GB1567971A - Data transmission systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO DATA TRANSMISSION SYSTEMS (71) We, THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex CMl 1PL, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to data transmission systems, and covers in particular data transmission control and monitoring systems.

In many data transmission systems data signals are passed between a plurality of remote units and a central unit. The data signals may relate to parameters monitored at the remote units, and/or they may be data signals relating to control functions to be carried out at any of the remote units on the command of said central unit. Some such systems are utilised not only for the purpose of data transmission as such but also as parts of various process control systems, where it is required to monitor various parameters at remote locations, display these at a central location, and from the central location effect control functions at the remote locations.

Hitherto, in order to achieve connections between the remote units and the central unit (or, in the case of a data transmission system as such, between units of a data highway), considerable quantities of cabling have been required, involving relatively complex layouts.

It has been common either to employ a separate cable running to each remote unit or to utilise a highway cable system (in which the cables for the separate units are united into one harness which extends from remote location to remote location, with the relevant cable connected to the particular remote unit).

One object of the present invention is to provide an improved data transmission system in which such complication is avoided.

Accordingly, this invention relates generally to a data transmission system wherein data signals are passed between a plurality of remotely located remote units and a central monitoring unit, in which system the remote units are connected in succession in a loop of transmission line terminated at either end by the central unit whereby data signals may be passed between any remote unit and the central unit over a common path in either direction around the loop.

The data signals may be data signals relating to parameters monitored at the remote units, and/or data signals relating to control functions to be carried out at any of the remote units by command of the central unit.

Preferably the transmission line is a multi-path transmission line one path of which is utilised for the passage of data signals and another path of which is utilised to supply operating power from the central unit in turn to the remote units in the loop in either direction around the loop.

Normally another of the paths is arranged to be at common potential.

The transmission line may take any convenient form, e.g. strip line. wire cable or co-axial.

Like all electrical systems. that of the invention is vulnerable to circuit breaks or to short circuits, but as regards the former it is very much less vulnerable than most because both power is transmitted and data is transmitted/received in both directions around the loop.

Thus, if anywhere in the loop there is a circuit break, there is still a connection one way round the loop to the units on that side of the break, and the other way to the units on the other side.

Even so, the system of the invention is still vulnerable to short circuits, and in order to minimise its degree of vulnerability (in a manner to be described hereinafter) it is provided that the central unit include means for monitoring the level of data voltage in the loop, and/or the level of power current supplied by it to the remote units, whereby if the former is below a predetermined value and/or the latter is above a predetermined value (either indicating a short circuit) the central unit interrupts the supply of power and data to the loop. The arrangement of the central unit is preferably such that, following an interruption as last mentioned, power and data are re-applied a predetermined number of times before they are permanently interrupted.

Moreover, in combination with the central unit monitoring means it is provided that each of the remote units include, in each of the data and/or power paths through the unit, a resistor shunted by a switch, the remote unit also including means for monitoring the level of data voltage across and/or power current through the resistor(s) and means for rendering the switch conductive if the data voltage level is above and/or the power current level below a predetermined value (indicating that the next following remote unit in the loop is not exhibiting a short circuit condition).

In each case the resistor advantageously has a value approximately ten times the relevant loading impedance of the next following remote unit.

In either direction around the loop the power and data paths are completed through each remote unit in turn; only if the monitoring means indicate that the next remote unit is not short-circuited are the connections to that next unit duly made.

The invention is illustrated in and further described with reference to the drawing accompanying the Provisional Specification. which drawing is a block schematic diagram of one process control and monitoring data transmission system in accordance with the present invention.

Referring to the drawing, the system is provided to monitor various parameters at a plurality of remote locations and transmit these for display at a central location, and also to transmit control signals from the central location to the remote locations. For this purpose, at the central location is provided a central control and monitoring unit 1, whilst at each remote location a remote monitoring and slave control unit 2A, 2B, 2C, 2D and 2E is provided.

The central unit 1 and each of the remote units 2A to 2E are connected in succession by a multi-path cable 3 in a single loop. The multi-path cable 3 includes a power path 4 for power supplied by the central unit 1, a data path 5 both for data control signals supplied by the central unit 1 to each of the remove units 2A to 2E and for data monitoring signals supplied by each of the remote units 2A to 2E to the central unit 1. and a common or reference potential path 6.

The data signals transmitted to and from the remote units 2A to 2E are multiplexed as known per se, the data signals for, and from, a particular remote unit being identified by a unique code. This aspect of the present invention will not be described in detail in view of the fact that standard procedures are followed.

As so far described, it will be apparent that if the multi-path cable 3 becomes completely disrupted between any two units (central or remote), or if one remote unit becomes open circuited. the whole system will continue to function normally since both power and communication with the central unit 1 will be maintained via the remaining two portions of the loop. Moreover, if the multi path cable 3 becomes disrupted in two places, or if two remote units become open circuited, only units between the two faults will be effected.

