Abstract:
Isolation components such as p-n junction or Schottky diodes are provided at pull-up resistors of each signal line of a Gunning Transceiver Logic (GTL) backplane bus in an electronic system for improved reliability, specifically to prevent momentary termination of the bus to ground when a circuit card incorporating the pull-up resistors is inserted into the system.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention is directed to communications systems such as data communications and telecommunications switching systems, although it is not limited thereto. In particular, the present invention is directed to a Gunning Transceiver Logic (GTL) backplane bus for such systems. 
       BACKGROUND OF THE INVENTION  
       [0002]    Communications systems, and in general many other types of electronic systems, rely on a backplane based architecture comprising a plurality of circuit cards that plug into, or are otherwise electrically connected to, a system backplane. The system backplane is a facility over which the circuit cards can communicate with each other and over which power is supplied to the circuit cards. Examples of the type circuit cards included in communications systems include switch fabric cards, control cards, input/output (I/O) cards, line cards, and processor cards. Due to the need for high fault tolerance in such systems, also referred to as high availability, one of more of these types of circuit cards may be replicated in the system for redundancy. 
         [0003]    A system backplane includes a backplane bus for inter communication between the circuit cards. A backplane bus commonly used for high speed systems is a Gunning Transceiver Logic (GTL) type bus. A GTL bus includes an open drain MOSFET transmitter circuit for each bit or signal line of the bus and one or more respective receiver circuits. A transmitter and receiver circuit may be included in one device thereby forming a GTL transceiver. The GTL bus has a low voltage swing, typically 0.8 volts, resulting in good performance at high data rates, low power consumption and minimal crosstalk between bus signals. Each bit line of a GTL bus is terminated to a termination voltage Vtt through a termination resistor Rtt, which is typically about 50 ohms. The termination voltage Vtt is typically 1.2 volts, although a newer version of the GTL specifications, GTL+, specifies a termination voltage of 1.5 volts. Herein after, the term GTL includes all such versions of the GTL buses unless stated otherwise. Additional details regarding GTL circuits and transceivers may be found by referring to U.S. Pat. No. 5,023,488 to Gunning et al. 
         [0004]    In the case of redundant systems, the transmitter of a GTL bus may be replicated on another circuit card for higher system reliability.  FIG. 1  depicts such a redundant GTL backplane bus as known in the prior art. Referring to  FIG. 1 , a redundant GTL backplane bus  10  includes a pair of termination cards  12 ,  14  labeled termination card A and B, that are both capable of driving electrical signals onto backplane connections  16  of the bus  10 . However, only one of the termination cards  12 ,  14  drives the bus at any given time, that card being designated as an active card of the pair of termination cards, while the other termination card  12 ,  14  remains in a standby mode and is designated as an inactive card of the pair. The termination card A  12  has a plurality of GTL transmitters  18  each of which is connected to one of the backplane connections  16 . Additionally, each of the GTL transmitters  18  is coupled at its output to a positive termination voltage Vtt A  via a corresponding one of a plurality of pull-up resistors  20 . The positive termination voltage Vtt A  is generated locally on the termination card A by a low dropout (LDO) voltage regulator  22 . Likewise, the termination card B  14  includes a plurality of GTL transmitters  24  each of which is connected to one of the backplane connections  16 . Additionally, each of the GTL transmitters  24  on the termination card B is coupled at its output to a positive termination voltage Vtt B  via a corresponding one of a plurality of pull-up resistors  26 . The positive termination voltage Vtt B  is generated locally on the termination card B by a low dropout (LDO) voltage regulator  28 . The positive termination voltages Vtt A  and Vtt B  are intended to be equal in value, and are also referred to herein as Vtt when specificity to a particular termination card A or B is not required. 
         [0005]    The redundant GTL backplane bus  10  may experience a problem if either one of termination cards A or B  12 ,  14  experiences an open or short circuit failure during normal operation. For example, if either one of the LDO voltage regulators  22 ,  28  or their associated components failed such that one of the termination voltages Vtt A  or VttB was at ground potential. In that case, rather than having a bus terminated to equal positive termination voltages Vtt A  and Vtt B  via parallel termination at both ends of the backplane connections  16 , each end would be terminated to a different voltage. This would likely cause the voltage levels on the bus  10  to be too low for successful data communications over the bus  10 . 
         [0006]    The type of failure just described with respect to the positive termination voltages Vtt A  and Vtt B  could be caused by various conditions such as component shorts/opens due to component aging or general malfunction of the card. However, another cause of particular concern is insertion of the inactive termination card. When the inactive termination card is inserted, a brief period of time passes before the LDO voltage regulator of that card can provide the termination voltage Vtt at its operational level. During this period of time, the operational effect is the same as having a termination voltage Vtt failure, and consequently the GTL backplane bus  10  is unable to reliably carry data communications between the various circuit cards of the system. 
