Method for entering and leaving a power-saving operation of a subscriber in an annular communication network

A method for entering and leaving a power-saving operation of a subscriber in an annular communication network includes transmitting data in the communication network in a format that prescribes a pulsed sequence of individual bit groups of identical length in which specific bit positions are reserved for a preamble which is decoded in a receive circuit of each subscriber. During a power-saving operation of a subscriber, at least part of the subscriber which contains the receive circuit remains turned on, and at least one special preamble code is provided, upon the reception of which a part of the subscriber turned off during the power-saving operation is turned on again.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The invention relates to a method for entering and leaving a power-saving 
operation of a subscriber in an annular communication network, in which 
data are transmitted in a format that prescribes a pulsed sequence of 
individual bit groups of identical length in which specific bit positions 
are reserved for a preamble that is decoded in a receive circuit of each 
subscriber. The invention also relates to such a method in a mobile 
annular communication system, in particular in a motor vehicle. 
The subscribers of the communication network are data sources and data 
sinks which are interconnected through data lines that form a ring feeder. 
Both source data and control data can be transmitted between the 
subscribers through the ring feeder by subdividing the bit groups into 
component bit groups which respectively form a data channel. A clock 
signal is generated by a single subscriber and transmitted through the net 
in the form of specific preamble codes which respectively identify the 
start of a block, which is formed in each case by a combination of a 
plurality of bit groups. All of the other subscribers detect the block 
start code and are synchronized with the clock signal with the aid of 
successive block start codes. It is noted that purely packet-oriented data 
transmission methods such as, for example, ATM methods, that is to say 
asynchronous transmission methods, are to be distinguished therefrom. 
Such an annular communication network as is described, for example, in 
Published European Patent Application 0 725 522 A1 serves for networking 
different types of electric and electronic devices which are intended to 
exchange information among one another in a partly complicated way, with 
the aid of data lines of physically simple construction. The subscribers 
can exchange both source data and control data through those data lines. 
For example, in the audio field it is possible to transmit audio data from 
data sources such as CD players, radio receivers and cassette recorders to 
data sinks such as amplifier/loudspeaker combinations, and control data 
can be transmitted at the same time, for example to control volume. In 
that case, a device can be constructed simultaneously as a data source and 
a data sink, such as is the case with a cassette recorder, for example. 
In order to realize power-saving functions in such a communication network, 
individual subscribers have previously been turned off completely upon 
external command or after the expiration of a preset time of inactivity. 
In order to reactivate such a turned-off subscriber, it has been necessary 
for the entire network to be turned off and turned on again. 
It is desirable to be able to reactivate a deactivated subscriber when 
required, in particular through the use of a subscriber which manages the 
network, without having to turn off the entire network and turn it on 
again or without carrying out a reset. 
There are special wakeup circuits for such a purpose which monitor the 
signal on a data line and which react to a specific modulated signal by 
again turning on the subscriber connected to the wakeup circuit. However, 
such a circuit, which each subscriber with a power-saving function must 
have, is relatively expensive and takes up a lot of space in relative 
terms. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a method for 
entering and leaving a power-saving operation of a subscriber in an 
annular communication network, which overcomes the hereinafore-mentioned 
disadvantages of the heretofore-known methods of this general type and 
with which it is possible to realize power-saving functions in an annular 
communication network in a particularly simple way. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a method for entering and leaving a 
power-saving operation of a subscriber in an annular communication 
network, which comprises providing subscribers having receive circuits in 
an annular communication network; transmitting data in a format 
prescribing a pulsed sequence of individual bit groups of identical length 
having specific bit positions reserved for a preamble to be decoded in the 
receive circuit of each subscriber; keeping at least part of a subscriber 
containing a receive circuit turned on during a power-saving operation of 
the subscriber; and turning on a part of the subscriber turned off during 
the power-saving operation, upon reception of at least one special 
preamble code by the receive circuit. 
The invention utilizes the fact that the possible information content of 
the preamble is not exhausted by the transmission of the clock signal. 
This holds in general even if further information is transmitted through 
the use of the preamble, such as the distinction between left-hand and 
right-hand channels, as is the case in the so-called "SPDIF format 
(Sony/Philips-Digital-Interface-Format)", the standard data transmission 
format for CD players. Consequently, the invention delivers an additional 
control function without loss of transmission capacity. 
