Flat cable bus

Abstract: A flat cable bus with fan-out patterns which provide equal length wires to eliminate clock skew is disclosed.

Description: This invention relates to a flat cable bus of the type to be connected between very high speed computers. Background of the Invention High speed computers often use parallel flat cable bus comprising a plurality of conductors arranged side by side in a common plane spaced and insulated from each other in a ribbon of dielectric material. In addition it is desirable that the cables go from a narrow, high density area to a wide transmission area. The wide cable area can maximize the signal propagation characteristics and increase the manufacturing yield of the flat cables. In the prior art generally these flat cables go from a broad terminal end to a narrow transmission section and then back to being broad at the other terminating end. The purpose of the broadening is to have connector pads and make easy access to connectors. The connectors at the terminating end would be roughly ten conductors per 2.54 cm (1 inch). DE-A-2 210 019 describes a method of manufacturing flat conductors with an insulating wrapping of thermoplastic or thermoelectric material. Flat conductors are shown which consist of several wires positioned adjacently to each other. Further an embodiment is shown in which the wires change direction to follow a path defined by a series of pegs. In this embodiment the wires spread out from each other at the two connection ends so as to facilitate easier connection. In an effort to reduce the space taken up by the connectors at the terminal end, especially for large high speed computers requiring multiple harnesses, it is desirable that the terminating end width be as small as possible and these terminating ends are made even closer than the middle transmission section. The transmission sections are wider so as to prevent resistive losses and other characteristic problems. This density at the terminating ends may be on the order of 100 conductors per 2.54 cm (1 inch). The conventional manner in which these cables are made is a fan-out pattern as shown in the prior art in Fig. 1. The wires on the outside of the cable are longer than the central wires. Specifically for 2.54 cm (one inch) connector widths to a 7.62 cm (three inch) transmission cable the maximum difference is 2.08 cm (0.82 inch). At expected future signal propagation velocities, this results in a time skew of 130 picoseconds. In a typical processor/memory/processor loop of a supercomputer this is multiplied by four to six consecutive flat cable buses. An example of such a system is described in US Application Serial No 176,495 filed March 31, 1988 (corresponding to EP-A-0 335 482, published on 04/10/1989) and entitled "High Performance Computer Package". The accumulated differential distance clock skew becomes a significant portion of the two to four nanosecond machine cycle that would be necessary for this type of computer. Summary of the Invention According to the present invention there is provided a flat cable bus comprising a plurality of conductors arranged side-by-side in a common plane in a dielectric body between two ends of the cable, the distance between adjacent conductors varying between the two ends of the cable, the cable being characterised in that in combination: said conductors within said body are relatively close at at least one of the ends and fan out from said at least one end to provide more widely spaced parallel conductors; and said conductors are configured so that, between the two ends of the cable, they all have equal path lengths. In the parallel cable bus of the present invention the wires fan out, and yet all the wires within the cable bus have equal path lengths to reduce clock skew. In accordance with preferred embodiments of the present invention the differential distance clock skew is eliminated. Description of Drawings Fig. 1 is a sketch of a prior art flexible flat cable with a fan-out pattern which has unequal path lengths; Fig. 2 is a view of a flexible flat cable of the type used in a harness in accordance with one embodiment of the present invention; Fig. 3 is a view taken along the lines 3-3 of Fig. 2; and Fig. 4 is a plan view of a flexible flat cable of the type used in a harness in accordance with a second embodiment of the present invention. Description of Preferred Embodiment Referring to Fig. 1 there is illustrated a sketch of the prior art fan-out flat cable 10 where the outboard conductor wires are substantially longer than the inner wires. For example, the central wire 11 may be six inches long with the