         [0007]    In view of the foregoing, it would be desirable to provide some means of protecting the operation of the GTL backplane bus against failures of either of the termination cards A or B, particularly with respect to the supply of the positive termination voltage Vtt. 
       SUMMARY OF THE INVENTION  
       [0008]    It is an object of the present invention to provide a GTL backplane bus having improved reliability over those known in the art, particularly with respect to supply of the positive termination voltage Vtt. 
         [0009]    Embodiments of the invention include an isolation component series coupled in a respective circuit path between a backplane connection and a termination voltage source in a manner so as to prevent current flow from the backplane connection to the termination voltage source. Hereinafter the term coupled is used in reference to a connection between a first component and a second component where the connection is not necessarily direct in that a third component could be in present in the connection between the two components. Otherwise, where the connection is intended to be a direct one the term connected is used. An advantage of the manner of the coupling of the isolation component is that it provides fault isolation between the backplane connection and the termination voltage source when the latter is shorted or otherwise driven to ground potential. 
         [0010]    Some embodiments of the invention include a plurality of isolation components each one of which is serially coupled in a respective circuit path between a respective backplane connection of a plurality of backplane connections and the termination voltage source. An advantage of having each isolation component coupled in a respective path, i.e. its own unique or solitary path, between the backplane connection and the termination voltage is that this coupling arrangement provides fault isolation between bit lines of the GTL bus. 
         [0011]    According to an aspect of the invention a GTL backplane bus is provided. The GTL backplane bus includes a plurality of backplane connections and a first termination card. The first termination circuit card includes a first termination voltage source; and a plurality of isolation components each one of which is serially coupled in a respective circuit path between a respective one of the backplane connections and the first termination voltage source in a manner so as to prevent current flow from the respective backplane connection to the first termination voltage source. 
         [0012]    According to another aspect of the invention a circuit card for a GTL backplane bus is provided. The circuit card includes a termination voltage source; a connector having a plurality of connection terminals each one of which is for connecting to a respective backplane connection of the GTL backplane bus; a plurality of GTL transmitters each one of which is coupled to a respective connection terminal of the connector; and a plurality of isolation components each one of which is serially connected in a respective circuit path between a respective one of the GTL transmitters and the termination voltage source in a manner so as to prevent current flow from the respective backplane connection to the termination voltage source. 
         [0013]    According to still another aspect of the present invention a termination circuit card for a GTL backplane bus is provided. The termination circuit card includes: a termination voltage source; a connector having a plurality of connection terminals each one of which is for connecting to a respective backplane connection of the GTL backplane bus; and a plurality of isolation components each one of which is serially connected in a respective circuit path between a respective one of the connection terminals and the termination voltage source in a manner so as to prevent current flow from the respective backplane connection to the termination voltage source. 
         [0014]    According to still another aspect of the present invention a GTL transceiver module for a GTL backplane bus having a plurality of bit lines is provided. The GTL transceiver module includes: an input for connecting to a termination voltage source; an input/output port for connecting to one bit line of the plurality of bit lines; a GTL transmitter for driving an electrical signal onto the one bit line; a GTL receiver for receiving electrical signals from the one bit line; and an isolation component connected between the input/output port and the input in a manner so as to prevent current flow from the input/output port to the input. 
         [0015]    Advantageously, embodiments of the invention could be used in a system having a GTL backplane bus for improved reliability of operation of the bus, especially with respect to failures or other actions that could affect bus termination voltages such as insertion of circuit cards that supply such voltages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where: 
           [0017]      FIG. 1  schematically depicts a redundant GTL backplane bus of the prior art; and 
           [0018]      FIG. 2  schematically depicts a redundant GTL backplane bus in accordance with an embodiment of the present invention. 
       
    
    
       [0019]    In the figures like features are denoted by like reference characters. 
       DETAILED DESCRIPTION  
       [0020]    Referring to  FIG. 2 , an improved redundant GTL backplane bus  29  in accordance with an embodiment of the invention includes a plurality of isolation components  30  each one of which is connected in series between a respective one of the backplane connections  16  and a corresponding one of the termination resistors  20  of the termination card A  12 . Similarly, the termination card B  14  includes a plurality of isolation components  32  each one of which is connected in series between a respective one of the backplane connections  16  and a corresponding one of the termination resistors  26  of the termination card B  14 . 
