The present invention relates to an interface circuit for connecting an electrical unit disturbance-free to an electrical bus, in which time-divided bus signals pass between several electrical units, said interface circuit comprising at the output thereof a three state buffer for each signal conductor to be connected to the bus of the electrical unit, said buffer being provided with a control input for bringing said buffer to a high-impedance state for connecting the electrical unit to the bus and for disconnecting it from the bus.

The data communications apparatus, such as data communi- cation means connected to telecommunications networks, fre¬ quently comprise a plurality of so-called plug-in units connected to the data communication bus in which data passes. Hereby, data is so transferred that the data to be transferred is supplied through a plug-in unit to a bus from which .another plug-in unit picks said data for trans¬ fer to a desired address. Thus, the plug-in units both supply data to the bus and pick data therefrom, as well as include logic for processing said data. In current digital data transfer networks, the time-division technique has more and more frequently been adopted, whereby time-divided synchronous signals are used. From the time-divided synchronous bus signals, the plug-in units pick given time intervals, and respectively, supply data of certain time intervals to the bus. In apparatus with large data transfer capacity the electrical plug-in units are ensured as re¬ gards their reliability, whereby a plug-in unit provided with an ensuring capacity has to be in connection with the bus in order to be switched on if another plug-in unit is damaged. When another plug-in unit is connected to the bus, or disconnected therefrom, the internal signals within the system become disturbed unless specific circuit designs are used. In the apparatus processing time-divided signals,

also the signals of the other time intervals become dis¬ turbed when plug-in units processing certain time intervals are added therein or removed therefrom unless specific cir¬ cuit designs are used.

The plug-in units may in practice be circuit cards pro¬ vided with a different logic, said cards being connected to the bus with connectors, e.g. so that the plug-in unit is easy to attach and detach. In order to avoid disturbances at the moment of attaching and detaching, so-called length¬ ened pins have been in use in the plug-in unit for the power supply of the plug-in unit (i.e. operating voltage) and/or respectively, shorter, pins for signal conductors in order to have the operating voltages connected to the plug- in unit prior to the connection of the signal lines to the bus, respectively, for the signal conductors to be discon¬ nected from the bus prior to the removal of the operating voltages of the plug-in unit. The use of said design is, however, difficult because the plug-in units usually com- prise a great number of conductors, and it is necessary to make sure that for each conductor, a pin of correct length with connectors has been provided. Another alternative is to use separate, so-called interface circuits, which are commercially available. Said interface circuits are pos- itioned between the logic of the plug-in unit and the bus, and they comprise, inter alia buffers, such as three state buffers, which are brought to a high-impedance state until the plug-in unit has at the signal conductors thereof been connected to the bus, and it is brought off from the high- impedance state when the plug-in unit is taken into use. A drawback of said separate interface circuits is that they include circuits for a great number of different appli¬ cations, whereby, with a view to a given application, they contain unnecessary circuits, thus taking too much space. The plug-in units are moreover usually provided with a greater number of signal conductors than there are buffers in commercially available interface circuits, so that

several interface circuits are required, these taking much space in the circuit card containing a plug-in unit, this card being made as small as possible so that the size of the data transfer means can be maintained small. Also, the interface circuits have to be brought externally at a right moment to their proper states in order to provide disturb¬ ance-free connection. Hence, disturbance-free connection may be uncertain.

The objective of the present invention is to implement disturbance-free connection to a time-divided bus to avoid the drawbacks mentioned above. This is possible with the aid of an integrated interface circuit comprising buffers and control circuits wherewith the automatic control of the logical states of the buffers and the minimisation of the auxiliary voltage required by the interface circuits have been implemented by disconnecting the clock signals of the logical circuits of the plug-in circuit automatically while being connected to the bus. In this manner, disturbance- free connection is ensured by means of a low power consump¬ tion, and thanks to the automatic control, the disturbance- free connection is well controlled and the disturbance of the signals can surely be avoided. In order to avoid said drawbacks, the interface circuit is moreover integrated to the same microcircuit as the actual operational logic of the plug-in unit.

The interface circuit according to the invention is char¬ acterized in that it comprises a control block to control the state of each three state buffer and in which the out¬ put states provided thereby have been preprogrammed, and which automatically sets the states of the three state buffers, and which is moreover provided with an input for programming it externally.

The interface circuit of the invention comprises a three state buffer connected to the bus of an electrical unit

with each signal conductor, a logic controlling each three state buffer and setting every time in a predetermined state according to what has been programmed in advance in the memory of the control logic. The memory and hence, the control logic may also be brought separately to desired logical states. In addition, means, e.g. a controllable switch, have been arranged in the interface circuit for disconnecting the clock signal of the logical circuit, said switch being controlled also by means of the logic con- trolling the three state buffer.

When a plug-in unit is connected to a bus, it receives from an external supply line a low-level auxiliary voltage with which to operate. For such auxiliary voltage line, a lengthened pin can be provided in the circuit card so that the interface circuit will receive the auxiliary voltage before the connection of the plug-in unit to the bus, whereby the interface circuit is in controlled state at the moment of connection. The control logic brings the three state buffers to a high-ohmic state, whereby the bus will neither be loaded nor disturbed by the plug-in unit, said unit controlling the disconnection of the clock signal of the logical circuit for minimizing the power consumption, in which case even a very insignificant auxiliary voltage suffices for the interface circuit. When the plug-in unit is taken into use, the power supply unit thereof is ex¬ cited, and when sufficient operating voltage is provided by the power supply unit, the disconnection of the clock sig¬ nals to the logical circuits is removed, and a signal is transmitted to the control logic about the changed state, whereby it releases the three state buffers from the high- impedance state so that input data to the plug-in unit can be supplied from the logical circuits out to the bus, res¬ pectively, a signal can be received from the bus. Hereby, the buffers observe the logical states of the data to and from the bus.