The receive circuit which is present in each subscriber must be able to 
decode the preamble completely automatically, in order to synchronize an 
internal oscillator of the subscriber, with the received clock pulse. For 
this purpose, the receive circuit contains some hardwired device, for 
example a switching mechanism, which is connected to a shift register, a 
FIFO circuit or the like, through which register or circuit each received 
bit group is directed. A received preamble code is detected, for example 
as a block start code, with the aid of the switching mechanism, and is 
processed appropriately. 
In accordance with the invention, this receive circuit, which is present in 
any case, is used in order to reactivate a deactivated subscriber without 
the need for any further components. In particular, it is not necessary 
for a microprocessor that is present in the subscriber to remain turned on 
in order to be able to detect any wakeup signal, nor is there a need for a 
processor in each subscriber, as in the case of an amplifier/loudspeaker 
combination, which participates essentially passively in the 
communication. 
When the power-saving operation of a subscriber is entered, only the 
receive circuit and an internal oscillator of the subscriber need remain 
turned on. The rest of the subscriber can be turned off. In order to leave 
the power-saving operation upon the receipt of the special preamble code, 
the receive circuit actuates some switch which turns on the rest of the 
subscriber again. 
During the power-saving operation, there is certainly a need for a small 
residual part of the subscriber to remain active, with the result that a 
low minimum current is required just as in the case of using external 
wakeup circuits. However, in accordance with the invention, a subscriber 
with a power-saving function has no need of any external components. 
Therefore, the internal changes are limited to the fact that the receive 
circuit additionally detects the wakeup preamble code and is capable of 
turning on the rest of the subscriber again. 
In order to be able to reactivate specific subscribers in a pinpointed 
fashion, there can be a plurality of special preamble codes which are 
respectively assigned to a subscriber or a group of subscribers, for 
example amplifier/loudspeaker combinations connected to the network. 
The ring feeder is preferably formed by optical fibers which interconnect 
two subscribers in each case, and permit high data transmission rates. 
Moreover, in the case of a communication system in a motor vehicle, the 
low weight of optical fibers is particularly advantageous. However, the 
invention is also suitable for purely electric annular communication 
networks in which the line sections are, for example, coaxial cables. 
It has proved effective, in particular, to transfer individual or several 
of the subscribers, apart from the clock pulse generator, into the 
power-saving operation by an instruction from the clock pulse generator. 
Due to the central instruction from the clock pulse generator the latter 
is always aware of whether one or more and, if appropriate, which 
subscribers are in the power-saving operation, and thus need to be 
transferred, if required, from this power-saving operation into the normal 
operation. It has also proved effective to provide the subscribers which 
are intended for the power-saving operation, with a unit which ensures 
that after a fixed time of inactivity of the subscriber, the subscriber 
goes over into the power-saving operation, preferably communicating this 
transition in advance to the clock pulse generator through the ring 
feeder. These two types of transition into the power-saving operation 
provide two simple and reliable methods for entering and leaving the 
power-saving operation of subscribers in a communication network. 
The use of the above-described methods proves to be particularly relevant 
in the case of a communication system in a motor vehicle, since it is 
precisely there that the power consumption of the electric components of 
the vehicle are to be kept as low as possible, in particular when the 
motor vehicle is at a standstill. The functionality of the vehicle can be 
kept reliable by using as low a power consumption as possible, even after 
lengthy operation of the electric components. Weak car batteries which 
fail to operate, particularly when starting in winter, are virtually 
excluded by this invention by virtue of the power consumption of the 
communication system in the motor vehicle. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
method for entering and leaving a power-saving operation of a subscriber 
in an annular communication network, it is nevertheless not intended to be 
limited to the details shown, since various modifications and structural 
changes may be made therein without departing from the spirit of the 
invention and within the scope and range of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures of the drawings in detail and first, 
particularly, to FIG. 1 thereof, there is seen an annular communication 
network with a subscriber 1 used as clock pulse generator, and three 
further subscribers 2, 3 and 4. The four subscribers 1 to 4 are 
interconnected annularly through optical fibers 5 to 8. The optical fibers 
5 to 8 form a ring feeder for a common transmission of source data and 
control data between the subscribers 1 to 4. The physical direction of the 
data transmission is represented by arrows. 
