Pulse or digital communications – Repeaters – Testing
Patent
1980-06-30
1984-04-24
Griffin, Robert L.
Pulse or digital communications
Repeaters
Testing
370 85, 307270, H04L 516
Patent
active
044452225
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a coupling circuit for transferring data signals at a high rate from a data transmitter to a bus common to several data transmitters. The invention concerns in particular coupling circuits for data transmitters that may be placed far from each other, which entails that there may be huge differences in potential between the individual data transmitters, and it is therefore required that the data transmitters are galvanically separated from the long bus by separator means and that the data signal is transmitted on the bus in the form of a differential signal. More particularly, the invention relates to a coupling circuit for transferring data at a high rate from a data transmitter to a bus common to several data transmitters, said circuit comprising separator means for galvanically separating each data transmitter from the bus and for producing an output signal on a signal output via switch means positioned between the signal output and the separator means.
When a single one out of many data transmitters connected to the same bus is to transmit data signals, the nontransmitting units must not load the bus noticeably, and it is therefore necessary that beside having two logic transmission states where the associated generator impedance is low, each transmitter has also a third state, called "high impedance state", where the transmitter has a high output impedance when it does not transmit data signals.
The separator means may be a transformer whose secondary windings constitute the terminals of the data output, or may be provided by means of capacitors, optical couplings, etc. The German Offenlegungsschrift No. 2 702 209 shows an example of optical separator means. They suffer from the drawback that they cannot transfer power and that the bus must be active, e.g. carry supply current to the phototransistors.
A transformer is preferred for most uses because, normally, higher signal amplitude is desired on the bus than the output voltage of most logic circuits, but for uses where no increase in amplitude is required the separator means will be cheaper in the form of a pair of capacitors. Below the invention will mainly be described in connection with a transformer as separator means.
Coupling circuits are known where one winding of the transformer is directly connected to the bus, while the other winding of the transformer is connected to a driver which for providing the high impedance state exhibits high output impedance when no data signals are transmitted. This technique, however, can only be employed in connection with high rate data signals if only few circuits of this type are connected to the common bus. The high output impedance in the high impedance state of the transmission circuit should of course be converted into a correspondingly high output impedance by the transformer, but owing to the high transmission frequency the iron core of the transformer will result in a heavy load on the bus. For many uses, such as for multiplex systems where the units alternately transmit many brief data blocks, it is desired that the coupling circuit have a change-over time which is significantly shorter than the change-over time for relays, and consequently a relay technique cannot be used for the purpose.
The object of the invention is to provide a rapidly reacting coupling circuit that can transfer data signals from a data transmitter to a bus common to many transmitters and which brings about an effective high impedance state, i.e. that many coupling circuits of the invention may be connected to a common bus without loading the bus noticeably when data signals are not transmitted.
This object is achieved in that the coupling circuit comprises a switch which is connected in series between each signal output and a separator means and is arranged to be actuated and driven solely by the data signal applied from the data transmitter through the separator means. Thus the circuit is interrupted at such a location that the iron core of the transformer or any other load via a separator capacitor do
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Chin Stephen
Christian Rovsing A/S
Griffin Robert L.
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