PROCESSING SYSTEM HAVING DISTRIBUTED RADIATED EMISSIONS

Abstract: A processing system (10) includes a backplane bus (13) that defines a plurality of locations (20-31) implemented as backplane slots each for connecting any option module (41) thereto. Each slot includes contact pins (201a-d) that carry binary logic levels forming a different digital number at each slot. Option modules include their own source of timing signals (220) comprising an oscillator (221) and an identical thermally-sensitive crystal (222) for driving the oscillator. Each option module includes receptacles (207a-d) for the contact pins. The receptacles are connected to the digital input port of a D/A converter (225) whose output port is connected to a heater (224) mounted in physical proximity to the crystal in the crystal's case (223). Depending on which slot an option module is mounted in, the D/A converter receives a different digital input and hence generates a different level of output. The heater generates heat in proportion to the converter's output and hence raises the temperature of the crystal to a different level at each slot, causing the crystal to generate somewhat different fundamental frequency at each slot. Hence option modules connected to different slots generate different fundamental frequencies and therefore emit different harmonic frequencies. Hence the amplitudes of the harmonics emitted by different option modules are non-additive, and radiated emission levels of the processing system equipped with all option modules are no higher than those of the system equipped with only one option module.

Description: PROCESSING SYSTEM HAVING DISTRIBUTED RADIATED EMISSIONS Technical Field This invention relates to a circuit arrangement for distributing radiated emissions comprising, a bus arrangement for connecting a plurality of circuit packages each at a different location'. Background of the Invention A processing system generally comprises a basic processor including a central processing module and a memory module, and a bus to which may be selectively connected one or more other functional modules that serve to enhance or expand the processor's capabilities and adapt the processor to its particular application. Such functional modules for example may include input and output (I/O) modules that interface the processor with equipment that allows it to communicate with the outside world. To properly time and synchronize operation of its internal circuits, each module commonly includes a source of timing, or clock, signals such as a crystal-driven oscillator. And to make the modules of a system compatible with each other, for example, such as to make them inter¬ changeable, and to make the modules capable of efficiently carrying on communications with each other across the bus, commonly the oscillators - and hence the driving crystals - of all modules of a system are substantially identical and generate signals of the same frequency. Only the one principal, or fundamental, frequency that is generated by an oscillator is required for the ' operation of a module. However, because of inherent imperfections in crystals that are used to generate the frequency, and because of nonlinearities of circuit components that are driven by the oscillator output, other frequencies that are integral multiples, or harmonics, of the fundamental frequency are generated within a module as well. The harmonics are undesirable because they are emitted, radiated, by the equipment that has generated them and thus can interfere with operation of other equipment and with communications in the vicinity. The greater the strength, or amplitude, of the radiated emissions, the greater the possibility of their causing interference, and the greater the distance at which they can cause interference. Hence it is desirable to keep the- level of the radiated emissions to a minimum, and governmental agencies set strict limits on the levels of emissions that are allowed for various kinds of equipment, including processing systems. Various schemes exist for limiting the radiated emissions of equipment, such as electromagnetic shielding of the emitting equipment. However, no effective scheme exists for eliminating the emissions completely. This presents a problem especially in systems having a plurality of identical frequency sources, such as the above-described processing systems, because the amplitudes of the plurality of emissions are additive at any one frequency. In spite of attempts to limit emissions by electromagnetic shielding, it may be difficult or impossible to meet limits imposed on emission levels with equipment having a plurality of like frequency sources. Because of this problem the above-described processing systems may be restricted in use to environments to which relatively high emission limits are applicable, or be restricted in their processing power, versatility, and capabilities because they cannot be equipped with more than a limited number of functional modules in order to meet applicable emission level limits. This problem is solved in accordance with this invention in a circuit arrangement in which the bus arrangement comprises, at least one connector for engaging each package at each location and arranged to identify the location, and a frequency-generating circuit responsive to being connected to a first location for generating a first frequency unique to that location. Summary of the Invention It is these and other disadvantages and problems of the prior art that the invention is directed to solving. Broadly according to the invention, apparatus ' that has a plurality of connecting arrangements, each for connecting to a frequency-generating apparatus, also has an arrangement for identifying the connecting arrangements, while the frequency-generating apparatus has an arrangement that is responsive to the connecting-arrangement- identifying arrangement and generates a frequency which is dependent upon the identity of the connecting arrangement to which the frequency-generating apparatus is