WO1988001459A1

WO1988001459A1 - Local area network interconnecting computer products via long telephone lines

Info

Publication number: WO1988001459A1
Application number: PCT/US1987/002099
Authority: WO
Inventors: Reese M. Jones
Original assignee: Farallon Computing
Priority date: 1986-08-22
Filing date: 1987-08-21
Publication date: 1988-02-25
Also published as: EP0289525A1; EP0289525A4; AU7914387A

Abstract

Standard interfaces of computers and peripherals are networked over telephone wires (2', 5') via a local area network (LAN) comprising a floating ground circuit. The LAN comprises modular interconnecting units and modular plugs (50) and jacks (12, 14) in combination with an isolation transformer (30) having a low interwinding capacitance and inertia. The LAN utilizes the ringing characteristic superimposed on signal pulses' leading or trailing edges for enhanced long range detectability and establishes a noise resistant droop of the pulses' leading or trailing edges. The modular LAN construction permits the use of standard 4-wire telephone line with one pair used for telephone and the other pair used for the network. The network can be connected in various topologies. The LAN system enables reliable interconnects to over 5,000 feet using in-place telephone grade wiring thereby substantially cutting the cost of installation or modification of a local area network.

Description

LOCAL AREA NETWORK INTRCONNECTING COMPUTER PRODUCT S VI A LONG

TELEPHONE LINES

FIELD OF THE INVENTION

This invention relates to microcomputer local area networks (LAN's) and modular coupling components thereof and more particularly to a LAN permiting the coupling together of computers and their peripherals through the use of common telephone lines which are unshielded multi-wire cables via modular connectors and jacks.

BACKGROUND OF THE INVENTION

In the past, it was thought that local area networks for connecting or interconnecting terminals, computers, and peripheral devices such as printers or disc drives over relatively long distances required dedicated shielded cable and amplifiers such that, without such devices wire runs of only 25 - 50 feet would reliably permit sufficient signal quality for the transmission of information, whether or not at relatively high data rates. Computers and their peripherals ineluding personal computers manufactured by IBM F, App le Computer Inc., e.g. the Macintosh^{( ™ )} and Macintosh-Plus^(™) computers and related hard disk drive and laser printer peripherals were and are networked via specialized connector units and cables provided e.g., by Apple Computer Inc. These connectors require networks, in a given local use area, to be run over separate shielded cables and to "daisy-chain" the devices to be connected together. Daisy-chaining refers to the serial connection of devices on the same cable, a party line. For operation of multiple computers sharing the same cables for communication some kind of polling or token passing protocol is required in order that the information can be transmitted, received and properly validated in the presence of other signals on the same line. Several network transmission protocols exist allowing multiple computers to communicate over a common signal carrier, a party line.

Interconnect systems for computers and their peripherals (computer products) require the ability to connect together the computer products to allow each to transfer data such as programs, files, mail, etc., from one machine to the next in a convenient manner. The data transfers are made between different types of computers with the aid of a standard file protocol for the network so that, for instance, a Macintosh^® computer and an IBM^® PC can easily share data given the right software for the network. In the past, Apple Computer Inc. had developed its "Apple Talk"^(™) brand network,, which allowed several computer products to share expensive or inf requently used peripherals such as modems, printers and mass storage devices. For instance, sharing one laser printer among several microcomputers is one of the most common usages for the AppleTalk connector network. Other uses for the network include electronic mail, sharing or transferring files, sharing mass storage devices and other peripherals.

In the past, if one were to transfer data between only two machines, one would simply utilize a compatible cable to plug the serial port of machines together and utilize speciated serial communication programs such as MocK terminal^{(™ )} or Free Term(™) to transfer the files from one machine to the next through the cable. If one wanted to connect more than two computers it was common practice to daisy-chain them with more cables and the use of Y adapters. One could even transfer data between any two computers assuming that only one computer was sending and the receiver was ready to receive the data, and also that no other transmission was already occuring on the line. State of the art systems, such as the Apple Talk system with its software protocol and its specialized cabling, keeps messages separate and makes sure they are delivered to the intended receiver and that the messages between devices do not interfere. Such systems utilize a transformer to electrically isolate each computer from the signal wires running between the various computers on the network while still allowing it to send and receive data over the wires. The rest of the transmission and reception screening protocol is embodied in executable software code.

