Abstract:
A bus includes at least a pair of terminators interposed between a pair of connectors. A first one of the terminators is located within a predetermined distance from a first one of the connectors. A second one of the terminators is located within the predetermined distance from a second one of the connectors. The second terminator is selectively disabled in response to the second terminator being interposed between the first terminator and a third terminator of the bus.

Description:
BACKGROUND 
     The disclosures herein relate in general to information processing systems and in particular to a system and method for connecting electronic circuitry in a computer system. 
     A small computer system interface (“SCSI”) bus is one technique for communicating information and signals (e.g. interfacing or bridging) between a central processing unit (“CPU”) and other devices (e.g. hard drives). A SCSI bus can include terminators and connectors. The portion of a SCSI bus between terminators forms a bus path. 
     If a portion of the SCSI bus is not bounded by terminators, such portion forms a stub. According to the SCSI specification, the maximum physical length of a stub is restricted to 0.1 meter. Such a restriction imposes limits on bus routing, and on the placement of terminators and connectors. 
     Accordingly, a need has arisen for a system and method for connecting electronic circuitry in a computer system, in which various shortcomings of previous techniques are overcome. More particularly, a need has arisen for a system and method for connecting electronic circuitry in a computer system, in which fewer limits are imposed on bus routing, and on the placement of terminators and connectors. 
     SUMMARY 
     One embodiment, accordingly, provides for a bus that includes at least a pair of terminators interposed between a pair of connectors. A first one of the terminators is located within a predetermined distance from a first one of the connectors. A second one of the terminators is located within the predetermined distance from a second one of the connectors. The second terminator is selectively disabled in response t 6  the second terminator being interposed between the first terminator and a third terminator of the bus. 
     A principal advantage of this embodiment is that (a) various shortcomings of previous techniques are overcome, and (b) fewer limits are imposed on bus routing, and on the placement of terminators and connectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a block diagram of a computer system according to the illustrative embodiment. 
     FIG. 2 is a block diagram of a computer of the computer system of FIG.  1 . 
     FIG. 3 is a block diagram of a first SCSI bus. 
     FIG. 4 is a block diagram of a second SCSI bus. 
     FIG. 5 is a block diagram of a first alternative version of the SCSI bus of FIG.  4 . 
     FIG. 6 is a block diagram of a second alternative version of the SCSI bus of FIG.  4 . 
     FIG. 7 is a block diagram of a third alternative version of the SCSI bus of FIG.  4 . 
     FIG. 8 is a block diagram of a SCSI bus of the computer of FIG.  2 . 
     FIG. 9 is a block diagram of a first alternative version of the SCSI bus of FIG.  8 . 
     FIG. 10 is a block diagram of a second alternative version of the SCSI bus of FIG.  8 . 
     FIG. 11 is a block diagram of a third alternative version of the SCSI bus of FIG.  8 . 
     FIG. 12 is a schematic electrical circuit diagram of a first terminator of the SCSI bus of FIG.  8 . 
     FIG. 13 is a schematic electrical circuit diagram of a second terminator of the SCSI bus of FIG.  8 . 
     FIG. 14 is a schematic electrical circuit diagram of a third terminator of the SCSI bus of FIG.  8 . 
     FIG. 15 is a block diagram of the SCSI bus of FIGS. 8 through 11. 
     FIG. 16 is a block diagram of an alternative version of the SCSI bus of FIG.  11 . 
     FIG. 17 is a block diagram of the SCSI bus of FIG.  16 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a computer system, indicated generally at  100 , according to the illustrative embodiment. System  100  includes input devices  104 , a display device  106 , a print device  108 , and a computer  102  for executing processes and performing operations (e.g. communicating information) in response thereto as discussed further hereinbelow. In the illustrative embodiment, system  100  is an IBM-compatible portable personal computer (“PC”) that executes Microsoft Windows 95 operating system (“OS”) software, or alternatively is any computer that executes any OS. All Microsoft products identified herein are available from Microsoft Corporation, One Microsoft Way, Redmond, Wash. 98052-6399, telephone (425) 882-8080. 
     Computer  102  is connected to input devices  104 , display device  106  and print device  108 . Print device  108  is, for example, a conventional electronic printer or plotter. Also, computer  102  includes internal speakers for outputting audio signals. In an alternative embodiment, the speakers are external to computer  102 . Moreover, system  100  includes (a) a first computer-readable medium (or apparatus)  110  which is a floppy diskette and (b) SCSI devices  111  such as a computer hard disk drive controller (and the associated hard disk operated by it) which is a second computer-readable medium (or apparatus). 
