Two-wire communication apparatus

A communication apparatus inserted in parallel in a two-wire communication line of a two-wire measuring instrument which consists of a transmitter and a receiver connected to each other through the two-wire communication line and transmits a process value detected by the transmitter to the receiver as an analog signal, the communication apparatus being adapted to communicate with the transmitter, comprising a switching device for separating a closed loop constituted by the transmitter and the receiver into a first loop including the transmitter and a second loop including the receiver, a power source for supplying power to the transmitter in the first loop, and dummy output means for outputtting a dummy signal to the receiver in the second loop.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to communication apparatus. More specifically 
the present invention is directed to a two-wire communication apparatus. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved two-wire 
communication apparatus. 
In accomplishing this and other objects, there has been provided, in 
accordance with the present invention, a communication apparatus inserted 
in parallel in a two-wire communication line of a two-wire measuring 
instrument which consists of a transmitter and a receiver connected to 
each other through the two-wire communication line and transmits a process 
value detected by the transmitter to the receiver as an analog signal, the 
communication apparatus being adapted to communicate with the transmitter, 
comprising a switching device for separating a closed loop constituted by 
the transmitter and the receiver into a first loop including the 
transmitter and a second loop including the receiver, a power source for 
supplying power to the transmitter in the first loop, and dummy output 
means for outputting a dummy signal to the receiver in the second loop.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a circuit diagram of a prior art two-wire measuring instrument 
comprising transmitter 1 installed at a measurement site 2, a power source 
4 arranged in a measuring chamber 3, and a reception resistor 5 serving as 
a signal receiver. The transmitter 1 serves to detect a process variable 
and to produce a representative output signal, e.g., a differential 
voltage, and to transmit the detected data. An analog current signal of 4 
to 20 mA is conventionally transmitted from the transmitter 1 along a 
two-wire communication line 7 to the measuring chamber 3 and is received 
by the reception resistor 5 as a voltage across the two resistor 
terminals. 
Generally, a communication apparatus 6 which can be additionally connected 
in parallel across the two-wire line 7 is of a portable type. For example, 
the communication apparatus 6 is inserted in parallel in a two-wire loop 
in the site 2, as shown in FIG. 2 and communicates with the transmitter 1. 
In this case, changes in parameters or adjustments for the transmitter 1 
are mainly communicated, Such a communication apparatus is disclosed in 
U.S. Pat. No. 4,520,488 entitled, "Communication System and Method". 
However, in such a conventional communication apparatus, a voltage across 
the two terminals of the receiver 5 is changed by a current flowing in a 
digital manner through the communication apparatus 6. This change in 
voltage is detected by the receiver 5 to perform communication. Therefore, 
while the communication apparatus 6 is operated, changes in voltage appear 
in the reception resistor 5 serving as the receiver. 
The communication apparatus according to the present invention, on the 
other hand, comprises switching means for separating a closed loop 
constituted by a transmitter and a receiver into a first loop including 
the transmitter and a second loop including the receiver, a power source 
for supplying power to the transmitter in the first loop, and dummy output 
means for outputting a dummy signal to the receiver in the second loop. 
After the loop is separated by the switching means, communication with the 
transmitter can be performed through the first loop. In addition, a 
communication signal during the communication does not influence the 
receiver. 
The present invention will now be described in detail with reference to an 
embodiment illustrated in FIG. 1 which is a circuit diagram having the 
same reference numerals in FIG. 1 to denote the same parts shown in FIG. 2 
and described above. A communication apparatus 10 includes a dummy output 
means 30 and a switching means 31. The switching means 31 separates a loop 
constituted by a transmitter 1 and a reception resistor 5 serving as a 
receiver, as needed, thereby forming different loops on a transmitter 1 
side and a reception resistor 5 side, respectively. 
The switching means 31 includes a transistor 20 serving as a switch and a 
switch 11. When the switch 11 is turned "off" and the transistor 20 is 
turned "on", a loop serving as a two-wire measuring instrument is formed. 
Conversely, when the switch 11 is turned "on" and the transistor 20 is 
turned "off", the loop serving as the two-wire measuring instrument is 
interrupted. At this time, a loop of switch 11, transmitter 1, power 
source 19, resistor 22 and switch 11 arranged in the order named is formed 
o the transmitter 1 side, and a loop of reception resistor 5, loop power 
source 4, transistor 17, resistor 18 and reception resistor 5 arranged in 
the order named is formed on the reception resistor 5 side. Reference 
numeral 12 denotes a switch for driving the switching transistor 20. 
The dummy output means 30 samples and stores a current value of the loop as 
the two-wire measuring instrument immediately before it is separated by 
the switching means 31, and outputs the corresponding current to the loop 
on the reception resistor 5 side as a dummy current value. The dummy 
output means 30 includes a sample/hold circuit 14 and a selector 15. When 
communication between the transmitter 10 and the receiver 1 is not 
performed, the dummy output means 30 and switching means 31 are in a 
non-operative state. More specifically, first and second switch signals 
respectively denoted by (DRIVE) and (HOLD) in FIG. 1 are in an OFF state. 
When the switch signal DRIVE is in the OFF state, the following three 
states are present, i.e., .circle. The switch 11 is turned "off" (open). 
.circle. An input B is selected as an input to the selector 15 and its 
output is set in a zero level. Therefore, the transistor 17 is turned 
"off". .circle. The transistor 12 is turned "off". That is, the 
transistor 20 is turned "on". 
When the switch signal HOLD is in the OFF state, the sample/hold circuit 14 
is set in a sampling state. 
When communication is to be performed from such a non-communication state, 
the switch signals HOLD and DRIVE are sequentially turned "on". Upon 
turning "on" the switch signal HOLD, a potential difference VRF 
corresponding to a loop current value at this time is held as an output 
from the sample/hold circuit 14. 
Then, the switch signal DRIVE is turned "on" to drive the switch 11, the 
selector 15, and the transistor 12. When the transistor 12 is turned "on", 
the transistor 20 is turned "off", thereby cutting off power supply from 
the loop power source 4 to the transmitter 1. When the selector 15 is 
operated, an input A (a hold value of the sample/hold circuit 14) is 
selected and a current value corresponding to the hold value flows into 
the reception resistor 5 through the transistor 17. More specifically, the 
current corresponding to the hold value flows through the reception 
resistor 5 side loop of reception resistor 5, loop power source 4, 
transistor 17, resistor 18 and reception resistor 5, and a dummy signal is 
applied to the reception resistor 5. When the switch 11 is turned "on", 
the transmitter 1 side loop of switch 11, diode 24, power source 19, 
resistor 22 and switch 11 is formed, and power is supplied from the power 
source 19 to the transmitter 1. 
In this state, when communication with the transmitter 1 is performed by 
supplying a digital signal between terminals 26 and 27, a communication 
signal does not influence the reception resistor 5. That is, a value of a 
current flowing through the reception resistor 5 during a communication 
can be held at a state immediately before the communication is started The 
arrangements of the switching means 31 and the dummy output means 30 are 
not limited to the embodiment of the present invention. For example, the 
switching transistor 20 in the switching means 31 can be replaced with a 
relay switch or the like. 
As has been described above, according to the communication apparatus of 
the present invention, after the loop serving as the two-wire measuring 
instrument is separated by the switching means into the transmitter and 
received side loops, communication with the transmitter is performed in 
the transmitter side loop while supplying a dummy output to the receiver 
in the receiver side loop. Therefore, a received current of the receiver 
is not influenced during the communication, and the received current is 
always kept constant during the communication 
Accordingly, it may be seen that there has been provided, in accordance 
with the present invention, an improved two-wire communication apparatus.