However, the system will still be prone to serious disruption if a short circuit occurs either in a remote unit or at some part of the cable 3. The detail of the present invention now to be described renders the system less subject to serious disruption in the event of such a short circuit.

The central unit 1 comprises: a console 7 including display, control and sensing apparatus; power supplies 8 and 9 feeding power to either end of path 4 of the multi-path cable 3; data transmitters 10 and 11 for applying data control signals to either end of the path 5 of the multi-path cable 3; and data receivers 12 and 13 for receiving data monitoring signals from either end of path 5 of the multi-path cable 3. The sensing apparatus forming part of console 7 is provided to sense power current above a predetermined maximum flowing into either end of path 4 and to sense data signal voltage below a predetermined minimum received from either end of path 5 of multi-path cable 3.

Each of the remote units 2A to 2E contains apparatus similar to that shown in detail only for remote unit 2A. Within each remote unit is the combination of a resistor 14 shunted by a switch 15 which completes power path 4 through the unit. The combination of a resistor 17 shunted by a switch 16 completes the data path 5 through the unit.

Resistors 14 and 17 each has a value equal to approximately ten times the loading impedance of the next remote unit in the loop.

Controlling the switches 15 and 16 (which are ganged to be open or closed together) is a sensing and control arrangement 18 which is connected to receive power from either end of the power path 4 via rectifiers 19 and 20. Sensing and control arrangement 18 is connected to derive an input from across resistor 14 to provide a power check and to derive an input from across resistor 17 to provide a data check.

Within the remote unit are also multiplexing circuits for introducing data signals onto data path 5 relating to parameters monitored at the remote location, and for taking off from data path 5 data signals from the central unit 1 and relating to control functions required to be carried out at the remote location. Also other processing functions and connections to sensing/controlling units may be included. These circuits are not shown, however.

The operation of the system will now be described.

Prior to "switch-on", switches 15 and 16 are open (i.e. non-conductive) in all of the remote units 2A to 2E, as represented. Central unit 1 applies power to both ends of the power path 4 of multi-path cable 3, and control data for the remote units 2A to 2E to both ends of the data path 5 of multi-path cable 3. The resultant power current and data voltage is sensed by console 7. If the power current is above, or at any time exceeds, a predetermined level, or if the data voltage as received by data receivers 12 and 13 is below, or at any time falls below, a predetermined value the arrangement is such that both power and data are interrupted (since either represents a possible short circuit condition). Power and data are re-applied for a pre-determined number of times before console 7 effects a permanent shut down pending investigation by an operator.

Assuming that the above power current and data voltage are satisfactory, and power and data are maintained, power and data will firstly reach remote units 2C and 2D. At each, either diode 19 or diode 20 applies energising power to sensing and control arrangement 18.

Sensing and control arrangement 18 thereupon monitors across resistors 14 and 17 to check that the power current and data voltage are satisfactory. If so, switches 15 and 16 are closed (rendered conductive), and power and data are passed on to the next remote unit in each case (i.e. remote unit 2B and 2E). The process is then repeated for as many remote units as there are in the loop, until the loop is complete.

If any remote unit senses that the next following remote unit is shorting, switches 15 and 16 of that unit will not close, and the faulty unit should be isolated automatically from each end of the loop.

As has already been mentioned, if a shorting fault ocurs at any time after the loop has been completed, console 7 at central unit 1 interrupts the supply of power and data. All of the switches 15 and 16 in the remote units open, and the starting up procedure is commenced, which should isolate the remote unit exhibiting or adjacent the fault. At the central unit 1 it is possible to determine which of the remote units is short circuiting or adjacent a short circuit in the cable 3 since of course data will not be recovered from that remote unit. Also, in the case of a single cable fault, console 7 can recognise that remote unit data is not received at both of receivers 12 and 13.

WHAT WE CLAIM IS: 1. A data transmission system wherein data signals are passed between a plurality of remotely located remote units and a central monitoring unit, in which system: the remote units are connected in succession in a loop of transmission line terminated at either end by the central unit whereby data signals may be passed between any remote unit and the central unit over a common path in either direction around the loop; the central unit includes means for monitoring the level of data voltage in the loop and/or the level of power current supplied by it to the remote units, whereby if the former is below a predetermined value and/or the latter is above a predetermined value the central unit interrupts the supply of power and data to the loop; and each of the remote units includes, in each of the data and or power paths through the unit, a resistor shunted by a switch, the remote unit also including means for monitoring the level of data voltage across and/or of power current through the resistor(s)and means for rendering the switch conductive if that data voltage level is above, and/or that power current level below, a predetermined value.