     A human user  112  and computer  102  operate in association with one another. For example, in response to signals from computer  102 , display device  106  displays visual images, and user  112  views such visual images. Also, in response to signals from computer  102 , print device  108  prints visual images on paper, and user  112  views such visual images. Further, user  112  operates input devices  104  in order to output information to computer  102 , and computer  102  receives such information from input devices  104 . 
     Input devices  104  include, for example, a conventional microphone, a conventional electronic keyboard, and a pointing device such as a conventional electronic “mouse”, rollerball or light pen. User  112  operates the keyboard to output alphanumeric text information to computer  102 , and computer  102  receives such alphanumeric text information from the keyboard. User  112  operates the pointing device to output cursor-control information to computer  102 , and computer  102  receives such cursor-control information from the pointing device. The microphone translates audio frequencies into electronic signals, and computer  102  translates the electronic signals into digital information. 
     A network  114  includes a network local area network (“LAN”) control manager server computer (“LCM”). For communicating with (i.e. outputting information to, and receiving information from) network  114  (including the LCM), computer  102  includes a network interface card (“NIC”) which is yet another type of computer-readable medium (or apparatus) connected to computer  102 . 
     FIG. 2 is a block diagram of computer  102 , which is formed by various electronic circuitry components. In the example of FIG. 2, such electronic circuitry components reside on a system printed circuit board (“PCB”). As shown in FIG. 2, the electronic circuitry components of computer  102  include (a) a central processing unit (“CPU”)  202  for executing and otherwise processing instructions, (b) core logic  204 , also referred to as a “host bus bridge,” which includes electronic circuitry for communicating information and signals (e.g. interfacing or bridging) between CPU  202  and other electronic circuitry and devices, (c) a system memory  206  such as random access memory device (“RAM”) and read only memory device (“ROM”) for storing information (e.g. instructions executed by CPU  202  and data operated upon by CPU  202  in response to such instructions), (d) a network interface  208  (e.g. Ethernet, token ring, fiber distributed data interface (“FDDI”), and/or asynchronous transfer mode (“ATM”) circuitry) for communicating information and signals between core logic  204  and network  114 , (e) small computer system interface (“SCSI”) input/output (“I/O”) controller circuitry  210  for communicating information and signals between core logic  204  and a SCSI bus  212 , and (f) miscellaneous other buses and interface devices  214  (e.g. video camera, floppy diskette  110  of FIG. 1) for performing other operations of computer  102 . Also, computer  102  may include various other electronic circuitry components that, for clarity, are not shown in FIG.  2 . 
     In the example of FIG. 2, core logic  204  includes other I/O controller circuitry for receiving information and signals from input devices  104  and for outputting information and signals to output devices  216  (e.g. speakers, display device  106  and print device  108  of FIG.  1 ). Also, in the example of FIG. 2, core logic  204  includes a basic input/output system (“BIOS”) electrically erasable programmable read only memory device (“EEPROM”) for storing firmware information (e.g. instructions). In response to such firmware information, computer  102  operates its various components, as for example by outputting and responding to interrupt requests (“IRQs”). 
     As shown in FIG. 2, core logic  204  is connected to (a) CPU  202  through a bi-directional processor bus and (b) SCSI I/O controller circuitry  210  through a bi-directional system bus (e.g. peripheral component interface (“PCI”)bus). Also, core logic  204  is connected to input devices  104 , system memory  206 , network interface  208 , other buses and interface devices  214 , and output devices  216 , as shown in FIG.  2 . Accordingly, core logic  204  communicates information and signals between CPU  202 , SCSI I/O controller circuitry  210 , input devices  104 , system memory  206 , network interface  208 , other buses and interface devices  214 , and output devices  216 . 
     In a significant aspect of the illustrative embodiment, SCSI I/O controller circuitry  210  is connected to a bi-directional SCSI bus  212 . SCSI bus  212  includes terminators  218  and  220 . As shown in FIG. 2, SCSI bus  212  is connected to various SCSI devices  111  such as a first hard drive (“HD0”)  111   a,  a second hard drive (“HD1”)  111   b,  a third hard drive (“HD2”)  111   c,  a compact drive ROM (“CD-ROM”)  111   d,  an optical scanner device  111   e,  and a tape backup device  111   f.  Although not shown in FIG. 2, each of SCSI devices  111   a-f  is connected to SCSI bus  212  through a respective associated connector. 