parallel segments at each terminal end being 1.27 cm (half an inch) long. The central section 11a of the center wire 11 is 7.62 cm (3 inches) long with both transition sections being 2.54 cm (an inch) on either end. The transition sections 16a, 16b and 17a and 17b for the outermost wires 16 and 17 is 3.58 cm (1.41 inches) making the outermost wires 16 and 17 have a total maximum increase of 2.08 cm (0.82 inches)over the central wire 11. At expected signal propagation velocities, this results in a time skew of 130 picoseconds. In a typical processor/memory/processor loop of a supercomputer type configuration as mentioned previously this could be multiplied by four to six consecutive flat buses and the accumulated clock skew could become a significant portion in a two to four nanosecond machine cycle. This represents the problem for which the present invention overcomes. Referring to Fig. 2 there is illustrated a flat cable bus 20 according to one embodiment of the present invention wherein the conductors 21 through 27 are parallel to each other at the terminal ends 28 and 29 and the arrangement of these conductors therebetween is spread out to accommodate and maximize the signal propagation characteristics as with the prior art cable in Fig. 1 but without the unequal length conductors. Cable 20 may be of commonly known type of flat conductor cable in which the individual conductors 21-27 are contained in plastic film material 19, such as mylar polyethyleneterephthalate). See Fig. 3. Cable 20 has one end 28 which is narrow and adapted to be connected, for example to a printed circuit board, and a second end 29 which is also narrow and adapted to connect to a circuit board. The number of conductors in the cable for a specific harnessing situation is equal to the number of positions on each row of a printed circuit board. The conductors are relatively thin or narrow between adjacent portions and remain separated by the plastic dielectric material. The differential distance clock skew is eliminated in the diagonal transition sections where the conductors are angled at 45 degrees as illustrated in Fig. 2. Fig. 2 shows a one to three to one fan-out design where the width of the line expands from 2.54 cm (one inch) at the terminal ends 28 and 29 to 7.62 cm (three inches) in the middle or in the main transmission line portion which is represented by the 2.54 cm (one inch) length in the sketch illustrating the 6.82 equal length lines. The middle or main transmission section 32 can be any desired length to accommodate the need. In the diagonal section 30 nearest end 28 the leftmost conductor 21 has no bend and is of minimum length (5.08 cm (2 inches)) and the rightmost conductor 27 has the most length and the longest diagonal portion (7.16 cm (2.82 inches)). Conductors 22 through 26 have increasingly longer diagonal sections and longer lengths. In the diagonal section 21 nearest end 29 the leftmost conductor 21 is of maximum added length (longest diagonal portion of (7.16 cm (2.82 inches)) which equals the diagonal portion of conductor 27 in section 30. Further, conductor 27 in transition section 31 is straight and of minimum length (5.08 cm (2 inches)). The middle conductor 24 has equal diagonal portions (3.58 cm (1.41 inches)) in sections 30 and 31 that total up to the diagonal portion of either conductor 21 or 27. The diagonal portions in section 30 progressively increase for conductors 22 through 27 and the diagonal portions in section 31 progressively decrease for conductors 22 through 26 such that all of the conductors 21-27 are of equal electrical length. In this way the conductor lengths are the same. For example, the left-most conductor 21 line remains straight for the first 8.89 cm (3.5 inches) (one inch representing the main transmission line section which may be any number of feet) then extends on a 7.16 cm (2.82 inch) diagonal to a 1.27 cm (half inch) portion at end 29. The total length of conductor 21 with the midsection being at the common 2.54 cm (one inch) is 17.32 cm (6.82 inches). Likewise the other conductors 22 through 27 have the same total length. Conductor 27 has the 7.16 cm (2.82 inch) diagonal portions in sections 30 and straight in the other sections. The middle conductor 24 has half of the diagonal portion in each section 30 and 31. Conductor 22 has the shortest diagonal portion (other than zero) in section 30 and conductor 26 has the same diagonal length section in section 31. The remaining length to provide the total of 17.32 cm (6.82 inches) is in the opposite section. Likewise this is true for conductors 23 and 25 as shown. Referring to Fig. 4 there is