Abstract:
A bridge accessible by a host processor can expand access over a first bus to a second bus. The first bus and the second bus are each adapted to separately connect to respective ones of a plurality of bus-compatible devices. Allowable ones of the devices include memory devices and input/output devices. The bridge has a link, together with a first and a second interface. The first interface is coupled between the first bus and the link. The second interface is coupled between the second bus and the link. The first interface and the second interface are operable to (a) send information serially through the link in a format different from that of the first bus and the second bus, (b) approve an initial exchange between the first bus and the second bus in response to pending transactions having a characteristic signifying a destination across the bridge, (c) exchange information between the first bus and the second bus according to a predetermined hierarchy giving the first bus a higher level than the second bus, and (d) allow the host processor, communicating through the first bus, to individually address different selectable ones of the bus-compatible devices on the second bus, including memory devices and input/output devices that may be present: (i) using on the first bus substantially the same type of addressing as is used to access devices the first bus, and (ii) without first employing a second, intervening one of the bus-compatible devices on the second bus.

Description:
This application is a continuation of application Ser. No. 90/130,058, filed Aug. 6, 1998, now U.S. Pat. No. 6,070,214. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to data processing systems, and more particularly, to bridge systems including mechanisms for transferring information between buses. 
     2. Description of Related Art 
     Computers can use buses to transfer data between a host processor and various devices, such as memory devices and input/output devices. As used herein an “input/output” device is a device that either generates an input or receives an output (or does both). Thus “input/output” is used in the disjunctive. These buses may be arranged in a hierarchy with the host processor connected to a high level bus reserved for exchanging the data most urgently needed by the processor. Lower level buses may connect to devices having a lower priority. 
     Other reasons exist for providing separate buses. Placing an excessive number of devices on one bus produces high loading. Such loading makes a bus difficult to drive because of the power needed and the delays caused by signaling so many devices. Also, some devices on a bus may periodically act as a master and request control over a bus in order to communicate with a slave device. By segregating some devices on a separate bus, master devices can communicate with other devices on the lower level bus without tying up the bus used by the host processor or other masters. 
     The PCI bus standard is specified by the PCI Special Interest Group of Hillsboro, Oregon. The PCI bus features a 32-bit wide, multiplexed address-data (AD) Maintaining a high data throughput rate (e.g., a 33 MHZ clock rate) on the PCI bus leads to a fixed limitation on the number of electrical AC and DC loads on the bus. Speed, considerations also limit the physical length of the bus and the capacitance that can be placed on the bus by the loads, while future PCI bus rates (e.g., 66 MHZ) will exacerbate the electrical load and capacitance concerns. Failure to observe these load restrictions can cause propagation delays and unsynchronized operation between bus devices. 
     To circumvent these loading restrictions, the PCI bus standard specifies a bridge to allow a primary PCI bus to communicate with a secondary PCI bus through such a bridge. Additional loads may be placed on the secondary bus without increasing the loading on the primary bus. For bridges of various types see U.S. Pat. Nos. 5,548,730 and 5,694,556. 
     The PCI bridge observes a hierarchy that allows an initiator or bus master on either bus to complete a transaction with a target on the other bus. As used herein, hierarchy refers to a system for which the concept of a higher or lower level has meaning. For example, a PCI bus system is hierarchical on several scores. An ordering of levels is observed in that a high level host processor normally communicates from a higher level bus through a bridge to a lower level bus. An ordering of levels is also observed in that buses at equal levels do not communicate directly but through bridges interconnected by a higher level bus. Also, an ordering of levels is observed in that data is filtered by their addresses before being allowed to pass through a bridge, based on the levels involved. Other hierarchical systems exist that may observe an ordering of levels by using one or more of the foregoing concepts, or by using different concepts. 
     Some personal computers have slots for add-on cards. Because a user often needs additional slots, expansion cards have been designed that will connect between the peripheral bus and an external unit that offers additional slots for add-on cards. For systems for expanding a bus, see U.S. Pat. Nos. 5,006,981; 5,191,657; and 5,335,329. See also U.S. Pat. No. 5,524,252. 
     For portable computers, special considerations arise when the user wishes to connect additional peripheral devices. Often a user will bring a portable computer to a desktop and connect through a docking station or port replicator to a keyboard, monitor, printer or the like. A user may also wish to connect to a network through a network interface card in the docking station. At times, a user may need additional devices such as hard drives or CD-ROM drives. While technically possible to a limited extent, extending a bus from a portable computer through a cable is difficult because of the large number of wires needed and because of latencies caused by a cable of any significant length. 