The invention is described below more in detail referring to the accompanying figure presenting the interface circuit according to the invention and the connections thereof to the bus and to the logical circuits of the plug-in unit.

In the circuit diagram shown in the figure, the interface circuit of the invention comprises a three state buffer 1 for each signal conductor S, a control block 2 ensuring controlled disturbance-free connection to the bus (not shown) , said block comprising a bi-stable logic 3 and a memory 4 setting the states of certain blocks in the in¬ terface circuit, preferably a transfer register 4 provided with the output states of the bi-stable logic 3 in the memory, and a disconnecting means 5,6 of a clock signal between at least one clock signal 15,16 and at least one logical circuit 7,8 of the plug-in unit. The logical cir¬ cuits 7,8 presented in the figure are not part of the in¬ terface circuit according to the invention, but part of the plug-in unit to which also the interface circuit is prefer- ably integrated.

When a plug-in unit is connected to a bus, it is provided with a small auxiliary voltage from an external supply line (not shown) , being needed by the interface circuit for the operations thereof, while the rest of the plug-in unit re¬ mains idle. For the auxiliary voltage line, a lengthened pin can be provided on the circuit card to provide the in¬ terface circuit with an auxiliary voltage prior to con¬ nection of the plug-in unit to the bus, whereby the inter- face circuit is in controlled state at the moment of con¬ nection. The control block 2 brings the three state buffers 1 into high-ohmic state, whereby the bus is neither loaded nor disturbed by the plug-in unit, said unit controlling the disconnection of the clock signals 15,16 of the logical circuits 7,8 in order to minimize the power consumption, so that even a very insignificant auxiliary voltage suffices for the interface circuit. The interface circuit is pro-

vided with a register access line 18 connected to a micro¬ processor (not shown) controlling the plug-in unit, to bring the output states of the bi-stable logic 3 control¬ ling the three state buffers 1 into the memory of the con- trol register 4. When the plug-in unit is connected to the bus, the control register 4 brings the outputs 10 of the bi-stable logic 3 to the states in the control register memory automatically, so that the three state buffers are set into high-impedance state, and the signals passing in the bus are not therefore disturbed by the plug-in unit connected to said bus. The bi-stable logic 3 provides at the same time the controls 11,12 to the clock signal dis¬ connection means 5,6, such as controllable switches, in order to disconnect the clock signals 15,16 of the logical circuits 7,8. When the plug-in unit is taken into use, the power supply unit thereof (not shown) is excited and when the power supply unit provides sufficient operating vol¬ tage, disconnection of the clock signals 15,16 from the logical circuits 7,8 is removed, and a signal is provided to the control block 2 about the changed state, whereby it removes the three state buffers 1 from the high-impedance state, so that input data S into the plug-in unit can be supplied from the logical circuits 7,8 out to the bus, respectively, a signal S can be received from the bus. Hereby, the buffers observe the state of the data to and from the bus. The connection of the clock signals 15,16 to the logical circuits 7,8 is so accomplished that when the power supply unit has been started to a requisite operating voltage level, the data thereof enters the interface cir- cuit to a hardware reset line 13 wherefrom the control is brought either directly to the clock signal disconnection means 6 and to the bi-stable logic 2, or merely to the bi¬ stable logic 2. When the bi-stable logic 2 receives a hard¬ ware reset signal 13, it controls 11 the disconnection means 5 to connect the clock signal to the logical circuit 8, bringing 10 the three state buffers 1 off from the high- ohmic state. Instead of that the data about the starting of

the power supply unit would enter the hardware reset-line 13, the microprocessor is made to provide the control block with data 18 thereabout by arranging new states in the con¬ trol register. In addition to what is said above, the hard- ware reset line 13 can be used as inhibit control 13 so that, when the three state buffers normally are automati¬ cally brought to high-impedance state when a plug-in unit is connected to a bus, this can be inhibited, if desired, by arranging a control for the bi-stable logic 3 and the disconnection means 5,6 in the inhibit control line 13, wherewith the disconnection of the clock signal and the output of the three state buffer are inhibited.

The figure shows no power supply unit, neither how to con- nect it to the circuits of the plug-in unit to supply the operating voltage. In practice, this can be solved in that between the auxiliary voltage line, the line from the power supply unit and the plug-in unit, a switch is arranged, this being controlled by the power supply unit. The control now acts so that the switch switches an auxiliary voltage on for the plug-in unit and the interface circuit for the time when the plug-in unit is being connected to the bus, and when the plug-in unit is taken into use, the micro¬ processor controlling said plug-in unit excites the power supply unit, which after being started sets the switch to a second position, whereby the plug-in unit and the interface circuit are provided with the operating voltage from the power supply unit, not from the auxiliary voltage line. Data from the power supply unit concerning the excitation thereof is simultaneously given to the hardware reset line 13 in the interface circuit.

With the aid of the present invention, electrical units, such as plug-in units, can be connected to a bus and dis- connected from the bus disturbance-free. The invention is particularly well suited for such data transfer apparatus, in the buses whereof the time-divided signals are processed

and transferred.