         [0021]    More specifically, when there are a number m of backplane connections  16   a  to  16   m  there are the same number m of isolation components  30   a  to  30   m,  which are shown here as Schottky diodes, however use of alternative types of isolation components is possible as will be described later. Each one of the isolation components  30   a  to  30   m  is series connected in a circuit path unique to a respective backplane connection  16   a  to  16   m  between that backplane connection  16   a  to  16   m  and a common termination voltage Vtt A  generated locally on the termination card A  12 . Each such circuit path is unique to a particular backplane connection  16   a  to  16   m  in that it is a solitary path for current to flow from the common termination voltage Vtt A  to the particular backplane connection  16   a  to  16   m.  The unique circuit path aspect of this connection arrangement provides fault isolation between the backplane connections  16   a  to  16   m.  The termination card A  12  also includes a number m of termination resistors  20   a  to  20   m,  each one of which having a typical value of 50 ohms and being connected to the common termination voltage Vtt A  and a respective isolation component  20   a  to  20   m.  For example, termination resistor  20   a  is connected between the termination voltage Vtt A  and the anode of isolation component  30   a,  which in this case is a Schottky diode. The cathode of the isolation component  30   a  is connected to the backplane connection  16   a  of the termination card A  12 . It should be noted that the positioning of any one of the isolation components  30   a  to  30   m  relative to its respective termination resistor  20   a  to  20   m  could be interchanged without appreciably affecting the operation of the bus  29 . For example, the termination resistor  20   a  could be connected between the backplane connection  16   a  and the cathode of isolation component  30   a,  with the anode of the isolation component  30   a  connected to the termination voltage Vtt A . 
         [0022]    The termination card B  14  has the same configuration as the termination card A  12  wherein there is a number m of termination resistors  26   a  to  26   m,  each one of which is connected between a common termination voltage Vtt B  and an anode of a respective isolation component  32   a  to  32   m.  The cathode of each of the isolation components  32   a  to  32   m  is connected to a respective backplane connection  16   a  to  16   m  of the termination card B  14 . 
         [0023]    In operation with termination card A  12  active, if the LDO voltage regulator  28  of the termination card B  14  fails causing Vtt B  to drop to zero volts, or at least below a minimum operational voltage, the isolation components  32  will block current flowing into the termination resistors  26  of the termination card B  14 . Without this protection preventing such current flow, logic high voltage levels on the backplane connections  16  would drop below threshold values resulting in failed data communications over the bus  29 . Likewise when termination card B  14  is active, a similar failure of LDO voltage regulator  22  of termination card A  12  is protected from affecting operation of the bus  29  by isolation components  30 . As well as protecting against failures of the LDO voltage regulator and associated components on the inactive termination, the invention also provides protection against a momentary voltage drop of the termination voltage Vtt when the inactive termination card is inserted into the system. The minimum operational voltage of the termination voltage Vtt is at least no lower than a voltage which causes errors in data communications over the bus  29 . 
         [0024]    Numerous modifications and adaptations may be made to the embodiment of the invention described above without departing from the scope of the invention, which is defined in the claims. For example, any GTL transmitter or any of the plurality of GTL transmitters could be replaced by a GTL transceiver or a plurality of GTL transceivers, respectively. A GTL transceiver comprises a GTL transmitter and a GTL receiver, and provides to a circuit card having said transceivers with the capability of communicating over the GTL backplane bus in a bidirectional manner. Multiple GTL transceivers could be implemented in one integrated circuit or other type of module. Alternatively, any of the plurality of GTL transmitters could be implemented on one module. Additionally, a circuit card may be augmented with one or more GTL receivers, some or all of which being either in discrete form or implemented in a module. A circuit card may have no capability to drive electrical signals onto the GTL backplane bus, but instead may only have capability to receive such signals, via one or more GTL receivers, and in which case the circuit card may be referred to as a slave card. A circuit card may not have any GTL transmission or GTL receiving capabilities, but instead only provide termination of backplane connections of a GTL backplane bus to a termination voltage source. 
         [0025]    Adaptations include replacing any of the Schottky diodes in the described embodiment with a p-n junction silicon or germanium diode. Other adaptations include replacing any of the Schottky diodes with any one of: a collector-base shunted NPN bipolar transistor, an collector-base shunted PNP bipolar transistor, or a drain-gate shunted PMOS or NMOS field effect transistor connected so as to block current flow from a corresponding backplane connection when the termination voltage Vtt of the termination card of the isolation component in question drops below an minimum operational voltage. 
         [0026]    It should now be apparent that the described embodiment, modifications, and adaptations provide an advantage over existing redundant GTL backplane buses by removing a single point of failure with respect to the termination voltage of the inactive termination card in a redundant system.