Circuit layout for a serial transmission and reception interface in particular for a vehicle occupant protection system as well as a procedure for controlling this interface

The invention relates to a circuit layout for a serial transmission and reception interface, in particular for a vehicle occupant protection system, as well as a procedure for controlling the same. Existing central units, in particular vehicle occupant protection systems, featured in most cases separate serial transmission and reception interfaces for the various modules to be connected, causing costs to increase whenever an additional diagnosis module is to be connected. By means of a switchable circuit layout for the interface, this will be connected either to a communication module or to the diagnosis module. The circuit layout is particularly simple and space-saving, being set up by means of diode switches, and preferably features a potential tapping point for automatic switchover into diagnosis mode.

BACKGROUND OF THE INVENTION 
The invention relates to a circuit layout for a serial transmission and 
reception interface, in particular for a vehicle occupant protection 
system, as well as a procedure for controlling this interface. 
From the state of the art, for instance DE 196 43 013 or DE 196 53 794, a 
large number of vehicle occupant protection systems, in particular for 
motor vehicles, are known where a central unit is connected to a 
communication module, e.g. for a sensor arrangement, via a serial 
transmission and reception interface. 
Based on the information determined by the sensor arrangement and passed to 
the central unit by the communication module, the central unit will decide 
on the triggering of vehicle occupant protection devices, e.g. air bags, 
belt tensioners, etc. 
In order to determine the operational condition of such a vehicle occupant 
protection system, e.g. for repair and maintenance purposes, it is 
necessary to be able to make appropriate evaluations by means of a 
diagnosis module within the central unit. 
SUMMARY OF THE INVENTION 
The invention provides a circuit layout for a serial transmission and 
reception interface by means of which such a diagnosis module can be 
connected as simply and economically as possible. In addition, a procedure 
for controlling such a serial transmission and reception interface will be 
stated. 
The basic idea of the invention is to connect the diagnosis module via the 
same serial transmission and reception interface and to link the central 
unit, via the multiplex operation circuit layout according to the 
invention, either with the communication module or with the diagnosis 
module. However, in order to implement such a multiplex operation as 
economically as possible, experience shows that it is better to use the 
circuit layout according to the invention, which is based on a diode 
logic, rather than a functionally equivalent multiplexer. Here, tristate 
outputs of the central unit will generate four control signals by means of 
which the circuit layout is controlled. At the same time, it will also be 
monitored whether a diagnosis module is switched on and active; if this is 
the case, switchover from the communication module to the diagnosis module 
takes place. Below, the invention will be elucidated further by means of 
embodiment examples and figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows, in a block chip, the central unit 1 complete with the serial 
transmission and reception interface (SCI) which can be connected, via the 
reception line 2 and the transmission line 3 and via the multiplexer 4, to 
the communication module M1 or the diagnosis module M2. An equivalent 
circuit diagram of the circuit layout according to the invention has been 
used showing the multiplexer 4 with the switches 4.1 and 4.2 in order to 
illustrate and elucidate more clearly the way in which the circuit layout 
operates; the switches 4.1 and 4.2 are controlled from the central unit 1 
by means of the control signals 4.3. The implementation of the circuit, 
however, is shown in FIG. 2. By means of the multiplexer 4 and its 
switches 4.1 and 4.2 it is easiest to illustrate the basic principle of 
operation; in accordance with this illustration, the reception line 2 of 
the central unit 1--depending on the switch state of the switches 4.1 and 
4.2 controlled by the central unit 1--is either connected to the 
transmission output T1 of the communication module M1 or, in the other 
switch state, to the transmission output T2 of the diagnosis module M2. 
Analog to this, the transmission line 3 of the central unit--depending on 
the switch state of the switch 4.2--is connected either to the reception 
input R1 of the communication module M1 or to the reception input R2 of 
the diagnosis module M2. FIG. 1 already shows the possibility 5 of 
interrogating the potential at the transmission output T2 of the diagnosis 
module M2, where the potential serves as indicator to the central unit 1, 
whether the diagnosis module M2 transmits any signals. Then, the central 
unit 1 can switch over the switches 4.1 and 4.2 of the multiplexer 4 by 
means of the control signals 4.3. 
The exact design of the circuit layout according to the invention is now 
shown in FIG. 2. Here, the reception line 2 and the transmission line 3 
are shown as seen from the central unit. The reception line 2 of the 
central unit 1 is connected to the transmission output T1 of the 
communication module M1, via a first diode switch 6. The diode switch 6 
features a diode D6 which, with its cathode, is connected to the reception 
line 2 of the central unit 1, as well as a voltage divider made up of the 
partial resistors R.sub.6.1 and R.sub.6.2 to the supply voltage U. The 
transmission output T1 of the communication module M1 is connected between 
the first partial resistor R.sub.6.1 and the second partial resistor 
R.sub.6.2. The diode switch 6 is controlled by means of the first control 
signal S1 which is supplied to the anode of the diode D6. Symmetrically to 
this, the second diode switch 7 is built up accordingly, complete with the 
partial resistors R.sub.7.1 and R.sub.7.2 and the diode D7, and the 
transmission output T2 of the diagnosis module M2 is connected between the 
partial resistors R.sub.7.1 and R.sub.7.2. In addition, interrogating the 
potential 5 is possible by connecting the central unit 1 between these 
partial resistors R.sub.7.1 and R.sub.7.2. 
