Test circuit for a protective coupler

The test circuit disconnects the active elements of the coupler from the input and output and applies a test signal to the input of the active elements. The output of the active elements is first compared with a first threshold which if exceeded or equaled provides a first indication. Within a predetermined time period the output of the active elements is compared with a second threshold to determine if the control function of the active element has reduced the input to the second threshold within the time period. If this condition prevails, the indication previously provided is removed; otherwise, the indication remains and a faulty coupler is indicated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to protective circuits in general and more 
particularly to a testing arrangement in which the efficacy of a 
protective circuit can be tested. 
2. Description of the Prior Art 
Protective coupling circuits for connecting a modem or data set to a public 
switched network are known in the art. These circuits limit the output 
voltage applied to the network regardless of the value of the input 
voltage within a predetermined time. The circuits generally use a program 
or standard resistor which when connected to a regulated voltage supply 
provides a reference voltage which is used for operating the circuits. 
Circuits of this type have not in the past been provided with on-line 
testing arrangements by which an operator can determine the efficacy of 
the circuit. Generally, the circuits have been removed from the operative 
environment and bench tested to determine whether or not they are 
functioning properly. This arrangement is both costly and inconvenient 
since it generally requires substitution and later test. When difficulty 
occurs in a communication path which may include a large variety of 
equipment such as business machines, modems, data sets and switched 
network facilities, as well as the protective coupling devices, it is 
incumbent to readily determine which of the devices in the end-to-end 
connection are malfunctioning. Removal of a device with either 
substitution or bench testing is not a practical solution. 
SUMMARY OF THE INVENTION 
The invention contemplates a test circuit for use with a protective coupler 
for coupling a modem to a switched network and for simultaneously limiting 
the maximum signal voltage applied to the network to a specified value and 
comprises first switching means for disconnecting the coupler from the 
modem and the network and for applying a voltage substantially in excess 
of the allowable network voltage to the input of the coupler, circuit 
means for generating first and second thresholds T1 and T2 corresponding 
respectively to the maximum allowable signal voltage level permitted in 
the network and to the voltage applied to the coupler by the first 
switching means, an electrically operated indicator, a first normally 
closed and a second normally open switch means serially connected for 
connecting the indicator to a power source, first comparator means 
responsive to T2 and to the coupler output for providing an output to the 
second normally closed switch means for energizing the indicator when the 
coupler output equals or exceeds T2 and a second comparator means 
responsive to T1, the coupler output and the prior energization of said 
indicator within a predetermined time period for providing an output 
signal to energize said first normally closed switch means to open which 
causes deenergization of the indicator when the coupler output is equal to 
or less than T1 within the above said predetermined time period following 
energization of the indicator.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 a conventional line amplitude control circuit 10 is connected 
between a modem not shown via a switched contact S1 and a network not 
shown by a switched contact S2. A pushbutton switch 11 connects a relay 12 
to a power supply 13 when actuated and held. Relay 12 is provided with six 
contacts, S1 and S2 previously described, and S3-S6 to be described below. 
These contacts are shown in the drawing in their normal position, i.e., 
with relay 12 deenergized. When the relay 12 is energized, the contacts 
assume the opposite position from that illustrated in the drawing. 
When pushbutton switch 11 is actuated and held closed, relay 12 is 
energized, switch contacts S1 and S2 open, thus disconnecting the line 
amplitude control circuit 10 from the modem and the switched network. At 
this time switch contacts S3 and S4 close. This applies power to an 
oscillator 14 and connects the oscillator output to the input of the 
amplitude control circuit 10. The signal from oscillator 14 is 
approximately +12 dB over the maximum allowable voltage on the network. 
The output of amplitude control circuit 10 is applied to a rectifier 15 
which provides a DC level at its output, the voltage of which is 
comparable to the maximum voltage permitted on the network. 
Resistors 16, 17 and 18 connected between a positive voltage source and 
ground form a voltage divider network. The voltage source is regulated, 
thus the voltage appearing between resistors 16 and 17 is used to control 
circuit 10 and provide a threshold voltage T2 while the voltage at the 
point between resistors 17 and 18 provides a threshold voltage T1. 
Resistors 16, 17 and 18 are selected such that the reference voltage T2 is 
approximately 11 dB over the maximum allowable voltage level on the 
network while T1 is 1 dB over the said maximum level. Threshold T2 is 
applied to one input of a comparator 19 along with the output of rectifier 
15. Comparator 19 provides a suitable output for energizing a latching 
relay 20 when the output of the rectifier 15 is equal to or greater than 
the threshold T2. When this condition prevails, relay 20 latches and 
closes an associated normally open switch 21. A contact S6 of relay 12 
which is normally open is used to release the latch on relay 20 when 
pushbutton 11 is released by the operator. As soon as contact 21 closes, a 
lamp 22 is connected to power source 13 and is illuminated thus indicating 
to the operator the beginning of the test. A single shot circuit 23 starts 
a timeout as soon as switch contact 21 closes and provides an output for a 
finite period of time, approximately 6 seconds, to a gate circuit 24. A 
second comparator 25 connected to T1 and the output of rectifier 15 
provides an output whenever the rectified output signal from rectifier 15 
is equal to or less than T1. This output is applied to gate 24. If the 
output occurs within the timeout of single shot circuit 23, it passes gate 
24 and actuates a latching relay 26 which is similar to relay 20 described 
above. This relay is released by contact S5 of relay 12 when pushbutton 
switch 11 is opened. When relay 26 is actuated and latches, a contact 27 
opens thus interrupting the energization of lamp 22. If the circuit 10 
does not reduce the amplitude of the signal applied via oscillator 14 
within the six second period, contact 27 does not open. Thus, the lamp 
remains lit indicating a circuit failure to the operator. If lamp 22 
extinguishes, the circuit is considered functioning properly. 
The graph illustrated in FIG. 2 shows the normal function of the line 
amplitude control circuit 10 as a function of voltage in response to an 
application of the output of oscillator 14. The signal from oscillator 14 
is 12 dB over the specified maximum level for the network. Within the 
period of 1-6 seconds the circuit must reduce the amplitude to no more 
than 1 dB over the specified voltage level. The first test, i.e., the 
output of comparator 19, is to determine that the circuit is functioning 
and providing an output comparable to the input at the earliest time 
period. The second test, i.e., the output of comparator 25, within the 
time period specified by the single shot circuit 23, is to determine that 
within the six second period of single shot circuit 23 the output of the 
amplifier is within the 1 dB limit of the specified voltage level, thus 
from the operator's point of view very shortly after the button is pushed, 
the light comes on and within six seconds the light goes out. If the light 
22 does not perform in this manner abnormal operation of the line 
amplitude control circuit 10 is indicated. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
therein without departing from the spirit and scope of the invention.