Patent Number: 047708425
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a common bus multinode sensor system that allows multiple remote units to simultaneously transmit over the common bus using frequency division multiplexing and, more particularly, to a system that provides power to the remote units via the bus and allows low speed digital, low frequency analog signals and high frequency analog signals to be transmitted over the bus to receivers which demodulate the analog signals and decode the digital signals. 2. Description of the Related Art Closed loop communication systems for facilities such factories and nuclear power plants require the capability of transmitting digital, low frequency analog and high frequency analog signals from a plurality of spaced apart points to a central location through an environment full of electromagnetic noise. Such systems typically require that the communication medium such as a twisted wire pair, coaxial cable or optical light guide be spread out through a very large complex requiring a media up to two kilometers in length. Prior art methods of providing such closed loop networks typically provide synchronous data transmission over a transmit channel and a receive channel. These systems poll each remote unit separately and, as a result, operate using a time division multiplexing scheme. The use of time division multiplexing does not allow plural sensors to be sampled simultaneously. The prior art systems also require power supplies at each remote unit producing ground loop problems as well as requiring that extra power wires be provided to the remote units. SUMMARY OF THE INVENTION It is an object of the present invention to provide a communications system that allows slow speed digital as well as low frequency and high frequency analog signals to be simultaneously transmitted for a plurality of sensors. It is another object of the present invention to power remote units over a common bus. It is an object of the present invention to provide a remote unit that can interface plural types of sensors. It is also an object of the present invention to provide separate communication channels for each remote unit. It is an additional object of the present invention to allow plural low speed analog signals to be multiplexed. It is a further object of the present invention to provide a low cost remote unit that takes advantage of medium scale integration. It is still another object of the present invention to allow asynchronous data collection. It is an object of the present invention to allow each remote unit to be flexibly assigned different channel frequencies for communication. It is an additional object of the present invention to allow easy addition of remote units. The above objects can be attained by a multinode system that transmits power down a common bus coaxial cable typically using an alternating current power source. Each remote unit connected to the coaxial cable converts the alternating current power to direct current power for an integrated circuit bus interface. The interface is externally pin programmable to provide a carrier at a frequency for a channel assigned to the remote unit thereby providing each remote unit with an individual identity. When plural low frequency analog signals are to be transmitted over the common bus an on-chip multiplexer multiplexes the signals to an off-chip, external analog-to-digital converter. The analog-to-digital converter loads an on chip parallel-to-serial out shift register that applies each bit of the digitized signal serially to an on chip Manchester encoder. The encoder modifies the input voltage of an on chip voltage controlled oscillator operating at the carrier frequency. The modulated frequency oscillator signal is applied to the coaxial cable. Receivers at the end of the coaxial cable can be tuned to the designated carrier frequencies to demodulate and then decode the encoded signal at any time. If a high frequency analog signal is supplied, it is used to directly modify the voltage applied to the voltage controlled oscillator, that is, the carrier frequency is modulated by the high frequency signal and the receiver operating at the carrier frequency demodulates the signal. The integrated circuit is arranged so that the digital circuitry is generally isolated from and on the opposite side of the chip from the analog circuitry so noise immunity is enhanced. The digital and analog circuitry have separate power supplies. The circuit is arranged in a carrier so that critical lead wire runs are held to a minimum. These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.