     In U.S. Pat. No. 5,696,949 a host chassis has a PCI to PCI bridge that connects through a cabled bus to another PCI to PCI bridge in an expansion chassis. This system is relatively complicated since two independent bridges communicate over a cabled bus. This cabled bus includes essentially all of the lines normally found in a PCI bus. This approach employs a delay technique to deal with clock latencies associated with the cabled bus. A clock signal generated on the expansion side of the cabled bus: (a) is sent across the cabled bus, but experiences a delay commensurate with the cable length; and (b) is delayed an equivalent amount on the expansion side of the cabled bus by a delay line there, before being used on the expansion side. Such a design complicates the system and limits it to a tuned cable of a pre-designed length, making it difficult to accommodate work spaces with various physical layouts. 
     U.S. Pat. No. 5,590,377 shows a primary PCI bus in a portable computer being connected to a PCI to PCI bridge in a docking station. When docked, the primary and secondary buses are physically very close. A cable is not used to allow separation between the docking station and the portable computer. With this arrangement, there is no interface circuitry between the primary PCI bus and the docking station. See also U.S. Pat. No. 5,724,529. 
     U.S. Pat. No. 5,540,597 suggests avoiding additional PCMCIA connectors when connecting a peripheral device to a PC card slot in a portable computer, but does not otherwise disclose any relevant bridging techniques. 
     U.S. Pat. No. 4,882,702 and show a programmable controller for controlling industrial machines and processes. The system exchanges data serially with a variety of input/output modules. One of these modules may be replaced with an expansion module that can serially communicate with several groups of additional input/output modules. This system is not bridge-like in that the manner of communicating with the expansion module is different than the manner of communicating with the input/output modules. For the expansion module the system changes to a block transfer mode where a group of status bytes are transferred for all the expansion devices. This system is also limited to input/output transactions and does not support a variety of addressable memory transactions. See also U.S. Pat. Nos. 4,413,319; and 4,504,927. 
     In U.S. Pat. No. 5,572,525 another bus designed for instrumentation (IEEE 488 General Purpose Instrumentation Bus) connects to an extender that breaks the bus information into packets that are sent serially through a transmission cable to another extender. This other extender reconstructs the serial packets into parallel data that is applied to a second instrumentation bus. This extender is an intelligent system operating through a message interpretation layer and several other layers before reaching the parallel to serial conversion layer. Thus this system is unlike a bridge. This system is also limited in the type of transactions that it can perform. See also U.S. Pat. 4,959,833. 
     U.S. Pat. No. 5,325,491 shows a system for interfacing a local bus to a cable with a large number of wires for interfacing with remote peripherals. See also U.S. Pat. Nos. 3,800,097; 4,787,029; 4,961,140; and 5,430,847. 
     The Small Computer System Interface (SCSI) defines bus standards for a variety of peripheral devices. This CSI bus is part of an intelligent system that responds to high-level commands. Consequently, SCSI systems require software drivers to enable hardware to communicate to the SCSI bus. This fairly complicated system is quite different from bridges such as bridges as specified under the PCI standard. A variety of other complex techniques and protocols exist for transferring data, including Ethernet, Token Ring, TCP/IP, ISDN, FDDI, HIPPI, ATM, Fibre Channel, etc., but these bear little relation to bridge technology. 
     See also U.S. Pat. Nos. 4,954,949, 5,038,320; 5,111,423; 5,446,869; 5,495,569; 5,497,498; 5,507,002; 5,517,623; 5,530,895; 5,542,055; 5,555,510; 5,572,688; and 5,611,053. 
     Accordingly, there is a need for an improved system for transferring information between buses. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrative embodiments demonstrating features, and advantages of the present invention, there is provided a bridge accessible by a host processor for expanding access over a first bus to a second bus. The first bus and the second bus are each adapted to separately connect to respective ones of a plurality of bus-compatible devices. Allowable ones of the devices include memory devices and input/output devices. The bridge has a link, together with a first and a second interface. The second interface is adapted to couple between the second bus and the link. The first interface and the second interface operating as a single bridge are operable to (a) send outgoing information serially through the link in a format different from that of the first bus and the second bus without waiting for an incoming acknowledgment over said link before inaugurating a transfer of said information over said link, (b) approve an initial exchange between the first bus and the second bus in response to a pending transaction having a characteristic signifying a destination across the bridge, and (c) allow the host processor, communicating through the first bus, to individually address different selectable ones of the bus-compatible devices on the second bus, including memory devices and input/output devices that may be present: (i) using on the first bus substantially the same type of addressing as is used to access devices the first bus, and (ii) without first employing a second, intervening one of the bus-compatible devices on the second bus. 