For the purpose of data exchange between the central unit 1 and the 
communication module M1, the first control signal S1 will be set to a high 
resistance state and the second control signal S2 to a low-voltage state; 
this will block the second diode switch 7 whilst the first diode switch 6 
will conduct the signals from the transmission output T1 of the 
communication module M1. The diode D6 will be connected in conducting 
direction whilst the diode D7 is connected in non-conducting direction. 
For data exchange between the central unit 1 and the diagnosis module M2, 
the first control signal S1 will be set to a low-voltage state and the 
second control signal S2 to a high resistance state; this causes the first 
diode switch 6 to block whilst the second diode switch 7 conducts the 
signals from the transmission output T2 of the diagnosis module M2. 
The voltage dividers R.sub.6.1 /R.sub.6.2 and R.sub.7.1 /R.sub.7.2 are 
selected here in accordance with the signal level voltages required. Thus, 
it has proven to be particularly advantageous for interrogating the 
potential 5 by the central unit 1 to select the partial resistor R.sub.7.2 
relative to the supply voltage U to be smaller (4.7 k.OMEGA. for example) 
than the partial resistor R.sub.7.1 in order to enable the voltage drop 
across the partial resistor R.sub.7.2 to be detected for interrogating the 
potential 5, even with a relatively low driver capacity of the 
transmission output T2. 
Naturally, it is in principle also possible to interrogate the potential of 
communication module M1, in analogy to the way it is done for the 
diagnosis module M2, and--if necessary--also to change the control 
priority. 
The transmission line 3 of the central unit 1 is respectively connected to 
the reception inputs R1 and R2 of the communication module M1 or the 
diagnosis module M2, via two decoupling resistors 8 and 9. From the 
transmission line 3, following the decoupling resistors 8 and 9, the 
control signals S3 and S4 are connected in, respectively, such that these 
will each act on the reception inputs R1 and R2 of the communication 
module M1 and the diagnosis module M2 but will not interact mutually with 
each other or with the transmission line 3. For data exchange between the 
central unit 1 and the communication module M1, the third control signal 
S3 will be set to the high resistance state and the fourth control signal 
S4 to the high-voltage state; this causes the reception input R2 of the 
diagnosis module M2 to be fixed at high level which means its quiescent 
condition whilst the reception input R1 of the communication module M1 
adopts the current signal of the signal line 3 from the central unit 1. In 
reverse, appropriately, for data exchange between the central unit 1 and 
the diagnosis module M2, the third control signal will be set to the 
high-voltage state and the fourth control signal will be set to a high 
resistance state; this causes the reception input R1 of the communication 
module to be fixed at high level which means its quiescent condition 
whilst the reception unit R2 of the diagnosis module M2 adopts the current 
signal of the transmission line 3 from the central unit 1. 
Due to the simple setup, using elementary components, this circuit layout 
can be implemented at significantly lower cost than pure multiplexer 
assemblies; and, by making skillful use of available free space, this 
circuit layout can even be arranged more compactly--and, that is, if 
necessary, distributed--on a printed circuit board. 
In contrast, FIG. 3 now shows the overall arrangement of the central unit 1 
as well as the communication module M1 and the diagnosis module M2, which 
are respectively connected to a transceiver T/R1 and T/R2--via the 
connection lines 11 and 12--which link the reception inputs (R1, R2) and 
the transmission outputs (T1, T2) with these bidirectional connection 
lines 11 and 12. Again, in accordance with FIG. 2, the circuit layout is 
shown as a multiplexer equivalent circuit 4 where the multiplexer is 
driven from the central unit via the signal lines S1 to S4. The central 
unit 1 is again connected to the transmission output T2 of the diagnosis 
module M2, via the potential tapping point 5. The transceivers T/R1 and 
T/R2, as well as the connection lines 11 and 12, and the corresponding 
transceiver assemblies in modules M1 and M2 provide for a spatial 
distribution of the communication module M1 and the diagnosis module M2 
within the vehicle, with the diagnosis module M2 also being connected 
in--for instance by means of a connector--only when required, e.g. when 
the vehicle is at the garage for routine servicing. Here, only a single 
connection line 11 or 12 will be required, respectively, if this is 
operated bidirectionally. In addition, the central unit 1 can be 
controlled via a common serial transmission and reception interface SCI; 
this causes the costs for this central unit 1 to be significantly reduced 
when compared with the use of several serial interfaces or even several 
central units. Interrogating the potential at point 5 enables the central 
unit 1 to provide for a switchover between the transmission and reception 
lines (2 and 3).