GB2254963A - Terminating arrangement for a backplane - Google Patents

Abstract

The signal lines, at each end of a backplane B each electrically connected to a powerbus 12, spaced from the rear face of the backplane and carried by and electrically insulated with respect to a H-shaped body 1 detachably secured to the backplane, via a discrete resistor R which is housed at least in part in a throughbore 4 in the body and which is electrically connected in series between the powerbus and the plated through hole in the group of holes at the end of the backplane connected to the signal line. Each of the ground lines of the backplane B is connected to the powerbus 12 via a discrete capacitor C which is housed at least in part in a throughbore 5 in the body 1 and which is electrically connected in series between the powerbus and the plated through hole of the group of holes connected to the ground line. <IMAGE>

Description

AN IMPROVED CIRCUIT BOARD In complex electronic apparatus incorporating a plurality of circuit boards arranged with their major faces lying in substantially parallel planes, it is the general practice to interconnect circuits of such boards by means of multi-circuits and associated connectors carried on a board, frequently referred to as a mother board or backplane; to which edge connectors of said plurality of circuit boards are permanently or detachably connected.

One kind of mother board or backplane which has hitherto been proposed and used comprises a substantially rigid board of electrically insulating material having extending through the board a multiplicity of holes whose boundary walls are coated with a layer of electrically conductive metal or metal alloy (such metal coated holes generally and hereinafter being referred to as plated through holes) and which are arranged in a plurality of discrete groups of plated through holes mutually spaced transversely of the board, the plated through holes of each group being arranged in a pattern of columns and rows of mutually spaced holes, the pitch between adjacent plated through holes of each column and of each row being n.Each of a major proportion of the plated through holes of each group is electrically connected in series to the correspondingly positioned plated through hole in each of the other groups by an elongate track of electrically conductive metal or metal alloy encapsulated in the electrically insulating material of the rigid board to form a signal line, the tracks electrically connected to the plated through holes of corresponding columns of the groups lying in common planes which are substantially parallel to and mutually spaced from one another and being electrically insulated from one another by the electrically insulating material of the board.Each of the remaining minor proportion of the plated through holes of each group is electrically connected to a substantially continuous layer of electrically conductive metal or metal alloy which is encapsulated in the electrically insulating material of the board and which constitutes a ground plane, to form a ground line.

A mother board or backplane of the aforesaid kind will hereinafter, for convenience, be referred to as "a backplane of the kind described". A backplane of the kind described may carry also a powerbus or a number of powerbuses; the or a powerbus may be an area of electrically conductive metal or metal alloy on a major surface of the rigid board and/or the or a powerbus may be a substantially continuous layer of electrically conductive metal or metal alloy constituting a power plane encapsulated in the electrically insulating material of the board and spaced and electrically insulated from the or each ground plane or from an adjacent power plane.

When a backplane of the kind described is to be used, at each end of the backplane the signal lines are electrically connected to a powerbus remote from the backplane or to the or a powerbus or power plane constituting a component part the backplane through resistors and the ground lines are electrically connected to said powerbus or power plane through capacitors.

In one backplane of the kind described which is currently widely used and which is generally known as the "Futurebus" backplane, each group of plated through holes comprises three columns each of 32 mutually spaced holes, the pitch n between adjacent holes in each column and in each row being 2.54mm. At each end of a "Futurebus" backplane, each signal line is terminated by a terminator assembly which carries a multi-socket connector for engaging with pins which have been electrically connected in the plated through holes of the end group of holes and which protrude from the rear of the board.A major proportion of the signal line sockets of the multi-socket connector carried by each terminator assembly are electrically connected via resistor packs mounted on the terminator assembly to the or a powerbus remote from the board and the remaining minor proportion of ground line sockets - which are substantially evenly distributed in the multi-socket connector - are electrically connected to this powerbus via capacitor packs mounted on the terminator assembly; each resistor pack is electrically connected to a plurality of signal lines, usually nine, and each capacitor pack is electrically connected to a plurality of ground lines, usually between ten and fourteen.