     In accordance with another aspect of the invention a bridge accessible by a host processor can expand access over a first bus to a second bus. The first bus and the second bus each are adapted to separately connect to respective ones of a plurality of bus-compatible devices. Allowable ones of the devices include memory devices and input/output devices. The bridge has a link, together with a first and a second interface. The first interface is adapted to couple between the first bus and the link. The second interface is adapted to couple between the second bus and the link. The first interface and the second interface are operable to (a) send information serially through the link in a format different from that of the first bus and the second bus, (b) exchange information between the first bus and the second bus according to a predetermined hierarchy giving the first bus a higher level than the second bus, and (c) allow the host processor, communicating through the first bus, to individually address different. selectable ones of the bus-compatible devices on the second bus, including memory devices and input/output devices that may be present: (i) using on the first bus substantially the same type of addressing as is used to access devices on the first bus, (ii) without first employing a second intervening one of the bus-compatible devices on the second bus, and (iii) without passing the information through an intervening hierarchical level. 
     In accordance with another, further aspect of the invention a bridge accessible by a processor can expand access over a first bus to a second bus. The first bus and the second bus each are adapted to separately connect to respective ones of a plurality of bus-compatible devices. The bridge has a link and a first and a second interface. The first interface is coupled between the first bus and the link. The second interface is adapted to couple between the second bus and the link. The first interface and the second interface operate as a single bridge and is operable to transfer information serially through the link in a format different from that of the first bus and the second bus without waiting for an incoming acknowledgment over the link before inaugurating a transfer of the information over the link. 
     By employing apparatus and methods of the foregoing type, an improved system is achieved for transferring information between buses. In one preferred embodiment, two buses communicate over a duplex link formed with a pair of simplex links, each employing twisted pair or twin axial lines (depending on the desired speed and the anticipated transmission distance). Information from the buses are first loaded onto FIFO (first-in first-out) registers before being serialized into frames for transmission over the link. Received frames are deserialized and loaded into FIFO registers before being placed onto the destination bus. Preferably, interrupts, error signals, and status signals are sent along the link. 
     In this preferred embodiment, address and data are taken from a bus one transaction at a time, together with four bits that act either as control or byte enable signals. Two or more additional bits may be added to tag each transaction as either: an addressing cycle; acknowledgment of a non-posted write; data burst; end of data burst (or single cycle). If these transactions are posted writes they can be rapidly stored in a FIFO register before being encoded into a number of frames that are sent serially over a link. When pre-fetched reads are allowed, the FIFO register can store pre-fetched data in case the initiator requests it. For single cycle writes or other transactions that must await a response, the bridge can immediately signal the initiator to wait, even before the request is passed to the target. 
     In a preferred embodiment, one or more of the buses follows the PCI or PCMCIA bus standard (although other bus standards can be used instead). The preferred apparatus then operates as a bridge with a configuration register that is loaded with information specified un er the PCI standard. The apparatus can transfer information between buses depending upon whether the pending addresses fall within a range embraced by the configuration registers. This scheme works with devices on the other side of the bridge, which can be given unique base addresses to avoid addressing conflicts. 
     In one highly preferred embodiment, the apparatus maybe formed as two separate application-specific integrated circuits (ASIC) joined by a cable. Preferably, these two integrated circuits have the same structure, but can act in two different modes in response to a control signal applied to one of its pins. Working with hierarchical buses (primary and secondary buses) these integrated circuits will be placed in a mode appropriate for its associated bus. The ASIC associated with the secondary bus preferably has an arbiter that can grant masters control of the secondary bus. This preferred ASIC can also supply a number of ports to support a mouse an keyboard, as well as parallel and serial ports. 
     When used with a portable computer, one of the ASIC&#39;s can be assembled with a connector in a package designed to fit into a PC card slot following the PCMCIA standard. This ASIC can connect through a cable to the other ASIC, which can be located in a docking station. Accordingly, the apparatus can act as a bridge between a CardBus and a PCI bus located in a docking station. Since the preferred ASIC can also provide a port for a mouse and keyboard, this design is especially useful for a docking station. Also, the secondary PCI bus implemented by the ASIC can connect to a video card or to a video processing circuit on the main dock circuit board in order to drive a monitor. 
