Interface for an I2C bus

The invention relates to an interface circuit for connecting a first apparatus to at least one second apparatus distant thereto by way of a bi-directional bus, in particular an I 2 C bus which comprises a data lead for the bi-directional transmission of data and a cycle lead for the bi-directional transmission of a cycle signal.

Such an interface circuit is known from EP 0 759 593 A2. The known interface circuit serves for transmitting the data of an apparatus controlled via an I 2 C bus to a computer and reverse via a so-called RS 232 interface. The data transfer is made by way of three control leads, wherein the one lead frees the other lead in its logic condition “0” and blocks it in its logic condition “1”. In this manner data may be transmitted from the computer to the controlled apparatus in “bits” and the controlled apparatus may be cycled.

Generally with the I 2 C bus, which consists of two leads SDA and SCL transmitting data and cycle signals in both directions, no error recognition mechanisms with respect to hardware are present. The recognition, and where appropriate alleviation, of transmission errors may only be realized through software (cf. the publication of “The I 2 C bus” of Ludwig Brackmann in ELRAD 1991, booklet 5, pages 44-47). The mentioned publication also specifies that the I 2 C bus serves as the communication link between the integrated circuits of a circuit board or between several circuit boards of an apparatus. In this application the I 2 C offers advantages with respect to the small surface area occupied on the circuit board, low operative costs and high reliability.

With medical applications there is often the requirement to exchange data or control commands between apparatuses distanced from one another. This is, for example, the case with a TEM insufflation system (TEM-transanal endoscopic microsurgery) subdivided into two apparatuses. The known apparatuses in the application must be connected to one another and be able to mutually receive and transmit information. With these types of medical applications it is extraordinarily important that all data and control commands are transmitted in an undisturbed and genuine manner. Since the apparatuses are to be as small and as inexpensive as possible, there is therefore no opportunity for software solutions for the recognition or alleviation of transmission errors. Such transmission errors may not only occur by electric coupling in the leads from the outside but also by potential differences between the apparatuses. Security against malfunctioning implemented through software, if it could successfully be carried out at all, would have too great of an expense.

Against this setting of the problem, it is the object of the invention to so form the interaction of apparatuses which are controlled by an I 2 C bus and which are distanced from one another, such that in a physical way malfunctioning of the data exchange is ruled out.

According to the invention such a solution is effected in that the data signal SDA and the cycle signal SCL of the I 2 C bus, before reaching the transmitter/receiver component and the transmission leads to the distanced apparatus, in each case are separated into a transmitting branch and a receiving branch and that, via a differential transmitter/receiver component, the signals arc transmitted between the apparatuses via differential collection leads.

At the differential transmission leads there are thus present identical signals which are, however, opposite in their polarity so that, based on a difference formation between the signals, potential data errors are eliminated. The transmitter/receiver components are terminated with characteristic impedance of the transmission leads in a manner free of reflection, by which means longer connection leads of several meters to a few kilometers may also be used.

The separation in each case into a transmitting and receiving branch is advantageously carried out by two Opto-couplers which are decoupled from one another via fast diodes and which at the same time also achieve a galvanic separation of the apparatuses communicating with one another.

These Opto-couplers are thus connected between the apparatus to which the interface is allocated and the differential bus driver. After the Opto-coupler, the now occurring signal is inputted to the differential bus driver (i.e. the transmitter/receiver component). The receiver of the differential bus driver is switched such that it always monitors the signals on the I 2 C bus. The transmitter, however, is by way of logic circuitry only activated when a ‘0’ is transmitted.

Of course, provided with the interface circuit according to the invention, not only two but more than two apparatuses may communicate with one another. Each apparatus must have the same interface (circuit arrangement). The multimaster capability and the transmission frequency of the I 2 C remain intact.

The invention is described in more detail by way of a block diagram (FIG. 1) of a preferred embodiment of the interface circuit according to the invention.

The interface circuit represented in FIG. 1 relates to the external connection of two apparatuses A and B by way of an I 2 C bus. The manner of functioning, the construction and the protocol of the I 2 C bus are assumed to be known (cf. e.g. the previously mentioned technical article “The I 2 C bus” by Ludwig Brackmann).

The interface circuit comprises a circuit arrangement allocated to each apparatus. The represented circuit arrangement is allocated to a first apparatus A symbolically represented on the left side of FIG. 1 . The interface circuit, via data lead SDA, receives data from the first apparatus A and transmits data to the second apparatus B. In a similar manner the circuit arrangement receives data from the distanced apparatus B and sends this data to the first apparatus A. The interface circuit also receives/transmits cycle signals from/to the first apparatus A via the cycle lead SCL.