     In the illustrative embodiment, SCSI bus  212  is (a) a multi-pin bus that includes a 32-bit data bus, or (b) a 68-pin wide bus that includes a 16-bit data bus, or (c) a 50-pin narrow bus that includes an 8-bit data bus. Also, SCSI bus  212  includes voltage supply line(s) (“power”), voltage reference line(s) (“ground”), and control lines. Each of hard drives  111   a-c  includes a respective hard disk drive controller (and the associated hard disk operated by it). 
     The portion of SCSI bus  212  between terminators (e.g. terminators  218  and  220  in FIG. 2) forms a bus path. If a portion of SCSI bus  212  is not bounded by terminators, such portion forms a stub. According to the SCSI specification, the maximum physical length of a stub is restricted to 0.1 meter. For example, in FIG. 2, the maximum physical length of any of stubs  222   a-f  is restricted to 0.1 meter, such as between SCSI device  111   a  and its respective associated connector to the bus path of SCSI bus  212 . Such a restriction imposes limits on bus routing, and on the placement of terminators and connectors. 
     FIG. 3 is a block diagram of a SCSI bus  300 . Although not shown in FIG. 3, each of SCSI devices D 1  and D 2  is connected to SCSI bus  300  through a respective associated connector. SCSI device D 1 , for example, may be a device controller such as a hard disk drive controller. 
     In FIG. 3, the portion of SCSI bus  300  between terminators T 1  and T 2  forms a bus path. Accordingly, in FIG. 3, the maximum physical length of any of stubs  302   a  and  302   b  is restricted to 0.1 meter, such as between SCSI device D 1  and its respective associated connector to the bus path of SCSI bus  300 . As discussed further hereinbelow in connection with FIGS. 4 through 7, such a restriction imposes limits on bus routing, and on the placement of terminators and connectors. 
     FIG. 4 is a block diagram of a SCSI bus  400 . Although not shown in FIG. 4, each of SCSI devices D 1  and D 2  is connected to SCSI bus  400  through a respective associated connector. In FIG. 4, the portion of SCSI bus  400  between terminator T and a connector C 1  is not fully bounded by terminators, because connector C 1  is not interposed between terminator T and a second terminator. Accordingly, such portion forms a stub. 
     Likewise, the portion of SCSI bus  400  between terminator T and a connector C 2  is not fully bounded by terminators, because connector C 2  is not interposed between terminator T and a second terminator. Accordingly, such portion forms a stub. According to the SCSI specification, the maximum physical length of each stub (e.g. between T and C 1 , and likewise between T and C 2 ) is restricted to 0.1 meter. Similarly, the maximum physical length of any of stubs  402   a  and  402   b  is restricted to 0.1 meter, such as between SCSI device D 1  and its respective associated connector to the bus path of SCSI bus  400 . Such a restriction imposes limits on bus routing, and on the placement of terminators and connectors. 
     FIGS. 5 through 7 are block diagrams of alternative versions of SCSI bus  400 . 
     In FIG. 5, a cable  500  is connected to connector C 2 , such that cable  500  forms a portion of SCSI bus  400 . Although not shown in FIG. 5, each of SCSI devices HD 0 , HD 1  and HD 2  (which are hard drives) is connected to cable  500  through a respective associated connector. Up to sixteen SCSI devices (including hard drives HD 0 , HD 1  and HD 2 , plus devices D 1  and D 2  which may also, for example, be hard drives) may be so connected to SCSI bus  400 . 
     As shown in FIG. 5, the portion of SCSI bus  400  between terminator T and a terminator T 2  is fully bounded by terminators (e.g. connector C 2  is interposed between terminator T and the second terminator T 2 ). Accordingly, such portion forms a bus path, and cable  500  may have a physical length substantially greater than 0.1 meter. But the maximum physical length of any of stubs  502   a ,  502   b  and  502   c  is restricted to 0.1 meter, such as between SCSI device HD 0  and its respective associated connector to the bus path of SCSI bus  400 . 