connected. More specifically, a processing system that has a communication bus defining locations each for connecting to a circuit package, such as a functional module, also has an arrangement for identifying each location, while the circuit package includes a frequency-generating arrangement, such as a crystal-driven oscillator, that is responsive to the location-identifying arrangement and generates a frequency that is a function of the identity of the location to which the circuit package is connected. Advantageously, a functional module of a processing system according to this invention generates a somewhat different fundamental frequency at each location of the bus, and hence modules connected to different locations generate somewhat different fundamental frequencies from each other even though all of the connected modules may have identical frequency-generating arrangements. Since the fundamental frequencies of the connected modules are different, their harmonics are also different, and hence the levels of the harmonics are not additive. Therefore, the amplitude of radiated emissions of a processing system equipped with a single module is no greater than that of the system equipped with a plurality of modules. Yet this result is accomplished while the • crystals of all modules of the processing system are allowed to remain identical, as the invention does not rely on differences among the modules and their crystals. Indeed, identical modules may be connected to different locations on the bus without affecting the level of the system's radiated emissions. And in processing systems wherein modules are interchangeable, in that any module may be connected to any location on the bus, the interchangeability of the modules is preserved, because the invention does not depend upon the positioning of modules. These and other advantages and features of the present invention will become apparent from the following description of an illustrative embodiment of the invention, taken together with the drawing. Brief Description of the Drawing FIG. 1 is a block diagram of a processing system embodying an illustrative example of the invention; FIG. 2 is a perspective view of the implementation of the backplane bus, with option modules, of FIG. 1. FIG. 3 is a circuit diagram of the connection of certain lines of the backplane bus of FIGS. 1 and 2; FIG. 4 is a diagram of a frequency signal source circuit of an option module of FIGS. 1 and 2; FIG. 5 is a table showing the operating characteristics of the digital-to-analog converter of the option module of FIG. 4; FIG. 6 is a graph of the thermal characteristic of the crystal of the option module of FIG. 4; and FIG. 7 is an illustrative graph of the distributed radiated emissions of the processing system of FIG. 1. Detailed Description Shown in FIG. 1 is an example of a processing system 10 that includes an illustrative embodiment of the invention. The processing system 10 includes a basic processor that comprises a central processing unit (CPU) 11 and a memory 12 connected to the central processing unit 11. Also connected to the CPU 11 is a backplane bus 13. The bus 13 defines a plurality - in this case twelve - locations numbered 20 through 31, at which option modules may be connected to the bus 13. The connecting arrangement allows the processing system 10 to be equipped with up to twelve option modules. The system 10 may be equipped with fewer than twelve option modules, or no option modules. Three option modules 40 through 42 are illustratively shown connected to the bus 13 at locations 20, 28, and 30, respectively. The bus 13 may define other locations as well. For example, the CPU 11 and the memory 12 may occupy a pair of locations on the bus 13. The bus 13 provides a communication path between the CPU 11 and the various option modules that may be connected to the bus 13. The option modules are circuit packages that provide various capabilities, such as I/O capabilities, to the processing system 10. For purposes of lower system cost, versatility, ease of manufacture, etc., the option modules are substantially identical, and any option module may be connected to the bus 13 at -any location 20-31. FIG. 2 illustrates the physical structure of the bus 13 and the option modules. As its name implies, the backplane bus 13 is implemented in a backplane 200. The backplane 200 comprises a circuit board 210 through which extend a plurality of half-connectors such as contact pins 201. The pins 201 are arranged in a plurality of groups to define a plurality of backplane slots. Each slot represents one of the locations 20-31 for connecting an option module to the bus 13. The portion of the backplane 200 shown in FIG. 2 represents locations 28-31. The option modules, of which the modules 41 and 42 are shown in FIG. 2, are circuit packages formed of printed circuit boards 205 that support various circuits, including frequency-generating arrangements, and printed conductors that interconnect the circuits. An edge of each circuit board 205 is adapted to electrically contact and engage the contact pins 201 of any location 20-31, thereby to make electrical contact between the conductors of the board 205 and the pins 201 of the location and also to mount the board 205 on the board 210. Any option module may be connected to the bus 13 at any location 20-31. The bus 13 is formed by the pins 201 and by conductors 202, such as conductors printed on the backplane circuit board 210 or wires extending along the board 210, that connect to various of the pins 201. The conductors 202 implement various functional lines of the bus 13, such as signal and power lines. Included among the functional lines of the bus 13 are a ground (GND) line and a positive voltage source (V+) line, shown in FIG. 3. Signal levels carried by these two lines"represent the binary logic zero and one signal levels, respectively. The GND and V+ lines are connected to certain ones of the pins 201 in a manner that defines a unique identification code for each of the locations 20-31. In