Whilst the "Futurebus" backplane and the terminator assemblies used with the "Futurebus" backplane have been found to be suitable for accommodating the bit rate of the digital signals transmitted by electronic apparatus currently in use, it has been found that they cannot accommodate the high bit rate of the digital signals transmitted by the more complex electronic apparatus now employed, with the result that the transmitted digital signals are inevitably delayed, attenuated or distorted.

Moreover, it has been found that the sockets of the multi-socket connectors of the terminator assemblies used with the "Futurebus" backplane are so arranged relative to one another that, when the transmitted digital signals have a high bit rate, there is invariably unacceptable interference, i.e. cross talk, between transmitted digital signals carried by separate signal lines of the board.

With a view to providing a backplane of the kind described which will have greater flexibility and enhanced performance, a backplane of the kind described has been proposed - known as the "Futurebus+" backplane - in which each group of plated through holes to which an edge connector of a circuit board can be electrically connected consists of four columns of 48 holes, the pitch n between adjacent holes in each column and in each row being 2mm. For the first time, at each end of each group additional plated through holes are provided for reception of a separate power connector.A signal connector to be connected to the 192 plated through holes of each group may comprise discrete modules of, say, 4 x 6 pins which can be assembled end to end to form a 192 pin connector which will be connected to the front face of the board and with which a female edge connector of a circuit board can readily engage.

Alternatively, the signal connector may comprise a single module of 4 x 48 pins. The pins of each module assembled to form the pin connector protrude a very short distance from the rear face of the board and may be soldered in place. As a consequence, the terminator assemblies currently employed to terminate a "Futurebus" backplane are not suitable for terminating the Futurebus+ backplane and a terminating arrangement for terminating a "Futurebus+" backplane is required which in no way will inhibit use being made of the greater flexibility and enhanced performance of the "Futurebus+" backplane.

It is an object of the present invention .to provide, in a backplane of the kind described, an improved arrangement for terminating the signal and ground lines of the backplane.

In the improved backplane of the kind described according to the invention, at each end of the backplane the signal lines are each electrically connected to a powerbus, spaced from the rear face of the backplane and carried by and electrically insulated with respect to a body detachably secured to the backplane, via a discrete resistor which is housed at least in part in a throughbore in said body and which is electrically connected in series between said powerbus and the plated through hole in the group of holes at said end of the backplane connected to said signal line.

By virtue of the fact that each signal line at each of its ends is connected to a discrete resistor, in the event that a resistor should be faulty only the signal line to which the faulty resistor is connected will be out of service until the faulty resistor is replaced. This is an important advantage not provided by the terminator assemblies of the "Futurebus" backplane.

Preferably, at each end of the backplane each of at least some of the ground lines is connected to said powerbus via a discrete capacitor which is housed at least in part in a throughbore in said body and which is electrically connected in series between said powerbus and the plated through hole in said group of holes connected to said ground line. In the event that a capacitor connected to one end of a ground line should be faulty only the ground line to which the faulty capacitor is connected will be out of service until the faulty capacitor is replaced.

Preferably, the body detachably secured to the rear face of the backplane has a plurality of throughbores which house the discrete resistors and capacitors, whose axes are substantially parallel to one another and substantially normal to the rear face of the backplane and which are so disposed within said body that the open ends of the throughbores nearer the rear face of the backplane are spaced therefrom. The body may also have two or more throughbores for accommodating screws or other fastening means to be employed to secure the body to the rear face of the backplane.In a preferred embodiment, the body has a cross-section of substantially H-shape with the throughbores passing through the bar extending between the two limbs of the H-shaped body and the body is so detachably mounted on the rear face of the backplane that one end face of each limb is in interfacial contact with the rear face of the backplane.