     In FIG. 6, a cable  600  is connected to connector C 1 , such that cable  600  forms a portion of SCSI bus  400 . Although not shown in FIG. 6, each of SCSI devices HD 3 , HD 4  and HD 5  (which are hard drives) is connected to cable  600  through a respective associated connector. Up to sixteen SCSI devices (including hard drives HD 3 , HD 4  and HD 5 , plus devices D 1  and D 2  which may also, for example, be hard drives) may be so connected to SCSI bus  400 . 
     As shown in FIG. 6, the portion of SCSI bus  400  between terminator T and a terminator T 1  is fully bounded by terminators (e.g. connector C 1  is interposed between terminator T and the second terminator T 1 ). Accordingly, such portion forms a bus path, and cable  600  may have a physical length substantially greater than 0.1 meter. But the maximum physical length of any of stubs  602   a ,  602   b  and  602   c  is restricted to 0.1 meter, such as between SCSI device HD 3  and its respective associated connector to the bus path of SCSI bus  400 . 
     In FIG. 7, cable  500  (of FIG. 4) is connected to connector C 2 , and cable  600  (of FIG. 5) is connected to connector C 1 , such that cables  500  and  600  form a portion of SCSI bus  400 . Notably, in FIG. 7, SCSI bus  400  includes three terminators (i.e. terminators T, T 1  and T 2 ). According to the SCSI specification, SCSI bus  400  may physically include more than two terminators, but a maximum of two terminators may be electrically enabled at any moment. Accordingly, in FIG. 7, terminator T is electrically disabled, and terminators T 1  and T 2  are electrically enabled, as discussed further hereinbelow in connection with FIGS. 12 through 14. 
     As shown in FIG. 7, the portion of SCSI bus  400  between terminator T 1  and terminator T 2  is fully bounded by terminators (e.g. connectors C 1  and C 2  are interposed between first terminator T 1  and second terminator T 2 ). Accordingly, such portion forms a bus path, and cables  500  and  600  are not restricted to a maximum physical length of 0.1 meter. Nevertheless, the maximum physical length between T and C 1 , and likewise between T and C 2 , is restricted to 0.1 meter, in order to preserve the option of either attaching or detaching cables  500  and/or  600  under various situations as shown in FIGS. 4 through 7. This restriction imposes undesirable limits on the number of SCSI devices that may be connected to SCSI bus  400  between connectors C 1  and C 2 . 
     FIGS. 8 through 11 are block diagrams of alternative versions of SCSI bus  212  of the illustrative embodiment. For clarity, FIGS. 2 and 8 through  11  show a subset of the SCSI devices actually connected to SCSI bus  212 . Moreover, any of the terminators shown in FIGS. 8 through 11 is capable of operating as terminator  218  or terminator  220  in FIG.  2 . 
     In a significant aspect of the illustrative embodiment, in FIG. 8, the portion of SCSI bus  212  between a terminator T 3  and a terminator T 4  is fully bounded by terminators, and such portion accordingly forms a bus path. Conversely, the portion of SCSI bus  212  between terminator T 3  and a connector C 1  is not fully bounded by terminators, because connector C 1  is not interposed between terminator T 3  and a second terminator. Accordingly, such portion forms a stub with a maximum physical length restricted to 0.1 meter. 
     Likewise, the portion of SCSI bus  212  between terminator T 4  and a connector C 2  is not fully bounded by terminators, because connector C 2  is not interposed between terminator T 4  and a second terminator. Accordingly, such portion forms a stub with a maximum physical length restricted to 0.1 meter. Similarly, the maximum physical length of any of stubs  802   a  and  802   b  is restricted to 0.1 meter, such as between SCSI device D 1  and its respective associated connector to the bus path of SCSI bus  212 . 
     In a significant aspect of the illustrative embodiment, in comparison to SCSI bus  400  (of FIGS.  4  through  7 ), more SCSI devices (in addition to D 1  and D 2 ) may be connected to SCSI bus  212  (of FIGS. 8 through 11) between connectors C 1  and C 2 . This is because the portion of SCSI bus  212  between terminators T 3  and T 4  forms a bus path, which may have a physical length substantially greater than 0.1 meter. 
     In FIG. 9, a cable  900  is connected to connector C 2 , such that cable  900  forms a portion of SCSI bus  212 . Notably, in FIG. 9, SCSI bus  212  includes three terminators (i.e. terminators T 2 , T 3  and T 4 ). In FIG. 9, because a maximum of two terminators may be electrically enabled at any moment, terminator T 4  is electrically disabled, and terminators T 2  and T 3  are electrically enabled, as discussed further hereinbelow in connection with FIGS. 12 through 15. 
