System for transmitting binary signals over a signal line

The invention relates to a system for transmitting binary signals over a signal line to a signal detecting device, in which a DC source is temporarily connectable to the signal line for generating binary signals. A discharge circuit is connected to the signal line. In order to keep the power loss very low in such a system, discharge circuit (7) contains a switchable current sink (8) connected to a signal line (3). Discharge circuit (7) also has a threshold value determination device (19) connected to signal line (3) in parallel with switchable current sink (8). Output (26) of threshold value determination device (19) is connected with a control input (17) of switchable current sink (8).

BACKGROUND OF THE INVENTION 
When binary signals, generated by temporarily connecting a DC source at 
relatively high voltage to a signal line, are used to transmit information 
over the signal line, the residual charge on the signal line must be 
reduced as quickly as possible after the signal level changes to achieve a 
high baud rate for transmission and to detect quickly a signal change. In 
addition, partial discharges on the signal line must be dissipated 
quickly. These requirements are especially important for applications in 
which signals are conducted over a long, capacitance-affected signal line. 
In such applications, a high discharge current must be assured for the 
charge stored on the signal line. 
For example, if a 220 VDC source is used and the length of the signal line 
is 1 km, corresponding to a line capacitance of 200 nF, a discharge 
current of 15 mA must flow if the voltage on the signal line is to be 
reduced sufficiently within 1 ms after each disconnection of the DC source 
(from 220 V to 150 V for example). The discharge current must be 
correspondingly high if a parasitic noise voltage is to be dissipated 
quickly. 
To solve this problem, in a system for transmitting binary signals over a 
signal line (see e.g., command and alarm module of the previously publicly 
used Siemens SINAUT-LSA telecontrol unit), a discharge circuit with a 
resistor and a downstream diode circuit was connected to the signal line. 
A high discharge current can flow continuously through such a discharge 
circuit even when the DC source is temporarily connected to the signal 
line. This results in a power loss of several watts. This power loss can 
cause problems with heat generation, particularly when several signal 
lines lead to one unit in which a corresponding number of discharge 
circuits must be available. 
German Patent No. 12 87 604 discusses a system for transmitting binary 
signals over a signal line to a signal detector, in which a DC source is 
temporarily connectable to the signal line as binary signals are 
generated, and in which a discharge circuit is connected to the signal 
line. In the arrangement of the '604 German patent, to compensate for 
parasitic line capacitances from the discharge circuit, the charge on the 
line is reversed between the binary signals formed by positive pulses, 
with a transistor that is connected so that it usually conducts, being 
blocked at the beginning of each positive pulse, and conducting again at 
the end of each pulse. In this way, another transistor is switched to 
conduct briefly, through which a negative voltage is applied to the line 
temporarily, resulting in a rapid polarity reversal of the line. 
SUMMARY OF THE INVENTION 
The goal of the present invention is to reduce considerably the power loss 
caused by the discharge circuit and to eliminate noise coupled into the 
signal line during any pauses between the binary signals. The present 
invention departs from a system for transmitting binary signals over a 
signal line to a signal detector in which a DC source is temporarily 
connectable to the signal line as binary signals are generated and a 
discharge circuit is connected to the signal line. 
To achieve the above mentioned goal, in a system of this kind according to 
the present invention, the discharge circuit contains a switchable current 
sink connected to the signal line. The discharge circuit has a threshold 
value determination device connected to the signal line, in parallel with 
the switchable current sink. The output of the current threshold value 
determination device is connected with a control input of the switchable 
current sink such that a current-determining resistance of the switchable 
current sink is changeable under control by the threshold value 
determination device. 
A threshold value determination device is discussed in German Patent No. 1 
237 173. However that device forms a component of a circuit for steepening 
the slopes of pulses, especially in radar and television technology. 
An important advantage of the system according to the present invention is 
that only a low discharge current flows through the discharge circuit 
during a binary "high" signal. The discharge current increases only when 
the voltage on the signal line drops after the binary signal ends. 
Therefore, the increased discharge current flows only during the discharge 
time of the signal line, during the pauses between the binary signals or 
after noise is introduced in the signal line. The system according to the 
present invention has a power loss of only about 5% of that of the 
aforementioned known system, while maintaining the dynamic properties of 
the known system. 
In the system according to the present invention, the current-determining 
resistance of the switchable current sink is advantageously variable under 
the control of the threshold value determination device. 
In the system according to the present invention, the threshold value 
determination device can take different forms. However, providing the 
threshold value determination device with a diode arrangement in series 
with a storage capacitor and forming the output of the threshold value 
determination device from a connection between the storage capacitor and 
the diode arrangement is especially advantageous. The diode arrangement 
thereby determines, with its diode thresholds, the activation threshold 
for the switchable current sink. 
In the system according to the present invention, the switchable current 
sink is advantageously provided with a current amplifier on its control 
input side. This arrangement permits the capacitances of the signal line, 
equal to the current amplification factor multiplied by the capacitance of 
the storage capacitor, to be rapidly discharged. An emitter follower can 
be used as the current amplifier, for example. 
In addition, providing the control input of the switchable current sink 
with a delay circuit, connected ahead of it, is advantageous because the 
delay circuit allows the response of the switchable current sink to be 
delayed. 
In the system according to the present invention, the switchable current 
sink is advantageously connected through an ohmic resistance to the signal 
line. As a result, a control voltage with hysteresis can be obtained that 
can be used for example to control a level evaluation circuit in the 
signal detector. 
Connecting switchable current sink with the signal line through a PTC 
thermistor is advantageous because a thermal overload on the discharge 
circuit, due for example to an excessive operating frequency, can thereby 
be avoided.

