Claims
- 1. A field instrument system comprising:
- a plurality of field instruments;
- a host instrument;
- a power source;
- a transmission line, at least a portion of which forms a current loop for connecting said field instruments, said host instrument, and said power source; and
- a communicator for communicating with said field instruments to monitor their status and communicating with said host instrument to inform said host instrument of the status of said field instruments;
- wherein said communicator communicates with said field instruments and said host instrument using digital electrical signals,
- wherein said transmission line carries said digital electrical signals among said field instruments, said host instrument, and said communicator,
- wherein said communicator is detachably connected to said transmission line,
- wherein electric power for operating said communicator is derived from said current loop, and
- wherein said field instruments are connected in parallel with one another along said current loop and transmit a measured physical quantity to said host instrument via said transmission line in digital signals.
- 2. The field instrument of claim 1, wherein said communicator includes a constant current circuit which controls the electric power derived from said current loop at a constant value, and wherein said constant current circuit is connected to said field instruments in parallel.
- 3. The field instrument system of claim 1, wherein said communicator includes a constant current circuit which controls electric power derived from said current loop at a constant value, said constant current circuit is connected in parallel with a voltage drop element which is connected in series with said current loop.
- 4. A communicator for use in a field instrument system which includes a plurality of field instruments, a host instrument, a power source, a transmission line forming a current loop connecting said field instruments, said host instrument and said power source, and carrying digital electrical signals, said field instruments being connected in parallel with one another along said current loop, said communicator comprising:
- a pair of terminals connected to said transmission line;
- signal input means, connected between said pair of terminals, for transforming a variation of current flowing between said pair of terminals into digital electrical signals; and
- signal output means, connected between said pair of terminals, for transforming current flowing between said pair of terminals in accordance with digital electrical signals,
- wherein said communicator monitors a status of said field instruments by communicating with said field instruments through said signal input means and said signal output means using digital electrical signals,
- wherein said communicator is detachably connected to said transmission line and has a DC-DC converter connected between said pair of terminals and has a constant current circuit for adjusting an output current of said DC-DC converter to a constant value, such that electric power drawn from said current loop is fed to said DC-DC converter, which then supplies the electric power to said communicator for consumption by said communicator, and
- wherein said communicator responds to said host instrument by communicating with said host instrument through said signal input means and said signal output means using digital electrical signals.
- 5. A field instrument system, comprising:
- a plurality of field instruments for measuring a physical quantity respectively;
- a host instrument;
- a power source;
- a transmission line which connects the field instruments, the host instrument and the power source in a current loop, wherein an electrical signal representing the measured physical quantity is transmitted from each one of the field instruments to the host instrument; and
- a communicator for monitoring the field instrument,
- wherein the electrical signal representing the measured physical quantity is a digital signal,
- wherein said communicator communicates with said field instruments and said host instrument using digital signals, and
- wherein said communicator includes means for deriving its electric power from the current loop and is freely connectable to or disconnectable from the transmission line without affecting communication of the digital signal transmitted thereon.
- 6. A field instrument system according to claim 5, wherein said communicator includes a constant current circuit for controlling the electric power derived from said current loop at a constant value, said constant current circuit being connected in parallel to said field instrument.
- 7. A field instrument system according to claim 5, wherein said communicator includes a constant current circuit for controlling the electric power derived from said current loop at a constant value, said constant current circuit being connected in parallel to said field instruments.
- 8. A field instrument system according to claim 5, wherein said communicator includes a constant current circuit for controlling the electric power derived from the current loop at a constant value, said constant current circuit being connected in parallel to a voltage drop element which is series connected in the current loop.
- 9. A field instrument system according to claim 5, wherein said communicator includes a constant current circuit for controlling the electric power derived from the current loop at a constant value, said constant current circuit being connected in parallel to a voltage drop element which is series connected in the current loop.
- 10. A communicator, comprising:
- a pair of terminals to be attached to a transmission line which forms a current loop connecting a plurality of field instruments, a host instrument and a power source;
- signal input means connected between said terminals for transforming a variation of the current flowing between the terminals into a first digital signal; and
- signal output means connected between said terminals for changing the current flowing between the terminals in accordance with a second digital signal,
- wherein said communicator communicates with said field instrument and said host instrument through said signal input means and said signal output means using digital signal so as to monitor said field instrument,
- wherein said communicator has a DC-DC converter connected to said pair of terminals to derive the electric power for operating said communicator from the current loop and a constant current circuit for adjusting the current drawn by the DC-DC converter from the current loop to a constant value, and
- wherein said communicator is freely connectable to or disconnectable from the transmission line without affecting communication over the transmission line of digital signals representing a physical quantity measured by the field instruments.
- 11. A communicator, comprising:
- a pair of terminals to be attached to a transmission line which forms a current loop connecting a plurality of field instruments, a host instrument and a power source;
- signal input means connected between said terminals for transforming a variation of a current flowing between the terminals into a first digital signal; and
- signal output means connected between said terminals for changing the current flowing between the terminals in accordance with a second digital signal,
- wherein said communicator communicates with said field instrument and said host instrument through the signal input means and the signal output means, using digital signals so as to monitor the field instrument,
- wherein said communicator has a DC-DC converter connected to the pair of terminals to derive the electric power for operating the communicator from the current loop,
- wherein said DC-DC converter includes means for maintaining the voltage across the pair of terminals constant while said electric power is drawn from the current loop, and
- wherein said communicator is freely connectable to or disconnectable from the transmission line without affecting communication over the transmission line of digital signals representing a physical quantity measured by the field instruments.
Priority Claims (1)
Number |
Date |
Country |
Kind |
1-267863 |
Oct 1989 |
JPX |
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Parent Case Info
This application is a continuation of application Ser. No. 07/594,983, filed on Oct. 10, 1990, now abandoned.
US Referenced Citations (8)
Foreign Referenced Citations (4)
Number |
Date |
Country |
0212897A3 |
Aug 1986 |
EPX |
02213767A2 |
Aug 1986 |
EPX |
0219120A2 |
Oct 1986 |
EPX |
0244808 |
May 1987 |
EPX |
Continuations (1)
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Number |
Date |
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Parent |
594983 |
Oct 1990 |
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