     In some embodiments, one ASIC will be mounted in the portable computer by the original equipment manufacturer (OEM). This portable computer will have a special connector dedicated to the cable that connects to the docking station with the mating ASIC. For such embodiments, the existence within the preferred ASIC of ports for various devices can be highly advantageous. An OEM can use this already existing feature of the ASIC and thereby eliminate circuitry that would otherwise have been needed to implement such ports. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic block diagram showing a bridge split by a link within the bridge, in accordance with principles of the present invention; 
     FIG. 2 is a schematic block diagram showing a bridge in accordance with principles of the present invention using the link of FIG. 1; 
     FIG. 3 is a schematic block diagram showing the bridge of FIG. 2 used in a docking system in accordance with principles of the present invention; 
     FIG. 4 is a cross-sectional view of the cable of FIG. 3; 
     FIG. 5 is a schematic illustration of the bridge of FIG. 3 shown connected to a portable computer and a variety of peripheral devices; and 
     FIG. 6 shows a docking station similar to that of FIG. 5 but with the portable computer modified to contain an application-specific integrated circuit designed to support a link to the docking station. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a bridge is shown connecting between a first bus  10  and a second bus  12  (also referred to as primary bus  10  and secondary bus  12 ). These buses may be PCI or PCMCIA 32-bit buses, although other types of buses are contemplated and the present disclosure is not restricted to any specific type of bus. Buses of this type will normally have address and data lines. In some cases, such as wit the PCI bus, address and data are multiplexed onto the same lines. In a dition, these buses will have signaling lines for allowing devices on the bus to negotiate transactions. For the PCI standard, these signaling lines will in lude four lines that are used either for control or byte enabling (C/BE[ 3 : 01 ). Others signaling lines under the PCI standard exist for gaining control over the bus, for handshaking, and the like (e.g., FRAME#, TRDY#, IRDY#, STOP#, DEVSEL#, etc.) 
     Buses  10  and  12  are shown connecting to a first interface  14  and second interface  16 , respectively (also referred to as interfaces  14  and  16 ). Bus information selected for transmission by interfaces  14  and  16  are loaded into registers  18  and  20 , respectively. Incoming bus information that interfaces  14  and  16  select for submission to the buses are taken from registers  22  and  24 , respectively. In one embodiment, registers  18 - 24  are each 16×38 FIFO registers, although different types of registers having different dimensions may be used in alternate embodiments. 
     In this embodiment, registers  18 - 24  are at least 38 bits wide. Thirty six of those bits are reserved for the  4  control bits (C/BE#[ 3 : 01 ]) and the 32 address/data bits (AD[ 31 : 0 ]) used under the PCI bus standard. The remaining two bits can be used to send additional tags for identifying the nature of the transaction associated therewith. Other bits may be needed to fully characterize every contemplated transaction. Transactions can be tagged as: addressing cycle; acknowledgment of a non posted write; data burst; end of data burst (or single cycle). Thus outgoing write transactions can be tagged as a single cycle transaction or as part of a burst. Outgoing read requests can also be tagged as part of a burst with a sequence of byte enable codes (C/BE) for each successive read cycle of the burst. It will be appreciated that other coding schemes using a different number of bits can be use in other embodiments. 
     The balance of the structure illustrated in FIG. 1 is a link designed to establish duplex communications between interfaces  14  and  16  through registers  18 - 24 . For example, encode  28  can accept the oldest 38 bits from register  20  and parse it into five bytes (40 bits). The extra two bits of the last byte are encoded to signify the interrupts, status signals and error signals that may be supplied from block  34 . 
     Each of these five bytes is converted into a 10 bit frame that can carry the, information of each byte, as well as information useful for regulating the link. F or example, these frames can carry comma markers, idle markers, or flow control signals, in a well-known fashion. A transceiver system working with bytes that were encoded into such 10 bit frames is sold commercially by Hewlett Packard as model number HDMP-1636 or -1646. Frames produced by encoder  28  are forwarded through transmitter  44  along simplex link  46  to receiver  48 , which supplies the serial information to decoder  30 . Likewise, encoder  26  forwards serial information through transmitter  38  along simplex link  40  to receiver  42 , which supplies the serial information to decoder  32 . 