Each subscriber 1 to 4 contains a receive and transmit circuit 9 to 12, 
each two of which are interconnected through a respective one of the 
optical fibers 5 to 8. Each receive and transmit circuit 9 to 12 contains 
an optoelectronic transducer for receiving optical signals from one of the 
optical fibers and converting them into electric signals, as well as an 
optoelectronic transducer for converting electric signals into optical 
signals which are fed to the optical fiber that is connected to the next 
subscriber in the ring. 
The received signals include continuously successive bit groups 13, as 
represented in FIG. 2, in which individual data channels are formed for 
source data or control data. The first four bit positions of each bit 
group 13 form a preamble 14. 
Each received bit group 13 is pushed in a receive and transmit circuit 9 to 
12 through a shift register, a FIFO memory or the like, in order to be 
able to selectively access the bits in the bit group 13. The bits of the 
preamble 14 are fed to a switching mechanism, a configuration of logic 
circuits, in which the preamble 14 is decoded. That is to say, the bits in 
the preamble 14 are used to detect that a preamble indicates a block start 
with which an oscillator or a PLL circuit of the subscribers 2 to 4 is 
synchronized. Moreover, in the case of an SPDIF format, the preamble 14 
can be used to detect whether a received bit group is assigned to a 
left-hand or a right-hand channel. 
The remaining bits of each bit group 13 can be fed to an electronic unit of 
the respective subscriber, for example an amplifier/loudspeaker 
combination, which converts audio data contained in the bit group 13 into 
acoustic signals. In this case, the bit group 13 is pushed essentially 
unchanged through the shift registers and fed into the ring feeder again. 
If the electronic device is a car radio, for example, it generally 
contains a microprocessor, which can write into the shift register in 
order, for example, to transmit audio data or control data to other 
subscribers. The bit groups which are thus changed are likewise fed again 
into the ring feeder. 
Each receive and transmit circuit 9 to 12 normally contains a switch for 
direct electric connection of the optoelectronic input and output 
transducers when the corresponding subscriber is turned off. 
In a first exemplary embodiment, this switch is not actuated when a 
subscriber enters a power-saving operation upon an external command or 
after the expiration of a preset time of inactivity. Rather, the receive 
and transmit circuit, including the internal oscillator of the subscriber, 
remains in operation, with the received signals being pushed in a 
transparent fashion through the shift register or the like and being fed 
again into the ring feeder. The switching mechanism in the receive and 
transmit circuits, through the use of which the preamble is decoded, 
likewise remains in operation. Apart from the clock pulse generator 1, all 
of the remaining parts of each subscriber 2 to 4, which are illustrated in 
a hatched manner in FIG. 1, can be turned off. 
A special, not yet allocated bit sequence is provided as a wakeup preamble 
code upon the reception of which the switching mechanism of a subscriber 2 
to 4 again switches on the hatched parts thereof. 
When another subscriber transmits a message to this subscriber and receives 
no confirmation, it transmits the wakeup preamble code. As an alternative, 
a subscriber can transmit a wakeup preamble code as a precaution whenever 
it has not transmitted to the same subscriber over a lengthy time. 
The four bits of the preamble shown in FIG. 2 permit a plurality of 
different wakeup preambles to be provided, for example one for waking up 
all of the subscribers in the ring, some for specifically waking up 
individual subscribers and, if appropriate, some for waking up a group of 
subscribers, for example all of the loudspeakers. 
In a second exemplary embodiment, the optoelectronic input and output 
transducers are directly and electrically interconnected, that is to say 
the switch mentioned above is actuated when a subscriber enters a 
power-saving operation upon an external command or after the expiration of 
a preset time of inactivity. However, the received bit groups or at least 
their first four bit positions are monitored for the wakeup preamble code, 
as in the first exemplary embodiment. Apart from the input and output 
transducers, which naturally have to operate continuously, in this second 
exemplary embodiment only the receive part of the receive and transmit 
circuit 9, 10, 11 or 12 and the internal oscillator remain in operation 
when a subscriber enters a power-saving operation.