Circuit arrangement for a digital telephone station

A circuit arrangement is provided for digital telephone stations in a digital private branch exchange, whereby control information are constantly exchanged via a subscriber line. The telephone station is controlled by a constantly interrogating and controlling microprocessor. The microprocessor can, for example, be arrested by a disturbing pulse. The invention provides the creation of a simple possibility of providing a reset pulse in an out-of-order case of the microprocessor. This is achieved in that repeated actuation of the cradle switch contact in brief succession charges the capacitor, due to multiple charging surges, the reaches the threshold voltage at the base of the transistor that is needed for this transistor, whereby the transistor is activated and generates the reset pulse.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a circuit arrangement for a digital 
telephone station in digital telephone systems, particularly private 
branch exchanges (PBX's), whereby the telephone station is controlled by a 
microprocessor and whereby inquiry and control signals are constantly 
exchanged via the leads of the subscriber line, whereby, furthermore, the 
feed voltage supply of the telephone station in the quiescent condition 
and in the voice condition occurs via the subscriber line or via a local 
voltage source, and whereby the cradle switch contact, in its actuated 
condition, connects a first control input of the microprocessor to ground 
via a first ohmic resistor. 
2. Description of the Prior Art 
Prior known telephone stations are fundamentally controlled by one or more 
microprocessors. Inquiry and control events, via the subscriber line, are 
thereby also sequenced during the quiescent condition of the telephone 
station when, for example, the handset is picked up at such a telephone 
station and the cradle switch contact is actuated as a result thereof, the 
microprocessor recognizes this event via a control input during a routine 
interrogation and interprets the same. 
Given disturbing influences, however, such microprocessors tend to block, 
so that an interrogation no longer occurs and, therefore, a connection 
cannot be produced after the handset is lifted. It is necessary to reset 
the microprocessor to an initial position via its reset input, or in some 
other form, and to allow the microprocessor to restart. This was 
heretofore carried out in that, for example, a resetting was effected on 
the basis of a brief disconnection of the local feed or of the remote 
feed. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide a possibility 
with which the out-of-order condition can be ended in a simple manner, 
particularly in providing this possibility as an action that is easy for 
the user of the telephone station. 
The above object is achieved, according to the present invention, in that a 
first capacitor is provided that is connected, first of all, to ground 
and, secondly, both to a first control input and to the feed voltage via a 
first ohmic resistor and via a second ohmic resistor and is also connected 
to ground via a collector-emitter path of a pnp transistor, via a diode, 
via a third ohmic resistor and via a second capacitor. Furthermore, the 
base of the transistor is connected to ground via a fourth ohmic resistor 
and via the cradle switch contact in the actuated condition of the cradle 
switch contact. The common junction of the third resistor and of the 
second capacitor is connected to the base of an npn transistor and is 
connected to ground via a fifth ohmic resistor. The npn transistor has its 
emitter connected to ground and has its collector, first of all, connected 
to the feed voltage via a sixth ohmic resistor and, secondly, to the reset 
input of the microprocessor. Furthermore, the fourth resistor is small in 
comparison to the first resistor and the first capacitor is small in 
comparison to the second capacitor. 
An advantage deriving therefrom is that a reset pulse for the 
microprocessor can be generated in a simple manner by merely multiply 
actuating the cradle switch contact, this reset pulse effecting a 
resetting of the processor program to an initial position, whereas a 
single actuation of the cradle switch contact does not effect any 
intervention in the processor program.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A normal operating condition is assumed first. The telephone station is 
situated in its quiescent condition. The handset is hung up and the cradle 
switch contact GU is open. In a manner not shown, the microprocessor MP 
routinely sequences its programs via the subscriber line (for example, 
indicating busy extension, indicating waiting subscribers, etc). 
According to FIG. 1, the potential U is available at the capacitor C1 by 
way of the resistors R1 and R2. High potential likewise is available at 
the input E of the microprocessor MP. 
When the handset is lifted, ground potential is applied to the junction of 
the resistors R1, R2 and R3 by way of the contact GU. Low potential, on 
the other hand, is applied to the input E and the microprocessor 
recognizes "handset lifted up" during its routine interrogation. 
The capacitor C1 is discharged to ground by way of the resistor R1, whereby 
the resistor R1 has a lower ohmic value than the resistor R2. Furthermore, 
the capacitor C1 is discharged to ground via the emitter-base path of a 
pnp transistor T1 and via a resistor R3 that has a value which is small in 
comparison to the value of the resistor R1. The transistor T1 is thereby 
briefly activated and a charge from the capacitor C1 flows into a 
capacitor C2 via the transistor T1, a diode D and a resistor R4. The 
capacitor C2 is significantly larger in capacitance value than the 
capacitor C1. Its voltage UC2 (FIG. 2, line b) assumes the value UX and is 
discharged to ground via a resistor R5. The charge of the capacitor C1 
(FIG. 2, line a) now lies at 0 and will not rise to the value U again 
until the handset is placed down at the end of the call, that is the hook 
switch is operated. The transistor T2 remains unaffected during this 
operation since its switching threshold (US in FIG. 2, line b) is not 
reached. A flow back and discharge of the charge of the capacitor C2 is 
prevented by the diode D. 
When it is now assumed that the microprocessor MP is no longer sequencing 
its program due to some type of disturbing pulse, i.e. has become stuck, 
then the subscriber can recognize this (abnormal flashing of the station 
lamps, no possibility of call set-up, etc.) and it is obvious that he will 
instinctly actuate the cradle switch contact repeatedly. 
When, in the out-of-order condition, the cradle switch contact is opened 
(pressing the support down), then the capacitor C1 charges as described 
(UC1 form 0 to the value U in FIG. 2, line a). 
When the cradle switch contact is opened, then the capacitor C1 discharges 
and the capacitor C2 assumes the charge UX (FIG. 2, line b). 
When the cradle switch contact GU is repeatedly closed and opened in brief 
succession, the capacitor C2 cannot discharge fast enough by way of the 
(high impedance) resistor R5 and now assumes the charge UY (FIG. 2, line 
b). This charge, however, lies above the switching voltage US (FIG. 2, 
line b) of the transistor T2, so that the transistor is briefly activated. 
A current flows from ground by way of the transistor T2 and the resistor 
R6 and a reset pulse (FIG. 2, line c) for the reset input Re of the 
microprocessor MP is formed therefrom. The charge of the capacitor C2 then 
flows off by way of the resistor R5. 
Whether the reset already occurs after a two-time actuation of the cradle 
switch contact or whether this must be done more frequently depends only 
on the rapidity of the operation of the contact. Given a one-time 
operation of the cradle switch contact (lifting the handset), by contrast, 
no resetting occurs. 
Although I have described my invention by reference to a particular 
illustrative embodiment thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. I therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of my contribution to the art.