Hot-swappable high speed point-to-point interface

Details Hot-swappable high speed point-to-point interface Hot-swappable high speed point-to-point interface

1999-12-09

2001-04-03

Lefkowitz, Sumati (Department: 2181)

Electrical computers and digital data processing systems: input/

Intrasystem connection

Bus access regulation

C710S106000, C326S021000, C326S090000

Reexamination Certificate

active

06212586

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a hot-swappable point-to-point interface for use in a high speed, differential, serial backplane.
BACKGROUND ART
With the advent of low cost Fibre Channel and Gigabit Ethernet transceivers, a communication or computer architecture using a low cost, high speed, serial backplane becomes increasingly feasible. These high speed transceivers, which are normally used to connect computers and other devices in networking applications, are well suited for application in a serial backplane. One advantage is a communication rate of 1 gigabit per second. A second advantage is operation using a single positive voltage power supply.
In a typical networking application of these transceivers, differential outputs of a transmitter are AC coupled to a differential transmission line. The transmission line is routed to a differential receiver. Termination and DC bias circuitry connected to the receiver input may include differential and common mode components, reducing reflections present on the transmission line and preventing conversion of differential mode propagation to common mode and vice-versa. The biasing structure further restores the DC portion of the signal that was lost through AC coupling, setting the incoming signal to a level appropriate for the receiver.
In a typical networking system, transmitters and receivers may be located on separate printed circuit cards. These cards are inserted into a backplane which provides electrical connectivity between the cards. For many reasons, including maintenance, reconfiguration, upgrades, and the like, it is desirable to remove cards from and insert cards in the backplane without removing power from the remainder of the system. The addition or removal of a card from a system without removing power is known as hot-swapping or live insertion.
As cards are hot-swapped, a transmitter may be connected to a transmission line with no terminator. This creates a source of electromagnetic interference (EMI). Because of the high speed edges generated by the drivers, this may also create a high-Q resonator that can damage the transmitter. Hot-swapping may also create a situation in which a receiver is not connected to a transmitter. This creates a differential input with a DC bias but no AC signal. For certain types of receivers such as crosspoint switches or a positive supply emitter-coupled logic (PECL) buffer, the input may oscillate or behave poorly.
What is needed is a system that enables cards used in a high speed point-to-point differential backplane to be hot-swapped. The cards should operate from only positive voltage supplies and should maintain proper signal termination. Various high speed logic families should be supported. Transmitter output EMI and ringing as well as receiver input oscillation created by removing a corresponding device should be eliminated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a point-to-point serial backplane interconnect that enables cards to be hot-swapped.
Another object of the present invention is to provide hot-swappability of cards using only positive power supplies.
Still another object of the present invention is to provide hot-swappability of cards while maintaining proper signal termination.
Yet another object of the present invention is to provide hot-swappability between elements of high speed, differential balanced logic families.
A further object of the present invention is to provide hot-swappability between transmitters and receivers used to create point-to-point serial bus connections.
A still further object of the present invention is to prevent EMI and resonance from a transmitter left unterminated because a corresponding receiver has been removed.
Yet a further object of the present invention is to prevent oscillation on the input of a receiver left disconnected by removal of a corresponding transmitter.
Yet a still further object of the present invention is to interface various differential logic families available now and in the future.
In carrying out the above objects and other objects and features of the present invention, a system for a hot-swappable point-to-point connection between a high speed transmitter on a first card and a high speed receiver on a second card is provided. The transmitter is disabled if the transmitter output is not connected to a fixed voltage through resistive elements. The receiver is disabled if the receiver input is not biased to a preset voltage. The first card and the second card can be inserted into a backplane, the backplane forming the connection between the transmitter and the receiver. The system includes a power indicator on the first card connected to the backplane when the first card is inserted in the backplane, the power indicator operable to assert a power signal when power is applied to the first card. A switch on the second card is connected to a bias network, the bias network providing a bias voltage to the input of the receiver. The switch has a control input connected to the backplane when the second card is inserted in the backplane. The switch enables the receiver when the control input is asserted and disables the receiver when the control input is unasserted by changing the bias voltage of the bias network. A connection through the backplane forms a path connecting the power indicator to the control input when the first card and the second card are inserted in the backplane. Therefore, the control input is unasserted if the second card is inserted in the backplane and the first card is not inserted in the backplane, disabling the receiver when the transmitter is not connected to the receiver. Likewise, a second power indicator on the second card is connected to the backplane when the second card is inserted in the backplane. A switch on the first card connects the transmitter output to a fixed voltage through at least one resistive element. The switch has a control input connected to the backplane when the first card is inserted in the backplane. The switch enables the transmitter when the control input is asserted and disables the transmitter when the control input is unasserted. Another connection through the backplane forms a path connecting the power indicator to the control input when the first card and the second card are inserted in the backplane. Therefore, the control input is unasserted if the first card is inserted in the backplane and the second card is not inserted in the backplane, disabling the transmitter when the transmitter is not connected to the receiver.
In one embodiment of the present invention, each power indicator may be a connection to a power bus on the card containing the power indicator. In an alternate embodiment, each power indicator may be a power-on reset generator operable to assert the corresponding power signal a preset time after power is applied to the card containing the power indicator.
A transmitter card is provided that includes a switch. The first end of the switch is connected to a fixed voltage. The control input of the switch is connected to the backplane when the transmitter card is inserted in the backplane. The switch closes when an asserted power signal is applied to the control input and is open otherwise. The transmitter card also includes at least one resistive element having a first end connected to the second end of the switch. A transmitter with an output is further included. The output is connected to the backplane when the transmitter card is inserted in the backplane. The output is further connected to the second end of the at least one resistive element. The transmitter is enabled if a path exists from the transmitter output to the fixed voltage through the at least one resistive element and is disabled otherwise. The transmitter card, backplane, and receiver card form a path between the transmitter and the receiver and a path between the power indicator and the control input when the transmitter card and the receiver card are inserted in the backplane. Thereby, if the transmitter card is in the backpl

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