A major problem of state of the art network systems is the perceived necessity to use shielded cable in order to preserve line quality and the quality of the transmission of the data such that breakup or loss of data would be minimized. Without amplification and other sophisticated cabling techniques specialized for each individual situation, it has been recommended that the use of standard flat multi-wire telephone type cable be limited to runs of 25 feet for use in a ground reference (non-differential) circuit. In laying out the state of the art connector type systems, special modules were utilized and special shielded cabling requiring a special plug was thought to be necessary to assure drop-out free transmissions. Additionally, j umper boxes were required where, for instance, at a given station there was no applicable computer or peripheral. Thus, the Apple Talk cabling system is a serial only system which requires sophisticated hardware and a shielded cable, along with shielded connectors in order, it was thought, to transfer error free data from one point to another at a given location.

It should be noted that typically, the telephone cable runs throughout an office exceed 1,000 feet and often are as long as 5 ,000 feet. It was therefore thought that standard telephone cables could not accommodate such long runs of cable without modem interconnection.

Other state of the art systems include coaxial cable linked systems of even greater cost and complexity, including, among others, the Xerox Corporation - licensed Ethernet^® system.

It is a principal object of the invention to provide a local area network (LAN) system and modular components therefor affording simple, effective, low cost, interconnection among similar or diverse microcomputers and peripherals and ever long running lengths, of ordinary telephone cable, often sharing a building's existing cables with the telephone system.

SUMMARY OF THE INVENTION

The present invention meets the foregoing object by provision of a LAN modular system which uses for interconnects: in-place telephone wires in conjunction as well as modular telephone connectors which are either already in place or easily installable. Rather than utilizing a special shielded system, it has been found that adequate communication among computer products (among microcomputers, per se, and/or their peripherals) can be achieved through the utilization of unshielded cable and a floating ground isolation circuit in which the impedance of the isolation transformer is minimized and appropriately coupled to, and constructed and arranged in relation to, the computer products' pulse modulation signal interface devices to achieve high detectability of signal pulses' and/or signal pulse groups' pulse edges. The system of the invention features the production of significant passed-through signal pulses with a superimposed high frequency (relative to the pulse modulation rate of the signal) ringing characteristic at the leading edges of pulse and/or of interpulse dwell ( i.e. , the trailing edge of the pulse) to enable enhanced detectability over long cable lengths notwithstanding degradation of the signal pulses per se. The ringing characteristic is originally produced in the interface transmission units of the computer products, e.g., in accordance with IEEE-RS422 standard. The LAN also comprises surge suppression and static voltage drains on each line so that the floating ground loop defined by the LAN is safe. Runs of up to 5 ,000 feet of cabling permit the system to be utilized throughout an office area without significant degradation of the signals passed between the various modules or units coupled to the line.

In addition to there being no need for the use of shielded cables or shielded connectors, the LAN of the present invention also permits the use of parallel, T, and Star network topologies as opposed to only daisy-chaining, although daisy-chaining or serial connection is possible without reconfiguring the module or the use of switches.

The transformer utilized in the LAN of the present invention is pref errably one having a 50 picofarad or less, and more preferably 10 picofarad or less, interwinding capacitance versus a 360 picofarad interwinding capacitance transformer used in the prior art common ground systems. The transformer is further of an industrial (power) grade with a large inertial mass of its former to constitute a common mode line choke, reducing line conducted interference (LCI) at 10kHz or higher, Phone systems per se operate substantially below this threshhold, typically rolled off at 5kHz or less. The typical computer signals to be imposed on the line have modulation rates of 10³ - 10⁶ baud (pulses per second). The use of only two wires to carry the network signals and wiring to be non-conflicting with the two wires used for telephone tip and ring usage in the telephone cable also permits the network to be run on the typically unused pair of wires in a building's four or more conductor telephone cabling. The telephone and network can each operate undisturbed by utilizing selected pairs of wires within the standard telephone cable. Thus, telephone interconnects that were previously made are not affected by the utilization of the other of the pair of wires and the subject module containing the isolation transformer.

Termination resistors of 20-200 preferrably 120 ohms are used in the LAN to significantly reduce electrical imbalances between the balanced pair in the phone wires line and reduce reflections at the ends of long runs and avoid phase summing collisions at T and star network connections of the LAN.

The foregoing structure has been found to provide a surprising response to the Quad driver-receiver circuits at the computer ends along the network, affording a significant. ringing to maintain raised leading (and trailing) edge corners of pulses to enhance detection sensitivity. Aside frαn pre serving this aid to detectability, the LAN of the present invention causes leading edges of pulses of signals passed through it to have an initial slower rise (droop) characteristic thereby further suppressing spurious signal or noise pickup.

