1. Field of the Invention
The invention relates generally to improvements in circuit boards useful for supporting and interconnecting computer devices, such as disk drives and processors. More specifically, the invention pertains to multi-layered circuit boards including differential buses and special via fields, to provide high speed signal transfers between and among electronic devices.
2. Description of the Prior Art
Modern computer systems use high speed digital buses to connect high performance computer devices, such as disk drives and processors. Generally, these buses comprise a plurality of parallel, conductive wires or traces. The bus wires are typically arranged on a printed circuit board, which may have inner or outer traces of bus wires separated by non-conductive core material. Active terminating loads, designed to match trace characteristic impedances, are provided at the end of each wire or trace. These loads include a load resistor, and may also include a voltage source or a current source.
The computer devices are connected to traces in the intermediate portion of the bus by means of wire stubs. The wire stubs extend across the bus, providing a parallel electrical connection for the transversely positioned device. Because stubs are transverse to the bus, stubs introduce signal perturbations and noise artifacts into the data transmitted through the bus of the computer system.
The use of stubs creates other problems in the design of circuit board layouts. It is typical to plug the computer devices into connectors or sockets, mounted to the printed circuit board. The stubs or other connection traces lead from the pins of the connectors to the bus. As the complexity of the computer system increases, the number of devices which must be connected to the bus also increases. However with a greater number of devices to mount on the board and to interconnect to the bus, the circuit layout of the board becomes increasingly difficult to implement. The long, parallel traces between devices must be routed to maintain a specific trace impedance, minimize crosstalk, minimize consumption of circuit board space, equalize the trace lengths, and use the least number of board layers. As a result, compromises are made in the performance of the computer system, so that all the devices can be interconnected as required.
Buses used for interconnecting devices may be either of single-ended or differential design Single-ended buses rely upon circuits comprised of a plurality of single conductors, spaced and insulated from a ground plane which is common to all conductors. Although single-ended buses are relatively easy to implement, they have significant limitations in data transfer rates, and are subject to adjacent channel noise interference, as well.
Differential buses, in contrast, use a pair of conductors, or trace-pairs, arranged in spaced relation and insulated from a ground plane. The predetermined spacing between the two conductors, in conjunction with the insulating dielectric and the ground plane, provide a controlled impedance layer for the rapid and efficient transfer of digital data. Both of the conductors are electrically above the ground plane at all times. Transmitters and receivers are employed at each end of the trace-pairs to encode and decode a predetermined signal differential between them. Consequently, differential buses provide the advantages of increased data rate transfer and greater immunity from noise, when compared to single-ended buses. However, because differential buses require two traces, they further restrain the options for circuit board layout and design.
In view of the above-noted difficulties in prior art designs, the need exists for a circuit board construction which allows the use of differential buses, and permits connectors for electronic devices and cables to be conveniently and efficiently located on the board.
The apparatus of the present invention employs a four-layered printed circuit board, for the purpose of mounting and electrically interconnecting two or more computer devices. These devices may include disk drives, processors, and external cables. Each of these devices is adapted to plug into a conventional device connector, such as a four row, twenty column SCSI (Small Computer System Interface) disk drive connector. The connectors may be located where desired on either side oft he circuit board, for example, in a line along one edge, or on opposing edges of the board.
Each connector includes at least one connector trace field, for connection to a differential bus. If the connector is to be connected to two connectors, then it includes two connector trace fields. The connector trace field provides a conductive transition between the pins of the connector and the bus. The differential bus is comprised of a plurality of trace-pairs of conductors, extending between adjacent computer devices. The first length of the differential bus is on a top side of the board, and the second length of the bus is on the bottom side of the board.
A via trace field is provided to interconnect the first and second lengths of the bus. The via trace field includes a top via field and a bottom via field. These via trace fields are substantially identical in configuration and size to the conductor trace fields. Vias, providing a conductive path through the circuit board, interconnect the top and the bottom via fields.
The via trace fields may include slight undulations so as to equalize the overall lengths of the trace-pairs in each differential bus. The via trace fields are also used selectively to reverse the trace or conductor order of groups of the trace-pairs, so that induced currents between the traces will cancel, minimizing crosstalk between them. This trace order reversal feature is also employed in the connector trace fields, to minimize crosstalk in an identical fashion.
The via fields may be oriented parallel with respect to collinearly arranged devices, or orthogonal with respect to devices or connectors which are parallel. Where a connector has laterally offset groups of pin connections, a via field may also include offset groups of vias to provide trace length equalization. These orthogonally oriented via fields also have the capability of equaling the lengths of the buses, and reversing the conductor order of the traces within the buses, to ensure the integrity of signal transfer between computer devices.
In the preferred microstrip construction, the trace-pairs are on the outside layers of the board, and the ground planes for each set of trace-pairs are buried in the board. Alternatively, in a stripline construction which is also disclosed, the trace-pairs are buried in the board, and the outer board surfaces are ground planes
These and other objects of the apparatus of the present invention will be apparent in the drawings and in the detailed description of the preferred embodiment, to follow.