this manner, each of the locations 20-31 is uniquely identified. This location-identifying arrangement is illustrated in FIG. 3. As FIG. 3 shows, the pins 201 that define the identity of each location 20-31 include four pins 201a-d to which are connected the GND and V+ lines. The arrangement in which the GND and V+ lines and the pins 201a-d are connected is different at each location 20-31. For example, at the location 20 the GND line is connected to all four pins 201a-d while the V+ line is not connected to any of the pins 201a-d. Because the GND line represents the logical zero level, the location 20 is marked by pins 201a-d with the digital identification code "0000". At the location 21 the GND line is connected to the pins 201a-c while the V+ line is connected to the pin 201d. The location 21 is thus marked with the digital identification code "0001". At the location 22 the GND line is connected to the pins 201a, b, and d, and the V+ line is connected to the pin 201c, thereby identifying the location 22 as "0010". The pins 201a-d of the other locations 23-31 are similarly connected to the lines GND and V+* to mark the locations with the digital identification codes "0011" through "1011", respectively. Thus each location 20-31 is identified by a different digital identification code. Returning to a consideration of the option modules, while the circuitry of different modules may support or implement different functions, the option modules have certain features in common. The common features that are relevant to an appreciation of this invention are shown and described in FIG. 4 which shows the option module 41. The module 41 is representative of all option modules. As was mentioned before, any option module may be connected to the bus 13 at any location 20- 31. For this purpose, the circuit board 205 of the option module 41 supports at one edge thereof an edge connector 206 which is adapted to engage and make electrical contact with the contact pins 201 of any location 20-31. The edge connector 206 comprises a plurality of half-connectors, which in this example are contact receptacles 207, each of which is adapted to mate with a corresponding pin 201 of any location 20-31. The connector 206 includes receptacles 207a-d which are configured to mate with the pins 201a-d. Like all option modules, the module 41 includes a frequency-generating circuit 220, for generating timing signals for controlling the operation of the various other circuits of the option module 41. The frequency-generating circuit 220 includes a conventional oscillator 221 which is driven in a conventional manner by a crystal 222. All option modules are designed to operate with timing signals having the same nominal frequency and to operate in a range of frequencies around the selected nominal frequency. Hence all option modules have substantially identical crystals 222 that output the same nominal frequency. For purposes of this illustrative example the nominal frequency output of the crystal 222 is about 8 MHz, but the various circuits of the option modules are designed to operate in a range of frequencies ranging from about 7.5 MHz to about 10.5 MHz. The frequency-generating circuit 220 includes an arrangement for varying the frequency of oscillation of the oscillator 221 by varying the frequency generated by the crystal 222. The crystal 222 is housed in a protective case 223. Included in the case 223 in physical proximity 5. to the crystal 222 is a heater 224, such as a diode. The physical proximity of the crystal 222 and the heater 224 and. their encapsulation in the common case 223 ensure that the crystal 222 is thermally coupled with the heater 224. The heater 224 is connected to the output port of a 0 conventional digital-to-analog (D/A) converter 225. The digital input port of the D/A converter 225 is connected to the receptacles 207a-d. The output of the D/A converter 225 powers the heater 224. The output of the D/A converter 225 is a function of the binary signal values 5 that it receives at its digital input port from the pins 201a-d via the receptacles 207a-d. The D/A converter 225 converts the digital signal input into a proportional current level output. FIG. 5 illustrates in tabular form the operating 0 characteristic of the D/A converter 225. FIG. 5 shows that the D/A converter generates a different level of output for every different digital input. For example, when the option module 41 is connected to the location 20, the D/A converter 225 receives at its digital inputs the digital 5 value "0000". The D/A converter 255 responds to that input by generating no output. When the option module 41 is connected to the location 21 , the D/A converter 225 receives the digital value "0001" and responds thereto by generating a current output level having a value of X. At 0 location 22 the D/A converter 225 receives the value "0010" and in response generates an output level of 2X. At location 23 the D/A converter 225 generates an output level of 3X, and so on, until at location 31 the D/A converter 225 generates an output level of 11X. Since the 5 digital input of the D/A converter 225 is the digital identification code of the location 20-31 to which the module 41 is connected, the output of the D/A converter is a function of the identity of the connected location 20-31. The output of the D/A converter 225 provides energy for powering the heater 224. The more current the converter 225 provides to the heater 224, the more heat the heater 224 generates and hence the more it raises the temperature of the crystal 222. Therefore, because the digital input to the D/A converter 225 is different at each • location 20-31, the temperature of the crystal 222 is different at each location 20-31 and is dependent upon the location 20-31 to which the option module 41 is connected. The D/A converter 225 is selected such that the difference X in its output level per unit change in its digital input causes the heater 224 to produce a temperature change Δt in the crystal 222. Hence the temperature of the