The powerbus preferably comprises a layer of electrically conductive metal or metal alloy on a surface of a substantially rigid board of electrically insulating material which is detachably or permanently secured to the body. Where the body has a cross-section of substantially H-shape, preferably the rigid board extends between the limbs of the H-shaped body on the side of the bar of the H remote from the backplane and, in this case, preferably also, the electrically conductive layer constituting the powerbus is on the surface of the board remote from the backplane.

Any convenient method may be employed to electrically connect one terminal of each discrete resistor or discrete capacitor to the powerbus and to electrically connect the other terminal of the resistor or capacitor to its associated plated through hole of said group of holes of the backplane and the particular methods used may be those most suitable for use with the forms of discrete resistors or discrete capacitors employed.Where, as is preferred, each discrete resistor and each discrete capacitor has a terminal lead at each of its ends, preferably one terminal lead of each resistor or capacitor is soldered in one of a plurality of plated through holes which extend through the rigid board and are electrically connected to the powerbus carried by the board and the other terminal lead is soldered in one of a plurality of plated through holes in a second substantially rigid board of electrically insulating material detachably or permanently secured to said body, each of which plated through holes is electrically connected by an electrically conductive track encapsulated in or carried on a surface of the second rigid board to one of said plurality of plated through holes in said second board in which one of the pins of a module, or of an assembly of modules, electrically connected in one of the plated through holes at said end of and protruding from the backplane, effects a detachable or permanent electrical connection, e.g. by effecting a pin and socket connection or by being soldered therein. Preferably, the plated through holes in the first and second rigid boards in which the terminal leads of a discrete resistor or discrete capacitor are soldered are in substantially axial alignment with the throughbore in said body in which the resistor or capacitor is housed.

Where, as is preferred, the body has a cross-section of substantially H-shape, preferably the second rigid board extends between the limbs of the H on the side of the bar of the H nearer the backplane.

The body housing the discrete resistors and capacitors may be made of electrically insulating material or it may be made of electrically conductive metal or metal alloy and electrically insulated from the powerbus and from the signal lines to which the discrete resistors are electrically connected.In a preferred embodiment, the body is made of electrically conductive metal or metal alloy and has a cross-section of substantially H-shape and is electrically connected to a ground plane, which is encapsulated in or carried on a surface of the first rigid board and which is electrically insulated with respect to the powerbus, and to a ground plane, which is encapsulated in or carried on a surface of the second rigid board and which is electrically insulated with respect to the signal lines, thereby effectively constituting a Faraday cage within which the discrete resistors and capacitors are accommodated.In this particular case, preferably each discrete capacitor has two terminal leads extending from the capacitor in the same directions and one terminal lead of each capacitor is soldered in one of a plurality of plated through holes which extend through the first rigid board and are electrically connected to the powerbus on one surface of the board and the other terminal lead is soldered in one of a plurality of plated through holes which extend through the first rigid board and which are electrically connected to the ground plane encapsulated in or carried on the other surface of the board.

The improved backplane terminating arrangement of the present invention is especially, but not exclusively, suitable for use with a "Futurebus+" backplane and, in this case, the bus will use recently developed, high current, open collector Backplane Transceiver Logic (BTL) on all signals.

The invention also includes, for use with a backplane of the kind described, a terminator assembly comprising a body having a cross-section of substantially H-shape and having passing through the bar extending between the two limbs of the H a plurality of mutually spaced throughbores whose axes are substantially parallel to the limbs of the H, each throughbore being of such a cross-sectional size that it can house a discrete resistor or capacitor and, extending between the limbs of and secured to the Hshaped body on one side of the bar, a substantially rigid board of electrically insulating material having on one of its major surfaces a layer of electrically conductive metal or metal alloy which is suitable for constituting a powerbus and which is electrically insulated with respect to the body. Preferably, the metal layer is on the surface of the board remote from the bar of the H.Preferably, also, the board has a plurality of plated through holes which extend through the board and are electrically connected to the metal layer carried by the board and which may be in axial alignment with the mutually spaced throughbores in the bar of the H.