     Although not shown in FIG. 9, each of SCSI devices HD 3 , HD 4  and HD 5  (which are hard drives) is connected to cable  900  through a respective associated connector. Up to sixteen SCSI devices (including hard drives HD 3 , HD 4  and HD 5 , plus devices D 1  and D 2  which may also, for example, be hard drives) may be so connected to SCSI bus  212 . The maximum physical length of any of stubs  902   a ,  902   b  and  902   c  is restricted to 0.1 meter, such as between SCSI device HD 3  and its respective associated connector to the bus path of SCSI bus  212 . 
     As shown in FIG. 9, the portion of SCSI bus  900  between terminator T 3  and terminator T 2  is fully bounded by terminators. Accordingly, such portion forms a bus path, and cable  900  may have a physical length substantially greater than 0.1 meter. 
     In FIG. 10, a cable  1000  is connected to connector C 1 , such that cable  1000  forms a portion of SCSI bus  212 . Notably, in FIG. 10, SCSI bus  212  includes three terminators (i.e. terminators T 1 , T 3  and T 4 ). In FIG. 10, because a maximum of two terminators may be electrically enabled at any moment, terminator T 3  is electrically disabled, and terminators T 1  and T 4  are electrically enabled, as discussed further hereinbelow in connection with FIGS. 12 through 15. 
     Although not shown in FIG. 10, each of SCSI devices HD 0 , HD 1  and HD 2  (which are hard drives) is connected to cable  1000  through a respective associated connector. Up to sixteen SCSI devices (including hard drives HD 0 , HD 1  and HD 2 , plus devices D 1  and D 2  which may also, for example, be hard drives) may be so connected to SCSI bus  212 . The maximum physical length of any of stubs  1002   a ,  1002   b  and  1002   c  is restricted to 0.1 meter, such as between SCSI device HD 0  and its respective associated connector to the bus path of SCSI bus  212 . 
     As shown in FIG. 10, the portion of SCSI bus  212  between terminator T 4  and terminator T 1  is fully bounded by terminators. Accordingly, such portion forms a bus path, and cable  1000  may have a physical length substantially greater than 0.1 meter. 
     In FIG. 11, cable  900  (of FIG. 9) is connected to connector C 2 , and cable  1000  (of FIG. 10) is connected to connector C 1 , such that cables  900  and  1000  form a portion of SCSI bus  212 . Notably, in FIG. 11, SCSI bus  212  includes four terminators (i.e. terminators T 1 , T 2 , T 3  and T 4 ). In FIG. 11, because a maximum of two terminators may be electrically enabled at any moment, terminators T 3  and T 4  are electrically disabled, and terminators T 1  and T 2  are electrically enabled, as discussed further hereinbelow in connection with FIGS. 12 through 15. 
     As shown in FIG. 11, the portion of SCSI bus  212  between terminator T 1  and terminator T 2  is fully bounded by terminators. Accordingly, such portion forms a bus path, and cables  900  and  1000  are not restricted to a maximum physical length of 0.1 meter. Nevertheless, the maximum physical length between T 3  and C 1 , and likewise between T 4  and C 2 , is restricted to 0.1 meter, in order to preserve the option of either attaching or detaching cables  900  and/or  1000  under various situations as shown in FIGS. 8 through 11. Advantageously, however, unlike SCSI bus  400  (of FIGS.  4  through  7 ), this restriction does not impose undesirable limits on the number of SCSI devices that may be connected to SCSI bus  212  between connectors C 1  and C 2 . This is because the portion of SCSI bus  212  between terminators T 3  and T 4  forms a bus path, which may have a physical length substantially greater than 0.1 meter. 
     FIG. 12 is a schematic electrical circuit diagram of a terminator  1200  of SCSI bus  212 . More specifically, terminator  1200  is a single ended (“SE”) terminator for a line  1202  (of SCSI bus  212 ) that communicates an SE signal. In response to an enable line  1204  having a logic 1 true state, each of transistors  1206  and  1208  is substantially turned on, so that line  1202  is electrically coupled (a) to a voltage reference node GND through transistor  1208  and a resistor R 2  and (b) to a voltage supply node Vcc (having a voltage equal to approximately 5 volts relative to GND) through transistor  1206  and a resistor R 1 . Conversely, in response to enable line  1204  having a logic 0 false state, each of transistors  1206  and  1208  is substantially turned off, so that line  1202  is electrically decoupled from GND and Vcc. For clarity, Vcc, GND and enable line  1204  are not shown in FIGS. 2 through 11. 