     Flow control may be necessary should FIFO registers  22  or  24  be in danger of overflowing. For example, if FIFO register  22  is almost full, it supplies a threshold detect signal  36  to encoder  26 , which forwards this information through link  40  to decoder  32 . In response, decoder  32  issues a threshold stop signal  50  to encoder  28 , which then stops forwarding serial information, thereby preventing an overflow in FIFO register  22 . In a similar fashion, a potential overflow in FIFO register  24  causes a threshold detect signal  52  to flow through encoder  28  and link  46  to cause decoder  30  to issue a threshold stop signal  54 , to stop encoder  26  from sending more frames of information. In some embodiments, the system will examine the received information to determine if it contains transmission errors or has been corrupted in some fashion. In such event the system can request a retransmission of the corrupted information and thereby ensure a highly reliable link. 
     In this embodiment, elements  14 ,  18 ,  22 ,  26 ,  30 ,  38  and  48  are part of a single, application specific integrated circuit (ASIC)  56 . Elements  16 ,  20 ,  24 ,  28 ,  32 ,  42  and  44  are also part of an ASIC  58 . As described further hereinafter, first ASIC  56  and second ASIC  58  have an identical structure but can be operated in different mode. It will be appreciated that other embodiments may not use ASIC&#39;s but may use instead alternate circuitry, such as a programable logic device, or the like. As shown herein, ASIC  56  is operating in a mode designed to service primary bus  10 , and (for reasons to be described presently) will be sending outputs to block  57 . In contrast block  34  of ASIC  58  will receive inputs from block  34 . 
     Encoders  26  and  28  have optional parallel outputs  27  and  29 , respectively, for applications requiring such information. Also for such applications, decoders  30  and  32  have parallel inputs  31  and  33 , respectively. These optional inputs and outputs may be connected to an external transceiver chip, such as the previously mentioned device offered by Hewlett Packard as model number HDMP-1636 or -1646. These devices will still allow the system to transmit serial information, but by means of an external transceiver chip. This allows the user of the ASIC&#39;s  56  and  58  more control over the methods of transmission over the link. 
     Referring to FIG. 2, previously mentioned ASIC&#39;s  56  and  58  are shown in further detail. The previously mentioned encoders, decoders, transmitters, receivers, and FIFO registers are combined into blocks  60  and  62 , which are interconnected by a duplex cable formed of previously mentioned simplex links  40  and  46 . Previously mentioned interface  14  is shown connected to primary bus  10 , which is also connected to a number of bus-compatible devices  64 . Similarly, previously mentioned interface  16  is shown connected to secondary bus  12 , which is also connected to a number of bus-compatible devices  66 . Devices  64  and  66  may be PCI-compliant devices and may operate as memory devices or input/output devices. 
     Interface  14  a shown connected to a first register means  68 , which acts as a configuration register in compliance with the PCI standard. Since this system will act as a bridge, configuration registers  68  will have the information normally associated with a bridge. Also, configuration registers  68  will contain a base register and limit register to indicate a range or predetermined schedule of addresses for devices that can be found on the secondary bus  12 . Under the PCI standard, devices on a PCI bus will themselves each have a base register, which allows mapping of the memory space and/or I/O space. Consequently, the base and limit registers in configuration registers  68  can accommodate the mapping that is being performed by individual PCI devices. The information on configuration registers  68  are mirrored on second configuration register  67  (also referred to as a second configuration means). This makes the configuration information readily available to the interfaces on both sides of the link. 
     In this embodiment, ASIC  58  has an arbiter  70 . Arbiters are known devices that accept requests from masters on secondary bus  12  for control of  25  the bus. The arbiter has a fair algorithm that grants the request of one of the contending masters by issuing it a grant signal. In this hierarchical scheme, secondary bus  12  requires bus arbitration, but primary bus  10  will provide its own arbitration. Accordingly, ASIC  56  is placed in a mode where arbiter  72  is disabled. The modes of ASIC&#39;s  56  and  58  are set by control signals applied to control pins  74  and  76 , respectively. Because of this mode selection, the signal directions associated with blocks  57  an  34  will be reversed. 
     In this embodiment, ASIC  58  is in a mode that implements a third bus  78 . Bus  78  may follow the PCI standard, but is more conveniently implemented in a different standard. Bus  78  connects to a number of devices that act as a port means. For example, devices  80  and  82  can implement PS/2 ports that  5  can connect to either a mouse or a keyboard. Device  84  implements an ECP/EPP parallel port for driving a printer or other device. Device  86  implements a conventional serial port. Devices  80 ,  82 ,  84  and  86  are shown with input/output lines  81 ,  83 ,  85  and  87 , respectively. Devices  80 - 86  may be addressed on bus  10  as if they were PCI devices on bus  12 . Also in this embodiment, a bus  88  is shown in ASIC  56 , with the same devices as shown on bus  78  to enable an OEM to implement these ports without the need for separate input/output circuits. 