The terminator assembly may also include any one or more of the optional features of the terminator assembly employed in the improved terminating arrangement hereinbefore described.

The invention is further illustrated by a description, by way of example, of a preferred arrangement for terminating the signal and ground lines of a backplane of the kind described, with reference to the accompanying diagrammatic drawing which shows a transverse cross-sectional view of a preferred terminator assembly mounted on the rear face of the backplane and employed to terminate the signal and ground lines thereof.

In the preferred backplane terminating arrangement illustrated in the diagrammatic drawing, a terminator assembly is detachably secured to each end of the rear face of a backplane which comprises a rigid board of electrically insulating material having extending through the board a multiplicity of plated through holes arranged in a plurality of discrete groups of plated through holes mutually spaced transversely of the board.

The plated through holes of each group are arranged in a pattern of columns and rows of mutually spaced plated through holes, the pattern of each group consisting of four columns each of 48 holes, the pitch between adjacent holes in each column and in each row being 2mm.

Each of a major proportion of the plated through holes of each group is electrically connected in series to the correspondingly positioned plated through hole in each of the other groups by an elongate copper track encapsulated in the electrically insulating material of the rigid board to form a signal line. The tracks electrically connected to the plated through holes of corresponding columns of the groups lie in common planes which are parallel to and mutually spaced from one another and are electrically insulated from one another by the electrically insulating material of the board.

Each of the remaining minor proportion of the plated through holes of each group is electrically connected to a continuous copper layer encapsulated in the electrically insulating material of the rigid board and constituting a ground plane, to form a ground line. At each end of each group of plated through holes, additional plated through holes are provided for reception of a separate power connector. A backplane as so described, generally referred to as a Futurebus+ backplane, is of a known construction which forms no part of the present invention and, as a consequence, the component parts of a Futurebus+ backplane are not illustrated and the backplane is identified by the general reference B.

At each end of the Futurebus+ backplane B, the 192 plated through holes of each group of plated through holes of the backplane are electrically connected to a multi-pin connector P comprising eight discrete modules of 4 x 6 pins assembled end-to-end to form a 192 pin connector which is mounted on the front face of the board and with which a female edge connector (not shown) of a circuit board can detachably engage. The majority of pins S of pin connector P (of which three are illustrated) protrude through and are in electrical contact with plated through holes in the backplane B electrically connected to signal lines of the backplane and a minority of pins G of the pin connector (of which one is illustrated) protrude through and are in electrical contact with plated through holes in the backplane electrically connected to ground lines of the backplane.

In the preferred back plane terminating arrangement, a terminator assembly T which is detachably secured to one end of the rear face of the Futurebus+ backplane B comprises an elongate body 1 which is made of electrically conductive metal and which is of Hshaped transverse cross-section, the body being detachably secured to the backplane by screws or other fastening means (not shown) passing through throughbores in the elongate body near its ends in such a way that end faces of the limbs 2 of the body bear against the rear face of the backplane and the bar 3 of the body is spaced from and extends parallel to the backplane. Extending through the bar 3 of the H-shaped body 1 is a plurality of throughbores 4,5 whose axes lie parallel to one another and to the limbs 2 of the body and normal to the rear face of the backplane B.Disposed between the limbs 2 of the H-shaped body 1 on the side of the bar 3 remote from the backplane B is a rigid board 11 of electrically insulating material carrying on the major surface of the board remote from the backplane a layer 12 of copper which constitutes a powerbus electrically insulated with respect to the body. The copper layer 12 extends into and lines each of a plurality of holes 15 in the board 11 in axial alignment with throughbores 4 in the bar 3 of the H-shaped body 1 and each of a plurality of holes 16 in the board (of which one only is illustrated) disposed opposite throughbores 5 in the bar to form-plated through holes, the copper layer forming, on the major surface of the board nearer the backplane, annular pads bounding the plated through holes 15 and 16.The major surface of the rigid board 11 nearer the backplane B carries a layer 14 of electrically conductive metal which is in electrical contact with the limbs 2 of the H-shaped body 1 and which extends into and lines each of a plurality of holes 17 in the board (of which one only is illustrated) disposed opposite throughbores 5 in the bar 3 of the H-shaped body 1 to form plated through holes, the metal layer 14 forming, on the major surface of the board remote from the backplane, annular pads bounding the plated through holes 17. On the major surface of the rigid board 11 remote from the backplane B, the copper layer 12 is electrically insulated from the annular metal pad bounding each plated through hole 17 by an annular gap 19; on the major surface of the board nearer the backplane, the metal layer 14 is electrically insulated from the annular copper pad bounding each of holes 15 and 16 by an annular gap 18.