     FIG. 13 is a schematic electrical circuit diagram of a terminator  1300  of SCSI bus  212 . More specifically, terminator  1300  is a low voltage differential (“LVD”) terminator for a pair of lines  1302   a  and  1302   b  (of SCSI bus  212 ) that together communicate an LVD signal. In response to an enable line  1304  having a logic 1 true state, each of transistors  1306 ,  1308 ,  1310  and  1312  is substantially turned on, so that: 
     line  1302   a  is electrically coupled (a) to Vcc through transistor  1306  and a resistor R 1  and (b) to line  1302   b  through transistor  1308 , a resistor R 2 , and transistor  1310 ; and 
     line  1302   b  is electrically coupled (a) to GND through transistor  1312  and resistor R 3  and (b) to line  1302   a  through transistor  1310 , resistor R 2 , and transistor  1308 . Notably, transistor  1308 , resistor R 2 , and transistor  1310  are electrically interposed between line  1302   a  and line  1302   b . Accordingly, line  1302   a  is capable of having a voltage that is significantly different from a voltage of line  1302   b.    
     Conversely, in response to enable line  1304  having a logic 0 false state, each of transistors  1306 ,  1308 ,  1310  and  1312  is substantially turned off, so that lines  1302   a  and  1302   b  are electrically decoupled from GND and Vcc. For clarity, Vcc, GND and enable line  1304  are not shown in FIGS. 2 through 11. 
     FIG. 14 is a schematic electrical circuit diagram of a terminator  1400  of SCSI bus  212 . More specifically, terminator  1400  is a multimode terminator for a pair of lines  1402   a  and  1402   b  (of SCSI bus  212 ) that together communicate either an LVD signal or an SE signal. In response to an LVD/SE line  1403  having a logic 1 true state, terminator  1400  operates in an LVD mode, and lines  1402   a  and  1402   b  together communicate an LVD signal. Conversely, in response to LVD/SE line  1403  having a logic 0 false state, terminator  1400  operates in an SE mode, and lines  1402   a  and  1402   b  together communicate an SE signal, such that: (a) line  1402   b  is electrically coupled to GND as discussed further hereinbelow; and (b) line  1402   a  communicates the SE signal relative to GND (i.e. relative to line  1402   b ). 
     Accordingly, in response to an enable line  1401  having a logic 1 true state and LVD/SE line  1403  having a logic 1 true state (i.e. LVD mode), logic 1  405  operates (a) a line  1404  to have a logic 1 true state and (b) a line  1406  to have a logic 0 false state. In that manner, each of transistors  1408 ,  1410 ,  1412  and  1414  is substantially turned on, and each of transistors  1416 ,  1418 ,  1420  and  1422  is substantially turned off, so that: 
     line  1402   a  is electrically coupled (a) to Vcc through transistor  1408  and a resistor R 1  and (b) to line  1402   b  through transistor  1410 , a resistor R 2 , and transistor  1412 ; and 
     line  1402   b  is electrically coupled (a) to GND through transistor  1414  and resistor R 3  and (b) to line  1402   a  through transistor  1412 , resistor R 2 , and transistor  1410 . Notably, transistor  1410 , resistor R 2 , and transistor  1412  are electrically interposed between line  1402   a  and line  1402   b . Accordingly, line  1402   a  is capable of having a voltage that is significantly different from a voltage of line  1402   b.    
     Conversely, in response to an enable line  1401  having a logic 0 false state, logic 1  405  operates each of lines  1404  and  1406  to have a logic 0 false state, irrespective of whether LVD/SE line  1403  has a logic 0 false state or logic 1 true state. In that manner, each of transistors  1408 ,  1410 ,  1412 ,  1414 ,  1416 ,  1418 ,  1420  and  1422  is substantially turned off, so that lines  1402   a  and  1402   b  are electrically decoupled from GND and Vcc. 