     Referring to FIG. 3, previously mentioned ASIC  58  is shown in a  15  docking station  130  connected to an oscillator  91  for establishing a remote and internal clock. ASIC  58  has its lines  81  and  83  connected through a connection assembly  90  for connection to a keyboard and mouse, respectively. Serial lines  85  and parallel lines  87  are shown connected to transceivers  92  and  94 , respectively, which then also connect to connection assembly  90  for connection to various parallel and serial peripheral, such as printers and modems. 
     ASIC  58  is also shown connected to previously mentioned secondary bus  12 . Bus  12  is shown connected to an a adapter card  96  to allow the PCI bus  12  to communicate with an IDE device such as a hard drive, backup tape drive, CD-ROM drive, etc. Another adapter card  98  is shown for allowing communications from bus  12  to a universal serial port (USB). A network interface card  100  will allow communications through bus  12  to various networks operating under the Ethernet standard, Token Ring standard, etc. Video adapter card  102  (also referred to as a video means) allows the user to operate another monitor. Add-on card  104  may be one of a variety of cards selected by the user to perform a useful function. While this embodiment shows various functions being implemented by add-on cards, other embodiments may implement one or more of these function on a common circuit board in the dock (e.g., all functions excluding perhaps the IDE adapter card). 
     ASIC  58  communicates through receiver/transmitter  106 , which provides a physical interface through a terminal connector  108  to cable  40 ,  46 . Connector  108  may be a  20  pin connector capable of carrying high speed signals with EMI shielding (for example a low force helix connector of the type offered by Molex Incorporated), although other connector types may used instead. The opposite end of cable  40 ,  46  connects through a gigabit, terminal connector  110  to physical interface  112 , which acts as a receiver/transmitter. Interface  112  is shown connected to previously mentioned first ASIC  56 , which is also shown connected to an oscillator  114  to establish a local clock signal. This specific design contemplates sing an external transmitter/receiver (external SERDES of lines  27 ,  29 ,  31 , and  33  of FIG.  1 ), although other embodiments can eliminate these external devices in favor of the internal devices in ASIC&#39;s  56  and  58 . 
     This embodiment is adapted to cooperate with a portable computer having a PCMCIA 32-bit bus  10 , although other types of computers can be serviced. Accordingly, ASIC  56  is shown in a package  116  having an outline complying with the PCMCIA standard and allowing package  116  to fit into a slot in a portable computer. Therefore, ASIC  56  has a connector  118  for  25  connection to bus  10 . Cable  40 ,  46  will typically be permanently connected to package  116 , but a detachable connector may be used in other embodiments, where a user wishes to leave package  16  inside the portable computer. 
     Power supply  120  is shown producing a variety of supply voltages used to power various components. In some embodiments, one of these supply lines can be connected directly to the portable computer to charge its battery. Referring to FIG. 4, the previously mentioned simplex links  40  and  46  are shown as twin axial lines  40 A an  46 A, wrapped with individual shields  40 B and  46 B. A single shield  122  encircles the lines  40  and  46 . Four parallel wires  124  are shown (although a greater number may be used in other embodiments) mounted around the periphery of shields  122  for various purposes. These wires  124  may carry power management signals, dock control signals or other signals that may be useful in an interface between a docking station and a portable computer. While twin axial lines offer high performance, twisted pairs or other transmission media may be used in other embodiments where the transmission distance is not as great and where the bit transfer speed need not be as high. While a hard wire connection is illustrated, in other embodiments a wireless or other type of connection can be employed instead. 
     Referring to FIG. 5, previously mentioned package  116  is shown in position to be connected to a PCMCIA slot in portable computer  126 . Computer  126  is shown having primary bus  10  and a host processor  128 . Package  116  is shown connected through cable  40 ,  46  to previously mentioned connector  108  on docking station  130 . 