Disposed between the limbs 2 of the H-shaped body 1 on the side of the bar 3 nearer the Futurebus+ backplane B is a rigid board 21 of electrically insulating material which on its major surface nearer the backplane carries a plurality of copper tracks 22 (of which one only is illustrated) electrically insulated with respect to the body. Each of the copper tracks 22 extends into and lines one of a plurality of holes 25 in the rigid board 21 in axial alignment with throughbores 4 in the bar 3 of the H-shaped body 1 and one of a plurality of holes 26 in the board disposed opposite throughbores 5 in the bar to form plated through holes, the copper tracks forming, on the major surface of the board remote from the backplane, annular copper pads bounding the plated through holes 25 and 26.

The major surface of the rigid board 21 remote from the backplane B carries a layer 24 of electrically conductive metal which is in electrical contact with the limbs 2 of the H-shaped body 1 and which extends into and lines each of a plurality of holes 27 in the board (of which one only is illustrated) disposed opposite throughbores 5 in the bar 3 to form plated through holes, the metal layer forming, on the major surface of the board nearer the backplane, annular metal pads bounding the plated through holes 27.On the major surface of the rigid board 21 nearer the backplane B, the copper tracks 22 are electrically insulated from the annular metal pads bounding each plated through hole 27 by annular gaps 28; on the major surface of the board remote from the backplane, the metal layer 24 is electrically insulated from the annular copper pads bounding the plated through holes 25 and 26 by annular gaps 29. The plated through holes 26 and 27 are in axial alignment with the additional plated through holes at the end of the Futurebus+ backplane B and each has a resilient metal socket tightly fitted therein.

Housed in each of the throughbores 4 in the bar 3 of the H-shaped body 1 is a discrete resistor R having at one end of the resistor a terminal lead which extends through a plated through hole 15 in the rigid board 11 and is soldered to the powerbus 12 and which at its other end has a terminal lead which passes through a plated through hole 25 in the rigid board 21 and is soldered to one of the copper tracks 22.In each of the throughbores 5 in the bar 3 of the H-shaped body 1 is housed a discrete capacitor C having two terminal leads extending in the same directions, one terminal lead of each capacitor extending through a plated through hole 16 in the rigid board 11 and being soldered to the powerbus 12 and one terminal lead of the capacitor extending through the plated through hole 17 and being soldered to the annular metal pad which is on the major surface of the board remote from the backplane B and which is contiguous with the metal layer 14.

Pins S of the multi-pin connector P electrically connected to signal lines of the Futurebus+ backplane B protrude through plated through holes of the additional plated through holes at the end of the backplane and effect plug and socket connections in the resilient sockets of the plated through holes 26 in the rigid board 21; pins G of the pin connector electrically connected to ground lines of the backplane protrude through plated through holes of the additional plated through holes at the end of the backplane and effect pin and socket connections in the resilient sockets of the plated through holes 27 in the rigid board 21. The metal layers 14 and 24, respectively, of the rigid boards 11 and 21 and the limbs 2 of the H-shaped body 1 together effectively constitute a Faraday cage within which the discrete resistors R and capacitors C are accommodated.