     By comparison, in response to enable line  1401  having a logic 1 true state and LVD/SE line  1403  having a logic 0 false state (i.e. SE mode), logic 1  405  operates (a) line  1404  to have a logic 0 false state and (b) line  1406  to have a logic 1 true state. In that manner, each of transistors  1416 ,  1418 ,  1420  and  1422  is substantially turned on, and each of transistor  1408 ,  1410 ,  1412  and  1414  is substantially turned off so that: 
     line  1402   a  is electrically coupled (a) to Vcc through transistor  1416  and a resistor R 4 , and (b) to line  1402   b  through transistor  1420 , a resistor R 5 , and transistor  1422 ; and 
     line  1402   b  is electrically coupled (a) to GND through transistor  1418  and (b) to line  1402   a  through transistor  1422 , resistor R 5 , and transistor  1420 . For clarity, Vcc, GND, enable line  1401 , LVD/SE line  1403  and logic 1  405  are not shown in FIGS. 2 through 11. 
     FIG. 15 is a block diagram of SCSI bus  212  of FIGS. 8 through 11. For clarity, dual termination control logic 1  500  is not shown in FIGS. 8 through 11. Nevertheless, logic 1  500  is present in the versions of SCSI bus  212  that are shown and described hereinabove in connection with FIGS. 8 through 11. 
     As shown in FIG. 15, logic 1  500  includes a first buffer/driver  1502  and a second buffer/driver  1504 . Buffer/driver  1502  receives a first signal from a line  1506  and, in response thereto, outputs a second signal (logically identical to the first signal) on a line  1508 . Likewise, buffer/driver  1504  receives a first signal from a line  1510  and, in response thereto, outputs a second signal (logically identical to the first signal) on a line  1512 . 
     Line  1506  is connected between connector C 1 and buffer/driver  1502 . Line  1508  is connected between buffer/driver  1502  and the enable line of terminator T 3 . In response to cable  1000  (FIG. 10) being disconnected from connector Cl, connector C 1 operates line  1506  to have a logic 1 true state. In response thereto, buffer/driver  1502  operates line  1508  to have a logic 1 true state, thereby enabling terminator T 3  as discussed hereinabove. 
     Conversely, in response to cable  1000  (FIG. 10) being connected to connector C 1 , connector C 1 operates line  1506  to have a logic 0 false state. In response thereto, buffer/driver  1502  operates line  1508  to have a logic 0 false state, thereby disabling terminator T 3  as discussed hereinabove. 
     Similarly, line  1510  is connected between connector C 2  and buffer/driver  1504 . Line  1512  is connected between buffer/driver  1504  and the enable line of terminator T 4 . In response to cable  900  (FIG. 9) being disconnected from connector C 2 , connector C 2  operates line  1510  to have a logic 1 true state. In response thereto, buffer/driver  1504  operates line  1512  to have a logic 1 true state, thereby enabling terminator T 4  as discussed hereinabove. 
     Conversely, in response to cable  900  (FIG. 9) being connected to connector C 2 , connector C 2  operates line  1510  to have a logic 0 false state. In response thereto, buffer/driver  1504  operates line  1512  to have a logic 0 false state, thereby disabling terminator T 4  as discussed hereinabove. 
     In an alternative embodiment, line  1506  is directly connected to line  1508  (and thereby to the enable line of terminator T 3 ), without buffer/driver  1502  being interposed between them. 
     In yet another alternative embodiment, buffer/driver  1502  is replaced by a first inverter, and buffer/driver  1504  is replaced by a second inverter. In such an alternative embodiment, terminator T 3  is modified to be enabled in response to line  1508  having a logic 0 false state and to be disabled in response to line  1508  having a logic 1 true state. Likewise, in such an alternative embodiment, terminator T 4  is modified to be enabled in response to line  1512  having a logic 0 false state and to be disabled in response to line  1512  having a logic 1 true state. 
     FIG. 16 is a block diagram of an alternative version of SCSI bus  212  of FIG.  11 . Unlike FIG. 11, SCSI bus  212  of FIG. 16 has an additional connector C 3 . The portion of SCSI bus  212  between terminator T 4  and connector C 3  is not fully bounded by terminators, because connector C 3  is not interposed between terminator T 4  and a second terminator. Accordingly, such portion forms a stub with a maximum physical length restricted to 0.1 meter. Also, unlike FIG. 11, cable  900  of SCSI bus  212  in FIG. 16 is connected to connector C 3  instead of connector C 2 . 