     Previously mentioned docking station  130  is shown connecting through PS/2 ports to keyboard  132  and mouse  134 . A printer  136  is shown connected to parallel port in docking station  130 . Previously mentioned video means  102  is shown connected to a monitor  138 . Docking station  130  is also shown with an internal hard drive  140  connecting to the adapter card previously mentioned. A CD-ROM drive  142  is also shown mounted in docking station  130  and connects to the secondary bus through an appropriate adapter card (not shown). Previously mentioned add-on card  104  is shown with its own cable  144 . 
     Referring to FIG. 6, a modified portable computer  126 ′ is again shown with a host processor  128  and primary bus. In this embodiment however, portable computer  126 ′ contains previously mentioned ASIC  56 . Thus there is no circuitry required (other than perhaps drivers) between ASIC  56  and cable  40 ,  46 . In this case, the laptop end of cable  40 ,  46  has a connector  142  similar to the one on the opposite end of the cable (connector  108  of FIG.  5 ).Connector  143  is designed to mate with connector  141  and support the highspeed link. As before, connectors  141  and  143  can also carry various power management signals, and other signal associated with a docking system. 
     An important advantage of this arrangement is the fact that ASIC  56  contains circuitry for providing ports, such as a serial port, a parallel port, PS/2 ports for a mouse and keyboard, and he like. Since portable computer  126 ′ would ordinarily provide such ports, ASIC  56  simplifies the design of the portable computer. This advantage is in addition to the advantage of having a single ASIC design (that is, ASIC&#39;s  56  and  58  are structured identically), which single design is capable of operating at either the portable computer or the docking station, thereby simplifying the ASIC design and reducing stocking requirements, etc. 
     To facilitate an understanding of the principles associated with the foregoing apparatus, its operation will be briefly described. This operation will be described in connection with the docking system of FIGS. 3 and 5 (which  20  generally relates to FIG.  2 ), although operation would be similar for other types of arrangements. For the docking system, a connection is established by plugging package  116  (FIG. 5) into portable computer  126 . This establishes a link between the primary bus  10  and SIC  56  (FIG.  3 ). 
     At this time an initiator (the host processor or a master) having access to primary bus  10  may assert control of the bus. An initiator will normally send a request signal to an internal arbiter (not shown) that will eventually grant control to this initiator. In any event, the initiator asserting control over primary bus  10  will exchange the appropriate handshaking signals and drive an address onto the bus  10 . Control signals simultaneously applied to the signaling lines of bus  10  will indicate whether the transaction is a read, write, or other type of transaction. 
     Interface  14  (FIG. 2) will examine the pending address and determine whether it represents a transaction with devices on the other side of the bridge (that is, secondary bus  12 ) or with the bridge itself. Configuration register  68  has already been loaded in the usual manner with information that indicates a range of addresses defining the jurisdiction of the interface  14 . 
     Assuming a write transaction is pending on bus  10 , interface  14  will transfer  32  address bits together with four control bits (PCI standard) to FIFO register  18  (FIG.  1 ). Encoder  26  will add at least two additional bits tagging this information as an addressing cycle. The information is then broken into frames that can carry flow control and other signals before being transmitted serially over link  40 . 
     Without waiting, interface  14  will proceed to a data cycle and accept up to 32 bits of data from bus  10  together with four byte enable bits. As before, this information will be tagged, supplemented with additional information and broken into frames for serial transmission over link  40 . This transmitted information will be tagged to indicate whether it is part of a burst or a single cycle. 
     Upon receipt, decoder  32  restore the frames into the original 38 bit format and loads the last two described cycles onto the stack of register  24 . Interface  16  eventually notices the first cycle as an addressing cycle in a write request. Interface  16  then negotiates control over bus  12  in the usual fashion and applies the address to bus  12 . A device on bus  12  will respond to the write request by performing the usual handshaking. 
     Next, interface  16  will drive the rite data stacked on register  24  into bus  12 . If this transaction is a burst, interface  16  will continue to drive data onto bus  12  by fetching it from register  24 . If however this transaction is a single cycle write, interface  16  will close the transaction on bus  12  and load an acknowledgment into register  20 . Since this acknowledgment need not carry data or address information, a unique code may be placed into register  20 , so that encoder  28  can appropriately tag this line before parsing it into frames for transmission over link  46 . Upon receipt, decoder  30  will produce a unique code that is loaded into register  22  and eventually forwarded to interface  14 , which sends an acknowledgment to the device on bus  10  that the write has  10  succeeded. 