Since each signal line of the Futurebus+ backplane B at each end of the backplane is connected to a discrete resistor R and each ground line of the backplane at each end of the backplane is connected to a discrete capacitor C, in the event that a resistor or capacitor should be faulty only the signal line or ground line to which the faulty resistor or capacitor is connected will be out of service until the faulty resistor or capacitor is replaced.

Claims (19)

Claims:

1. An arrangement for terminating the signal and ground lines of a backplane of the kind described, wherein at each end of the backplane the signal lines are each electrically connected to a powerbus, spaced from the rear face of the backplane and carried by and electrically insulated with respect to a body detachably secured to the backplane, via a discrete resistor which is housed at least in part in a throughbore in said body and which is electrically connected in series between said powerbus and the plated through hole in the group of holes at said end of the backplane connected to said signal line.

2. A backplane terminating arrangement as claimed in Claim 1, wherein at each end of the backplane each of at least some of the ground lines is connected to said powerbus via a discrete capacitor which is housed at least in part in a throughbore in said body and which is electrically connected in series between said powerbus and the plated through hole in said group of holes connected to said ground line.

3. A backplane terminating arrangement as claimed in Claim 1 or 2, wherein the body detachably secured to the rear face of the backplane has a plurality of throughbores which house discrete resistors and capacitors, whose axes are substantially parallel to one another and substantially normal to the rear face of the backplane and which are so disposed within said body that the open ends of the throughbores nearer the rear face of the backplane are spaced therefrom.

4.- A backplane terminating arrangement as claimed in Claim 3, wherein the body also has two or more throughbores for accommodating screws or other fastening means employed to secure the body to the rear face of the backplane.

5. A backplane terminating arrangement as claimed in Claim 3 or 4, wherein the body has a cross-section of substantially H-shape with the throughbores passing through the bar extending between the two limbs of the H-shaped body and the body is so detachably mounted on the rear face of the backplane that one end face of each limb is in interfacial contact with the rear face of the backplane.

6. A backplane terminating arrangement as claimed in any one of the preceding Claims, wherein the powerbus comprises a layer of electrically conductive metal or metal alloy on a surface of a substantially rigid board of electrically insulating material which is detachably or permanently secured to the body.

7. A backplane terminating arrangement as claimed in Claim 5, wherein the powerbus comprises a layer of electrically conductive metal or metal alloy on a surface of a substantially rigid board of electrically insulating material which extends between the limbs of and is detachably or permanently secured to the H-shaped body on the side of the bar of the H remote from the backplane, the electrically conductive layer constituting the powerbus being on the surface of the board remote from the backplane.

8. A backplane terminating arrangement as claimed in Claim 7 in which each discrete resistor and each discrete capacitor has a terminal lead at each of its ends, wherein pins of a module are electrically connected in the plated through holes at said end of and protrude from the rear face of the backplane and effect plug and socket connections in holes in a second substantially rigid board of electrically insulating material extending between the limbs of and secured to the H-shaped body on the side of the bar of the H nearer the backplane, and wherein one terminal lead of each resistor or capacitor is soldered in one of a plurality of plated through holes which extend through the first rigid board and are electrically connected to the powerbus carried by the first rigid board and the other terminal lead is soldered in one of a plurality of plated through holes in the second rigid board, which plated through hole is electrically connected by an electrically conductive track encapsulated in or carried on a surface of the second rigid board to one of the pins of said module effecting a plug and socket connection in one of said holes in the second rigid board.

9. A backplane terminating arrangement as claimed in Claim 8, wherein the H-shaped body is made of electrically conductive metal or metal alloy and is electrically connected to a ground plane, which is encapsulated in or carried on a surface of the first rigid board and which is electrically insulated with respect to the powerbus, and to a ground plane which is encapsulated in or carried on a surface of the second rigid board and which is electrically insulated with respect to the signal lines.