2. A system as claimed in claim 1 and wherein the transmission line is a multi-path transmission line, one path of which is utilised for the passage of data signals and another path of which is utilised to supply operating power from the central unit in turn to the remote units in the loop in either direction around the loop.

3. A system as claimed in claim 2 and wherein another of the paths is arranged to be at common potential.

4. A system as claimed in any of the above claims and wherein the arrangement of the central unit is such thàt, following an interruption power and data are re-applied a predetermined number of times before they are permanently interrupted.

5. A system as claimed in any of the above claims and wherein each remote unit resistor has a value approximately ten times the relevant loading impedance of the next following

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. Controlling the switches 15 and 16 (which are ganged to be open or closed together) is a sensing and control arrangement 18 which is connected to receive power from either end of the power path 4 via rectifiers 19 and 20. Sensing and control arrangement 18 is connected to derive an input from across resistor 14 to provide a power check and to derive an input from across resistor 17 to provide a data check. Within the remote unit are also multiplexing circuits for introducing data signals onto data path 5 relating to parameters monitored at the remote location, and for taking off from data path 5 data signals from the central unit 1 and relating to control functions required to be carried out at the remote location. Also other processing functions and connections to sensing/controlling units may be included. These circuits are not shown, however. The operation of the system will now be described. Prior to "switch-on", switches 15 and 16 are open (i.e. non-conductive) in all of the remote units 2A to 2E, as represented. Central unit 1 applies power to both ends of the power path 4 of multi-path cable 3, and control data for the remote units 2A to 2E to both ends of the data path 5 of multi-path cable 3. The resultant power current and data voltage is sensed by console 7. If the power current is above, or at any time exceeds, a predetermined level, or if the data voltage as received by data receivers 12 and 13 is below, or at any time falls below, a predetermined value the arrangement is such that both power and data are interrupted (since either represents a possible short circuit condition). Power and data are re-applied for a pre-determined number of times before console 7 effects a permanent shut down pending investigation by an operator. Assuming that the above power current and data voltage are satisfactory, and power and data are maintained, power and data will firstly reach remote units 2C and 2D. At each, either diode 19 or diode 20 applies energising power to sensing and control arrangement 18. Sensing and control arrangement 18 thereupon monitors across resistors 14 and 17 to check that the power current and data voltage are satisfactory. If so, switches 15 and 16 are closed (rendered conductive), and power and data are passed on to the next remote unit in each case (i.e. remote unit 2B and 2E). The process is then repeated for as many remote units as there are in the loop, until the loop is complete. If any remote unit senses that the next following remote unit is shorting, switches 15 and 16 of that unit will not close, and the faulty unit should be isolated automatically from each end of the loop. As has already been mentioned, if a shorting fault ocurs at any time after the loop has been completed, console 7 at central unit 1 interrupts the supply of power and data. All of the switches 15 and 16 in the remote units open, and the starting up procedure is commenced, which should isolate the remote unit exhibiting or adjacent the fault. At the central unit 1 it is possible to determine which of the remote units is short circuiting or adjacent a short circuit in the cable 3 since of course data will not be recovered from that remote unit. Also, in the case of a single cable fault, console 7 can recognise that remote unit data is not received at both of receivers 12 and 13. WHAT WE CLAIM IS:

1. A data transmission system wherein data signals are passed between a plurality of remotely located remote units and a central monitoring unit, in which system: the remote units are connected in succession in a loop of transmission line terminated at either end by the central unit whereby data signals may be passed between any remote unit and the central unit over a common path in either direction around the loop; the central unit includes means for monitoring the level of data voltage in the loop and/or the level of power current supplied by it to the remote units, whereby if the former is below a predetermined value and/or the latter is above a predetermined value the central unit interrupts the supply of power and data to the loop; and each of the remote units includes, in each of the data and or power paths through the unit, a resistor shunted by a switch, the remote unit also including means for monitoring the level of data voltage across and/or of power current through the resistor(s)and means for rendering the switch conductive if that data voltage level is above, and/or that power current level below, a predetermined value.

2. A system as claimed in claim 1 and wherein the transmission line is a multi-path transmission line, one path of which is utilised for the passage of data signals and another path of which is utilised to supply operating power from the central unit in turn to the remote units in the loop in either direction around the loop.

3. A system as claimed in claim 2 and wherein another of the paths is arranged to be at common potential.

4. A system as claimed in any of the above claims and wherein the arrangement of the central unit is such thàt, following an interruption power and data are re-applied a predetermined number of times before they are permanently interrupted.

5. A system as claimed in any of the above claims and wherein each remote unit resistor has a value approximately ten times the relevant loading impedance of the next following

remote unit.

6. A data transmission system substantially as hereinbefore described with reference to the drawing accompanying the provisional specification.