     If the initiator instead sets its control bits during the address cycle to indicate a read request, interface  14  would also accept this cycle, if it has jurisdiction. Interface  14  will also signal the initiator on bus  10  that it is not ready to return data (e.g., a retry signal, which may be the stop signal as defined under the PCI standard). The initiator can still start (but not finish) a data cycle by driving its signaling lines on bus  10  with byte enable information. Using the same technique, the address information, followed by the byte enable information, will be accepted by interface  14  and loaded with tags into register  18 . These two lines of information will be then encoded and transmitted serially  20  over link  40 . Upon receipt, this information will be loaded into the stack of register  24 . Eventually, interface  16  will notice the first item as a read request and drive this address information onto secondary bus  12 . A device on bus  12  will respond and perform the appropriate handshaking. Interface  16  will then forward the next item of information from register  24  containing the byte  25  enables, onto bus  12  so the target device can respond with the requested data. This responsive data is loaded by in  16  into register  20 . If pre-fetching is indicated, interface  16  will initiate a number of successive read cycles to accumulate data in register  20  from sequential addresses that may or may not be requested by the initiator. 
     As before, this data is tagged, broken into frames and sent serially over link  46  to be decoded and loaded into register  22 . The transmitted data can include pre-fetched data that will be accumulated in register  22 . Interface  14  transfers the first item of returning data onto primary bus  10 , and allows the initiator to proceed to another read cycle if desired. If another read cycle is conducted as part of a burst transaction, the requested data will already be present in register  22  for immediate delivery by interface  14  to bus  10 . If these pre-fetched data are not requested for the next cycle, then they are discarded. 
     Eventually the initiator will relinquish control of bus  10 . Next, an initiator  10  on bus  12  may send a request for control of bus  12  to arbiter  70  (FIG.  2 ). If arbiter  70  grants control, the initiator may make a read or write request by driving an address onto bus  12 . Interface  16  will respond if this address does not fall within the jurisdictional range of addresses specified in configuration register  67  (indicating the higher level bus may have jurisdiction). In the same manner as before, but with a reversed flow over links  40 ,  46 , interface  16  may accept address and data cycles and communicate them across link  40 ,  46 . Before being granted bus  10 , interface  14  will send a request to an arbiter (not shown) associated with bus  10 . 
     In some instances, an initiator on primary bus  10  will wish to read from, or write to, port means  80 ,  82 ,  84 , or  86 . These four items are arranged to act as devices under the PCI standard. Interface  16  will therefore act as before, except that information will be routed not through bus  12 , but through bus  78 . 
     Other types of transactions may be performed, including reads and writes to the configuration registers  67  and  68  (FIG.  2 ). Other types of transactions, as defined under the PCI standard (or other bus standards) may be performed as well. 
     Interrupt signals may be generated by the ports or other devices in ASIC  58 . Also external interrupts may be received as indicated by block  34 . As noted before, interrupt signals may be embedded in the code sent over link  46 . Upon receipt, system  60  decodes the interrupts and forwards them on to block  57 , which may be simply one or more pins from ASIC  56  (implementing, for example, INTA of the PCI standard). This interrupt signal can either be sent over the bus  10  or to an interrupt controller that forwards interrupts to the host processor. System errors may be forwarded in a similar fashion to produce an output on a pin of ASIC  56  that can be routed directly to bus  10  or processed using dedicated hardware. The designer may wish to send individual status signals, which can be handled in a similar fashion along link  40 ,  46 . 
     It is appreciated that various modifications may be implemented with respect to the above described, preferred embodiment. In other embodiments the illustrated ASIC&#39;s may be divided into several discrete packages using in some cases commercially available integrated circuits. Also, the media for the link may be wire, fiber-optics, infrared light, radio frequency signals, or other media. In addition, the primary and secondary buses may each have one or more devices, and these devices may be in one or more categories, including memory devices and input/output device. Moreover, the devices may operate at a variety of clock speeds, bandwidths and data rates. Furthermore, transactions passing through the bridge may be accumulated as posted writes or as pre-fetched data, although some embodiments will not use such techniques. Also, the bridge described herein can be part of a hierarchy using a plurality of such bridges having their primary side connected to the same bus or to buses of an equivalent or different level. Additionally, the illustrated ports  25  can be of a different number or type, or can be eliminated in some embodiments. Also, the illustrated arbiter can be eliminated for secondary buses that are not design to be occupied by a master. While a sequence of steps is described above, in other embodiments these steps may be increased or reduced in number, or performed in a different order, without departing from the scope of the present invention. 
     Obviously, many modifications an variations of the present invention are possible in light of the above teachings It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.