crystal 222 is a function of the identity of the connected location 20-31. Turning to FIG. 6, there is shown in graph form the thermal characteristic of the crystal 222. The crystal 222 has a high thermal coefficient. The crystal 222 is therefore thermally responsive, in that its frequency output f changes significantly for a given change Δt in temperature. The thermal characteristic of the crystal 222 is rather linear, in that equal-size changes Δt in temperature of the crystal 222 produce approximately equal-size changes Δf in the frequency output of the crystal 222. The crystal 222 is selected to produce a nominal frequency f- j at ambient operating temperature T-* . The ambient operating temperature T-* is the temperature in the case 223 with the heater 224 not powered. The nominal frequency f-* in this illustrative example is about 8 MHz. This is the fundamental frequency generated by the crystal 222 when the option module 41 is connected to the location 20. The harmonics generated by the option module 41 in the location 20 are therefore integral multiples of f•■ . At location 21 , the D/A converter 225 generates an output level of X, causing the heater 224 to raise the temperature of the crystal 222 by Δt over the ambient temperature T- j . The crystal 222 responds to the temperature increase of Δt by increasing the fundamental frequency that it generates by Δf over the nominal frequency of f•• . In location 21 the module 41 therefore generates harmonics that are integral multiples of (f- j + Δ). In this illustrative example, Δf may be, for example, 200 KHz. At location 22, the temperature of the crystal 222 is (T-* + 2 Δt), the fundamental frequency generated by the crystal 222 is (f* | + 2 Δf) , and hence the harmonics generated by the option module 41 are integral multiples of (f- j + 2 Δf). The temperature and frequencies increase correspondingly at the other locations 23-31 of the bus 13. Hence the fundamental and harmonic frequencies generated by the crystal 222 are a function of the identity of the connected location 20-31. Ambient temperature changes will also cause shifts in the frequency output of the crystals. However, changes in the ambient temperature will generally be common to all modules and hence will affect the crystals of all boards substantially equally. Hence the frequency outputs of all modules' crystals will tend to drift in unison and maintain their relative frequency differences. FIG. 7 shows the net result of this invention as applied in the above-described illustrative embodiment of the processing system 10. As a result of each option module with which the processing system 10 is equipped operating at a slightly different fundamental frequency, the harmonics emitted by these modules likewise fall at slightly different frequencies. And because harmonics of different modules are not of the same frequencies, the amplitudes of the harmonics are not additive. Therefore the harmonics of different modules do not reinforce each other, and hence the peak harmonic signals produced by the processing system 10 equipped with all option modules are substantially no higher than those produced by the processing system 10 equipped with only one option module. Yet the various option modules still retain their common characteristics, such as being driven by substantially identical crystals, and the system still retains its versatility of accepting connection of any option module at any of the locations 20-31. Of course, various changes and modifications to the illustrative embodiment described above will be apparent to those skilled in the art. For example, each option module may have an assigned location on the backplane bus, depending upon the modules' function. The system may be capable of accepting fewer than or more than twelve option modules. The modules may operate at a different nominal frequency or with different frequency changes among the bus locations than the ones disclosed. The disclosed linear D/A converter and diode may be replaced by a D/A converter that generates a current level output proportional to the square root of the digital signal input and by a resistor, respectively. Or the D/A converter and heater may be replaced by a resistive network that may be powered directly from the V+ signal line. Such changes and modifications can be made without departing from the spirit and the scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the following claims.

Claims: Claims 1. A circuit arrangement for distributing radiated emissions comprising: a bus arrangement for connecting a plurality of circuit packages each at a different location? CHARACTERIZED IN THAT the bus arrangement (200) comprises: at least one connector for engaging each package at each location and arranged to identify the location; and a frequency-generating circuit (205) responsive to being connected to a first location for generating a first frequency unique to that location. 2. The circuit arrangement in accordance with claim 1 CHARACTERIZED IN THAT the bus arrangement comprises: a prescribed number of connectors at each location for engaging each package and arranged to be connected to the circuit package in a prescribed manner that identifies the location of the prescribed number of connectors. 3. The circuit arrangement in accordance with claim 1 CHARACTERIZED IN THAT the bus arrangement comprises: a plurality of connectors for engaging each package. 4. The circuit arrangement in accordance with claim 1 CHARACTERIZED IN THAT the bus at each location comprises: circuitry for identifying each location. 5. The circuit arrangement in accordance with claim 4 CHARACTERIZED IN THAT the circuitry indicates a digital identification code of the location. 6. The circuit arrangement in accordance with claim 1 CHARACTERIZED IN THAT the connector at each location carries one of a plurality of voltage levels.

Inventors: AULT CYRUS FRANK, AULT, CYRUS, FRANK
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1985
Main classification: G06F1
IPC codes: G06F1, G06F12, H05K1