The represented interface circuit is incorporated into a TEM insulation system. The first apparatus A is a Laparo-Pneu and the second apparatus is a TEM pump. With TEM it is specifically necessary to unfold the rectum with CO 2 gas and, when required, to rinse it. The TEM pump and the Laparo-Pneu of this application must be connected to one another and be able to mutually receive and transmit information. With this application it is particularly important that the data to be transferred is transmitted in as undisturbed and genuine a manner as possible. Since, as already mentioned, the I 2 C bus in itself does not provide any error recognition mechanism with regard to hardware, the interface circuit according to the invention provides physical (i.e. circuit) means which are able to practically exclude disturbances in the data exchange occurring on the transmission path.

The interface circuit for each apparatus A, B comprises identical circuit arrangements, of which that belonging to apparatus A is shown in FIG. 1 . For the data lead SDA and the cycle lead SCL there is provided a separating means consisting of Opto-couplers 1 a , 1 b , 2 a , 2 b , which separate the data and cycle signals of the I 2 C bus in each case into a transmitting branch S and a receiving branch E. The Opto-couplers 1 a , 1 b and 2 a , 2 b used for this separation simultaneously carry out a galvanic separation between the first apparatus A and the second apparatus B. By way of this measure, possible disturbances because of diverting currents or differences in potential are eliminated.

The Opto-couplers 1 b , 2 b of the receiving branches E arc only active when the Opto-coupler 1 a and 2 a is inactive (i.e., SDA is not sending a “0” to the differential transmitter)and a “0” is received by the differential receiver and input to the Opto-coupler 1 b , 2 b in the corresponding receiving branch E. It must also be prevented that the respective Opto-coupler 1 b , 2 b in the receiving branches E does not simultaneously couple back a transmitted “0” at the SDA input to the apparatus A, since otherwise a self-blocking would arise. The Opto-couplers 1 a , 1 b and 2 a , 2 b of the transmitting and receiving branches S and E are decoupled from one another by fast diodes, which, however, are not shown in the circuit diagram of the FIG. 1 . The data signals SDA from/to the Opto-couplers 1 a , 1 b (i.e. the data signals SDA allocated to the transmitting and receiving branches S, E), as well as the cycle signals SCL from/to the Opto-couplers 2 a , 2 b (i.e. the cycle signals SCL allocated to the transmitting and receiving branch S, E), are in each case applied separately to two differential bus drivers 4 , 6 comprising transmitters 4 a , 6 a and receivers 4 b , 6 b.

Each receiver 4 b and 6 b of the differential bus drivers 4 , 6 always monitors the signals on the I 2 C bus. The transmitters 4 a and 6 a are however only actively switched by way of a release logic 7 a and 8 a respectively when a “0” is transmitted from SDA, (i.e. the transmitters 4 a and 6 a , during transmission, are respectively switched on and off). For this, the signal SDA and the cycle signal SCL in the transmitting branches S are led from the Opto-coupler via a respective inverting member 3 and 5 , respectively, to the release logic 7 a and 8 a , respectively, of the differential bus driver component 4 and 6 respectively. This means that the inverting members 3 and 5 merely ensure that the bus driver components 4 and 6 respectively are started or stopped. The respective data entrances D of the transmitters 4 a , 6 a are always kept fixed at ground.

The differential bus driver components 4 , 6 convert the I 2 C bus signals SDA and SCL into physical differential signal levels SDA+, SDA− and SCL+ and SCL− respectively. The signal lines SDA+, SDA− and SCL+, SCL− are connected with resistors R 1 , R 2 and R 3 , R 4 respectively, whose values are the respective characteristic impedances, and also to the ends of the differential bus leads lying at the interface. The resistances R 1 and R 4 determine the level of SDA+, SDA− and SCL+, SCL− respectively, while the transmitter is deactivated.

In the receiving branches E no inversion is necessary nor is desired, since indeed the signal arriving at the receiving point should be read unchanged, (i.e. not inverted).

The second apparatus B contains an exactly identical interface circuit arrangement as that described herein for the first apparatus A.

The above details show that the interface circuit according to the invention 5 is suitable for an error-free transmission of data or control signals between distanced apparatuses of micro-surgery. In particular the interface circuit according to the invention is suitable for two apparatuses of a TEM insufflation system, which are separated from one another, since the apparatuses separated from one another are galvanically separated and the transmission between the apparatuses may be effected in a error-free manner by the differential bus driver components and the data signal leads SDA+, SDA− and cycle signal leads SCL+, SCL− which are connected to the bus-driver components. The transmission frequency as well as the known protocols for the data exchange via an I 2 C bus remain intact.