, which pass information to be modulated to the modulator (Figure 7). The line 904 is the signal line which controls the LED 60 (see Figure 9).

The RS232 interface is shown at upper right, and the electronics connected between the interface and the microprocessor act to provide and accept signals to and from the RS232 interface which comply with the standard specifications. The used data-sockets of the interface and their corresponding lines are 1120 (received data), 1140 (transmitted data), 1130 (received handshake) and 1150 (transmitted handshake). The handshake lines are generally low; when the line 1130 is set high by a source external to the interface (for example an instructing computer), the microprocessor 1020 is informed that the computer is ready to receive data. Likewise, the line 1150 is set by the microprocessor to indicate whether the unit is ready to accept data.The receiving lines 1120 and 1130 are connected to the relevant terminals of the microprocessor 1020 via resistors 1180 and 1190, and they are also clamped between the OV and +5V rails by diodes 1200, 1210, 1220 and 1230. These act to protect the microprocessor against unexpected inputs. The output lines 1140 and 1150 are switched between high and low values by means of amplifiers 1240 and 1250. These are driven by lines 1260 and 1270 attached to their inverting inputs. The non-inverting inputs are held at a fixed level by means of a potential divider comprising resistors 1280 and 1290. Protection resistors 1300 and 1310 are also provided.

Some of the data connections 1320 of the microprocessor are unused. Thse are, at present, linked to a line 1330, but could, if desired be provided with similar electronics to that described above and used as lines for accepting addressing information. (Thus, the line 1330 is intended to represent suitable conventional electronics). The connections could for example, pass information on the address of the unit from which data has been received, or to which it is to be sent. They could also be used to pass other control information to enable a user to control a remote unit in a variety of ways. In each case, the only modifications needed would be to the software and the provision of (conventional) electronics to ensure RS232 specification 1/0.

Figures 11 to 14 illustrate, in flow diagram form, the method used for receiving and transmitting data.

Figures 11 shows the start-up procedure and the main loop. On power-up, the control lines are set to their correct state and the memory initialized. Then follows the main loop. A test is made for data on the mains (by reading the lines 1100 and 1110; see Figure 10). If there is some it is read in, if not the microprocessor checks whether it is expecting a reply from any earlier message it may have sent. If a reply is expected, a timer (based on the 6mHz crystal 1080; see Figure 10) will be in operation. If it is the program returns to point A to await it unless a "timeout" has occured, when it will repeat the message and try again. If no reply is expected, a test is made of the RS232 input lines to see if data are waiting to be input. If so, they are read in; otherwise the program loops back to point A.It should be noted that while in the timing loop awaiting a reply, the unit will not send out any further data until the reply is received. A timeout effectively means that it has given up awaiting a reply, and the LED 60 (see Figures 1 and 9) will flash to signify this.

Figure 13 shows the steps followed if data are detected on the mains. They are first read by the microprocessor via the line 1100 (Figure 10), and the packet checksum and status character are read. If an error is detected in the data, they are ignored entirely; control then passes to the main control loop if a reply is not being awaited, otherwise the message is transmitted again. On receipt of an uncorrupted packet the status character is checked to see whether it consists of data or is simply a reply to an earlier message. If the latter no action need be taken, and control returns to the main loop. Otherwise the data are checked to see whether they consist of simply a new message or whether they are a combined message and reply to an earlier one. If there is a new message it is output to the RS232 port. A flag is set to indicate that a message has been received to which a reply will be required. A test is then made whether the program is awaiting a reply from an earlier sent-out message. If so it is sent out immediately. Otherwise a test is made for any waiting RS232 data and they are read in if any exist; the message to be sent out is then output to the mains.

Figure 12 indicates the procedure for reading in RS232 data. The data are first input along the line 1120 (Figure 10) and are then checked. If no understandable data are found, control returns to the main loop, otherwise a message counter is incremented by 1 and the reply flag is cleared. If the mains line is clear at this point the message just received from the RS232 is output, otherwise the message is held and any further messages on the mains are read in first.

Figure 14 illustrates message output along the mains wiring. If, when a message is to be sent, there are no RS232 data waiting the message must be a simple reply. A packet is formed including a status word indicating that the message is a reply, and a checksum. If, on the other hand, data are to be sent, the status word is set to reflect this, a checksum is formed and a reply request set. The timer is then randonized and started: this will later be used to indicate whether a reply to this message has been received. The message, or reply, is sent out when the mains line is clear (any incoming message being read in first).