illustrated, a design wherein the fan-out is at only one end of a cable 39 wherein there is a wide terminal end 40 such as 7.62 cm (3 inches) for example and a narrow terminal end 32 such as 2.54 cm (one inch) wherein the conductors at terminal 40 are widely spaced and parallel and the conductors at the terminal 32 are closely spaced and parallel. In the arrangement of Fig. 4 the conductors are again embedded in a plastic mylar tape with the conductors parallel to each other and spaced by the dielectric and it may be a flexible medium. In this path length adjustment the conductors are again equalized with the conductor 41 having a length of 17.32 cm (6.82 inches) with a 45 degree diagonal section 7.16 cm (2.82 inches) long. The conductor 47 at the opposite side has a bi-directional section of a 45 degree bend, a 90 degree bend an 45 degree bend to provide a v-loop where each of the legs are 3.58 cm (1.41 inches). The central conductor 44 has an asymmetrical v-loop with 5.36 cm (2.11 inches) in the diagonal nearest end 32 and only 1.78 cm (0.7 inch) nearest end 40. The v-loops in conductors 42 to 47 total 7.16 cm (2.82 inches) and get increasingly shorter at end 32 and longer at end 40 with 90 degree bend between legs of the bend as shown in Fig. 4. The matter set forth in the accompanying drawings is being offered by way of illustration only. The density may be for example 100 conductors in 2.54 cm (per inch) at the narrow end to 100 conductors in 7.62 cm (to three inches). Only seven are shown for illustration purposes.

Claims: A flat cable bus comprising a plurality of conductors (21 through 27) arranged side-by side in a common plane in a dielectric body (19) between two ends of the cable, the distance between adjacent conductors varying between the two ends of the cable, the cable being characterised in that in combination: said conductors within said body (19) are relatively close at at least one of the ends and fan out from said at least one end to provide more widely spaced parallel conductors; and said conductors are configured so that, between the two ends of the cable, they all have equal path lengths. A cable as claimed in Claim 1, further comprising a parallel transmission line section (32), wherein between said two ends (28, 29) of the cable and the parallel transmission line section there are tapering portions (30, 31) of the cable, within each tapering portion the conductors being of unequal lengths. A cable as claimed in Claim 2 wherein said conductors in said tapering portions (30, 31) are angled at a 45 degree angle from the direction of the conductors at the termination end (28, 29). A cable as claimed in Claim 1 wherein at said two ends (28, 29) said conductors are closely spaced and parallel, and wherein adjacent and inboard each of said ends there is a separate tapering portion with a first conductor (21) nearest one side of the cable in a first tapering portion (30) near the first end (28) being straight and in a second tapering portion (31) nearest the second end (29) being the longest diagonal section, and wherein a second conductor (27) nearest the side of said cable opposite said one side has the longest diagonal section in said first tapering portion (30) and is straight in said second tapering portion (31), such that the overall lengths of the two conductors are equal. A cable as claimed in Claim 4 wherein a central conductor (24) has equal length diagonal lines in both tapering portions (30, 31) that together equal the length of said longest diagonal line. A cable as claimed in Claim 4 or Claim 5 wherein in the first tapering portion (30) the first conductor (21) nearest one side of said cable is straight and each of said conductors toward an opposite side of the cable have progressively longer diagonal sections, and in the second tapering portion (31) the first conductor (21) has the longest diagonal section and each of said conductors toward said opposite side have progressively shorter diagonal sections. A cable as claimed in Claim 1 wherein at the first end (40) of the cable the conductors (41 through 47) are widely spaced and at the second end (32) said conductors are more narrowly spaced and wherein the conductor (47) nearest one side of the cable has a symmetrically v-shaped bend and said conductors toward an opposite side have a progressively less symmetrical bend, the total length of the legs of the bend being constant.

Inventors: GILLETT JOHN BRIAN, LOHREY FRED HELMUT, LORENZEN JERRY ALAN, GILLETT, JOHN BRIAN, LOHREY, FRED HELMUT, LORENZEN, JERRY ALAN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01B7
IPC codes: H05K1, H01B7