10. A backplane terminating arrangement as claimed in Claim 7 in which each discrete resistor has a terminal lead at each of its ends and each discrete capacitor has two terminal leads extending from the capacitor in the same directions, wherein pins of a module are electrically connected in the plated through holes at said end of and protrude from the rear face of the backplane and effect pluged socket connections in holes in a second substantially rigid board of electrically insulating material extending between the limbs of end secured to said H-shaped body on the side of the bar of the H nearer the backplane, wherein one terminal lead of each resistor is soldered in one of a plurality of plated through holes which extend through the first rigid board and are electrically connected to the powerbus carried by the first rigid board and the other terminal lead is soldered in one of a plurality of plated through holes in the second rigid board, which plated through hole is electrically connected by an electrically conductive track encapsulated in or carried on a surface of the second rigid board to one of the pins of said module effecting a plug and socket connection in one of said holes in the second rigid board, and wherein one terminal lead of each capacitor is soldered in one of said plurality of plated through holes which extend through the first rigid board and are electrically connected to the powerbus on one surface of the board and the other terminal lead is soldered in one of a plurality of plated through holes which extend through the first rigid board and which are electrically connected to a ground plane encapsulated in or carried on the other surface of the board.

11. A backplane terminating arrangement as claimed in Claim 10, wherein the H-shaped body is made of electrically conductive metal or metal alloy and is electrically connected to the ground plane encapsulated in or carried on a surface of the first rigid board and to a ground plane which is encapsulated in or carried on a surface of the second rigid board and which is electrically insulated with respect to the signal lines.

12. A backplane terminating arrangement as claimed in any one of Claims 8 to 11, wherein the plated through holes in the first and second rigid boards in which the terminal leads of a discrete resistor are soldered are in substantially axial alignment with the throughbore in said body in which the resistor is housed.

13. For use with a backplane of the kind described, a terminator assembly comprising a body having a cross section of substantially H-shape and having passing through the bar extending between the two limbs of the H a plurality of mutually spaced throughbores whose axes are substantially parallel to the limbs of the H, each throughbore being of such a cross-sectional size that it can house a discrete resistor or capacitor and, extending between the limbs of and secured to the Hshaped body on one side of the bar, a substantially rigid board of electrically insulating material having on one of its major surfaces a layer of electrically conductive metal or metal alloy which is suitable for constituting a powerbus and which is electrically insulated with respect to the body.

14. A terminator assembly as claimed in Claim 13, wherein the metal layer is on the surface of the board remote from the bar of the H.

15. A terminator assembly as claimed in Claim 13 or 14, wherein the board has a plurality of plated through holes which extend through the board and are electrically connected to the metal layer carried by the board.

16. A terminator assembly as claimed in Claim 15, wherein said plurality of plated through holes are in axial alignment with mutually spaced throughbores in the bar of the H.

17. A terminator assembly as claimed in any one of Claims 13 to 16, wherein a second rigid board of electrically insulating material extends between the limbs of and is secured to the H on the opposite side of the bar of the bar H to the first rigid board, the second rigid board having a plurality of plated through holes extending therethrough and electrically connected to tracks of electrically conductive metal or metal alloy encapsulated in or carried on one major surface on the second rigid board.

18. A terminator assembly as claimed in Claim 17, wherein the plurality of plated through holes in the second rigid board are in axial alignment with the plurality of plated through holes in the first rigid board.

19. A terminator assembly as claimed in Claim 17 or 18, wherein the body is made of electrically conductive metal or metal alloy; the first rigid board has encapsulated in or carried on the other of the major surfaces of the board a layer of electrically conductive metal or metal alloy which is suitable for constituting a ground plane, which is electrically insulated with respect to the metal layer suitable for constituting a powerbus and which is electrically connected to the body; and the second rigid board has encapsulated in or carried on the other of the major surfaces of the board a layer of electrically conductive metal or metal alloy which is suitable for constituting a ground plane, which is electrically insulated with respect to the electrically conductive tracks encapsulated in or carried said one major surface of the second rigid board