Reference to a time interval of random length is intended to include a time internal chosen according to any pseudo-random process or algorithm. References to a computer to be connected to the unit are intended to include references to any device capable of acting in response to a stream of coded digital data, such as a printer or memory, as well as devices which generate such streams of coded digital data.

Claims (12)

1. A method of transmitting data between computers which comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires by frequency-modulating the impressed signal.

2. A method of transmitting data between computers which comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires by frequency modulating the impressed signal, sensing packets received at each address location, and on so sensing, transmitting a packet received signal addressed to the transmitting location, the transmitting location sensing packets received and on not receiving a packet received signal, retransmitting the packet, after a time interval, and on still receiving no packet received signal retransmitting again, sensing retransmissions and providing an indication of such a condition.

3. A method as claimed in Claim 2, in which the time interval is of random length.

4. A data communication unit provided with plug connectors for a mains socket and providing, within a housing of a size such as to allow direct plugging of the unit into a mains socket: a data input/output interface and a mains socket for receiving or transmitting data from or to a computer and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains; means for frequency-modulating the impressed signal; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet; means for transmitting a signal to denote the reception of a packet of data;; means for sensing the receipt of a message received signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval.

5. A unit as claimed in Claim 4, in which the time interval is of random length.

6. A system for transmitting data between computers comprising an a.c. mains supply circuit, and a number of power input/output locations in the circuit, and N data communication units (where N is greater than one) each being directly connectable to the mains circuit at any of the power input/output locations, each data communication unit providing within a housing of a size such as to allow direct unsupported connection to a power input/output location; a data input/output interface and a mains socket for receiving or transmitting data from or to a computer adapted to communicate via the interface and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains; means for frequency modulating the impressed signal;; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet; means for transmitting a signal to denote the reception of a packet of data; means for sensing the receipt of a messagereceived signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval.

7. A system as claimed in Claim 6, in which the time interval is of random length.

8. A mains-socket-connectable communication device arranged to communicate with another such device at another socket in a ring main circuit via the neutral and earth wires and provided with an RS232 interface by which data can be passed to or from a data producing or receiving unit, the device being arranged to produce packets of information and to transmit these via a frquency modulated carrier impressed on the neutral and earth wires.

9. A method of transmitting data between computers substantially as specifically herein described with referene to the drawings.

10. A data communication unit, or a system for transmitting data between computers, or a mains socket connectable communication device substantially as specifically herein described with reference to the drawings.

11. A method of transmitting data between computers on a computer network which comprises impressing, at an address location on the mains, a low power a.c. signal on the neutral and earth wires of the mains, converting the data into addressed packets and transmitting the data along the neutral and earth wires to the selected address location for the packet by frequency-modulating the impressed signal, sensing all packets received at each address location, identifying any packets addressed to that location and on so sensing, transmitting a packetreceived signal addressed to the transmitting location, the transmitting location sensing all packets received and on not receiving a packet received signal, retransmitting the packet after a time interval of random length, and on still receiving no packet received signal retransmitting again, sensing retransmissions and providing an indication of such a condition.

12. A system for transmitting data between computers on a computer network comprising an a.c. mains supply circuit, and a number of power input/output locations in the circuit, and N data communcation units (where N is greater than one) each being directly connectable to the mains circuit at any of the power input/output locations, each data communication unit providing within a housing of a size such as to allow direct unsupported connection to a power input/output location; a data input/output interface and a mains socket for receiving or transmitting data from or to a computer adapted to communicate via the interface and for powering the computer, or only a data input/output interface; means for impressing a low power a.c. signal on the neutral and earth wires of the mains while isolating the unit electrically from the mains; means for frequency modulating the impressed signal; means for converting data from the computer into packets or for converting packets received from the mains into data usable by the computer; means for addressing the packets; means for sensing the address of a received packet; means for detecting whether the packet is addressed to the receiving unit and, if not, for rejecting the packet; means for transmitting a signal to denote the reception of a packet of data; means for sensing the receipt of a messagereceived signal; and means for retransmitting the last transmitted packet if no message received signal is sensed after a time interval of random length.