DETAILED DESCRIPTION 
The system shown in FIG. 1 comprises a DC source 1 that can deliver a DC 
voltage of between 24 and 250 V for example. The DC source 1 is 
temporarily connectable, by means of an alarm contact 2, which can also be 
an electronic signal switch, to a signal line 3, thereby generating binary 
signals. The signal line 3 can be a capacitance-affected line with 220 
nF/km, for example. A signal detector 5 is arranged at an end 4 of the 
signal line 3. The signal detector 5 is associated with an overvoltage 
protection system 6 and a discharge circuit 7. Both the overvoltage 
protection system 6 and the discharge circuit 7 are likewise connected to 
the end 4 of signal line 3. 
In a preferred embodiment of the present invention, the discharge circuit 7 
is designed as shown in FIG. 2. The discharge circuit 7 of FIG. 2 includes 
a current sink 8 having a depletion-type n-channel field effect transistor 
9. A series circuit, composed of a first resistor 11 with resistance 
R.sub.1 and a second resistor 12 with resistance R.sub.2, is connected 
between ground and the source electrode 10 of the transistor 9. The 
resistance R.sub.1 is much less than the resistance R.sub.2. The gate 
electrode 13 of the field effect transistor 9 is connected to ground. The 
drain electrode 15 of the depletion-type field effect transistor 9 is 
connected to the end 4 of the signal line 3 via an ohmic resistance or a 
PTC thermistor 14. 
The emitter-collector lead of a p-n-p transistor 16 is connected in 
parallel with the second resistor 12. The base of the transistor 16 is 
connected to a control input 17 of the switchable current sink 8. A 
high-ohmage resistor 18, which blocks the transistor 16 when the voltage 
on signal line 3 increases or remains constant, is located between the 
emitter and the base of transistor 16. When the transistor 16 is caused to 
conduct by a corresponding signal at the control input 17 of the 
switchable current sink 8, the second resistor 12 is shunted, thereby 
considerably reducing the current-determining resistance of switchable 
current sink 8 formed by resistors 11 and 12 since R.sub.1, is much less 
than R.sub.2. Accordingly, in such an instance, a high discharge current 
flows through the current sink 8. 
A threshold value determination device 19 is connected, in parallel with 
the switchable current sink 8, to the end 4 of the signal line 3. The 
threshold value determination device includes a series circuit composed of 
a resistor 20 and a storage capacitor 21, and a diode arrangement 22 
having a Zener diode 23 and a flux diode 24. The resistor 20 serves as a 
protective resistor while the storage capacitor 21 is charging, when the 
voltage from the DC source 1 is applied to the signal line 3. The diode 
arrangement 22 is dimensioned so that the zener diode voltage and flux 
diode voltage roughly correspond to the gate-source voltage of field 
effect transistor 9. The Zener diode 23 can be replaced with a 
reverse-connected flux diode. 
An output line 26 is connected to the threshold value determination device 
19 at a node 25 between the diode arrangement 22 and storage capacitor 21. 
A delay circuit 27, containing a resistor 28 and a capacitor 29, is 
connected between the output 26 of the threshold value determination 
device 19 and the control input 17 of switchable current sink 8. By 
suitably dimensioning elements 28 and 29 of delay circuit 27, a response 
delay of switchable current 8 can be adjusted. 
The system and the discharge circuit of the present invention operate as 
follows. If the DC voltage on the signal line 3 or at its end 4 decreases 
after the alarm contact 2 opens, the transistor 16 of the switchable 
current sink 8 is triggered by a threshold value on the transistor 16 of 
the switchable current sink 8, set by the diode arrangement 22. 
Consequently, the discharge current through switchable current sink 8, 
which has been very small up to this point, is considerably increased 
because the second resistor 12 is shunted. Consequently, the voltage drop 
on signal line 3 is accelerated. In these instances, the transistor 16 is 
activated up to its saturation voltage, so that the discharge current is 
determined only by the value R.sub.1, of resistor 11. Accordingly, a very 
high discharge current, 20 mA for example, flows through the current sink 
8 to ground. When alarm contact 2 closes again, the voltage at output 26 
of threshold value determination device 19 rises once again and transistor 
16 is blocked (i.e., is opened or does not conduct). Now the second 
resistor 12 is no long shunted. Consequently, the discharge current is 
very low because of the increased resistance. 
Finally, a signal can be tapped from an output 30 and hence at the 
connection of resistor 14 to switchable current sink 8, said signal 
exhibiting hysteresis. A level evaluation, not shown here, in signal 
detector 5 can be controlled by this signal. 
The present invention relates to a system for transmitting binary signals 
over a signal line to a signal detecting device, in which a DC source is 
temporarily connectable to the signal line for generating binary signals. 
A discharge circuit is connected to the signal line. To keep the power 
loss very low in such a system, a discharge circuit contains a switchable 
current sink connected to the signal line. The discharge circuit also has 
a threshold value determination device connected to the signal line in 
parallel with the switchable current sink. An output of the threshold 
value determination device is coupled with a control input of switchable 
current sink.