     FIG. 17 is a block diagram of SCSI bus  212  of FIG.  16 . For clarity, control logic 1  700  is not shown in FIG.  16 . Nevertheless, logic 1  700  is present in the version of SCSI bus  212  that is shown and described hereinabove in connection with FIG.  16 . 
     As shown in FIG. 17, logic 1  700  includes an AND gate  1702  that receives a first signal from a line  1704  and a second signal from a line  1706 . In response to the first and second signals, AND gate  1702  outputs a third signal (logically ANDed in response to the first and second signals) on a line  1708 . 
     Through logic 1  700 , a line  1710  is directly connected to a line  1712 . In that manner, an output of connector C 1  is directly connected to the enable line of terminator T 3 . 
     In the embodiment of FIG. 17, connector C 1  operates line  1710  to have (a) a logic 1 true state in response to cable  1000  (FIG. 10) being disconnected from connector C 1 , thereby enabling terminator T 3  as discussed hereinabove, and (b) a logic 0 false state in response to cable  1000  (FIG. 10) being connected to connector C 1 , thereby disabling terminator T 3  as discussed hereinabove. 
     Similarly, connector C 2  operates line  1704  to have (a) a logic 1 true state in response to cable  900  (FIG. 9) being disconnected from connector C 2  and (b) a logic 0 false state in response to cable  900  (FIG. 9) being connected to connector C 2 . Also, connector C 3  operates line  1706  to have (a) a logic 1 true state in response to cable  900  (FIG. 9) being disconnected from connector C 3  and (b) a logic 0 false state in response to cable  900  (FIG. 9) being connected to connector C 3 . 
     Line  1704  is connected between connector C 2  and AND gate  1702 , and line  1706  is connected between connector C 3  and AND gate  1702 . Line  1708  is connected between AND gate  1702  and the enable line of terminator T 4 . 
     Accordingly, if cable  900  is disconnected from both connectors C 2  and C 3 , then logic  1700  enables terminator T 4 . Conversely, if cable  900  is connected to either connector C 2  or connector C 3 , then logic 1  700  disables terminator T 4  as discussed further hereinbelow. 
     In the illustrative embodiment, each connector of SCSI bus  212  is either a 68-pin connector for a wide bus or a 50-pin connector for a narrow bus. The 68-pin wide bus includes “high byte” signals and “low byte” signals, whereas the 50-pin narrow bus includes only the low byte signals. The low byte signals include data signals D 0  through D 7  (which form an 8-bit data bus), plus control signals and a low byte parity signal. The high byte signals include data signals D 8  through D 15  (which, in addition to data signals D 0  through D 7 , form a 16-bit data bus), plus a high byte parity signal. 
     SCSI bus  212  of FIG. 17 includes terminator circuitry (e.g. the circuitry of FIG. 14) for each signal of SCSI bus  212 . Accordingly, because the wide bus includes nine signals additional to the narrow bus, each terminator of the wide bus includes nine instances of terminator circuitry (e.g. the circuitry of FIG. 14) additional to the instances of terminator circuitry that are included in the narrow bus. 
     As discussed hereinabove in connection with FIG. 14, each signal is communicated by two lines. Accordingly, because the high byte signals include nine signals additional to the low byte signals, the wide bus (68-pin connector) has eighteen lines additional to the narrow bus (50-pin connector). 
     In the example of FIG. 17, C 2  is a 68-pin connector, and C 3  is a 50-pin connector. If cable  900  is disconnected from both connectors C 2  and C 3 , then logic 1  700 : (a) enables all portions of terminator T 4  that communicate high byte signals; and (b) enables all portions of terminator T 4  that communicate low byte signals. If cable  900  is connected to connector C 2  and disconnected from connector C 3 , then logic 1  700 : (a) disables all portions of terminator T 4  that communicate high byte signals; and (b) disables all portions of terminator T 4  that communicate low byte signals. If cable  900  is connected to connector C 3  and disconnected from connector C 2 , then logic 1  700 : (a) enables all portions of terminator T 4  that communicate high byte signals; and (b) disables all portions of terminator T 4  that communicate low byte signals. The enabling and disabling is performed as discussed further hereinabove, as for example in connection with FIG.  14 . 
     Although illustrative embodiments have been shown and described, a wide range of modification, change and substitution is contemplated in the foregoing disclosure and, in some instances, some features of the embodiments may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and go in a manner consistent with the scope of the embodiments disclosed herein.