The combination of the foregoing features affords increased detection reliability over substantially longer length (by orders of magnitude) networks compared to state of the art preceding this invention and enables beneficial use of instal- led public utility and private telephone network wiring. Regarding the latter feature, the relative low grade of phone network cabling compared to typical dedicated computer network-wiring is accommodated by the robust network system of the present invention. After having set up either a temporary or a permanent system, terminating resistors are utilized by adding them to the first and last wall boxes and at the center of star connections of a large multi-user installation. This is easily done and, as mentioned before, the terminating resistors are 120 ohms which provides exceptionally good matching to standard modular telephone cabling. The utilization of the termination resistor beneficially reduces noise on the network. Its use loads the terminal ends and reduces reflections which cause noise. In the present invention, utilization of a floating ground avoids questions relating to whether or not proper grounding is in fact attained in a nominally grounded installation with individual computers attached to different power circuits. Rather than requiring coaxial cables or shielded pairs, the local area network of the present invention permits util ization of standard telephone cabling over distances greater by far than previously attained in any volume usage. Running a network without grounds and/or without shielding permits longer cable lengths and the utilization of isolation transformers with less than 10 picofarad interwinding capacitance, rather than the prior art systems' 360 picofarads improves signal response.

The foregoing is explained in further detail and other objects, features, and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof, taken in conjunction with the accomparying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of the principal elements of a network circuit established in accordance with a preferred embodiment of the invention. FTG. 2 is a qualitatively illustrated pulse modulation waveform produced using the network of FIG. 1 in conjunction with typical, e.g. , quad driver-receiver, circuitry of the computer components attached to the network as shown in FIG. 3; and FIGS. 4 - 6 are diagrams and drawings of phone system inter-relation of the system of FIGS. 1-3, in a mode allowing mechanically convenient interconnection for serial continuity.

DETAILE DESCRIPATIO N OF PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown a network according to a preferred embodiment of the invention incorporating a modular subassembly 10 comprising a part 12 of an RJ-11 standard connector connectable to selected pins (2 and 5) of the "wall" portion of a phone system's portion 14 of the connector. The pins "2" and "5" are those leading to wires 2', 5' (i.e., yellow and black, respectively under longestablished U.S./AT&T standards) , which are not part of the essential tip and ring signal wires of the telephone system

(pins 3 and 4, red and green) .

The subassembly 10 further comprises a cable 20, with leads 22, 24 to a transformer 30 with windings 32, 34 about a high mass metal former 36. A further cable 40 leads to a plug 50 appropriate for the computer or peripheral, e.g., DB-9,

DIN-8, or DE-25 among others.

The system includes a line termination resistor 16, e.g., 120 ohm, limiting line reflections. The system further includes high impedance (e.g. one Megohm) resistors 26 for draining static voltages and varsitors 28 (zinc oxide resistors, e.g., Panasonic EZR-C05 Dk201) for shorting transient surges to ground and balance resistors 38 as may be appropriate for the interface circuitry attached to plug end 50 as recommended by the manufacturer of such circuitry -- typically IK each to match any of the Am 26LS29-32 Quad Driver/Receivers used in party line configuration producing (and detecting) differential signals from a balanced circuit (e.g., two wire pair) .

The transformer 36 is typically a Coilcraft^(TM) P104 -- i.e., a 1.0:1.0 turns ratio transformer with 100 - 1000 turns in each of windings 32, 34 of 20 - 30, gauge wire about an Eform closed loop former of cross-section about equal to a winding loop cross-section, affording a high electromagnetic inertial mass reducing conductive line interference. The interwinding capacitance of the transformer is typically 7pF and the voltage isolation between windings is well over 350 volts, typically 3500 V-RMS, further isolating static charges on the network. The DC resistance of each of windings 32, 34 is under an ohm, preferrably 0.2 ohms maximum. FIG. 2 shows, in portion 2A, a computer generated signal, with some ringing of its own apparent. FIG. 2B shows the down-line signal as detected after passage through two legs (and intermediate phone cabling) of a network as indicated in FIG. 1. At leading edge LE, there is a slight droop D and slower initial rise (reducing sensitivity to transient noise spikes) . At the peak of the leading edge is a pronounced ringing leading to a sharper corner (compared to prior art networks of the class involved here).

FIGS. 2C and 2D illustrates degradation of the pulse over a long (e.g. , 1000 feet) cable run without (FIG. 2C) and with (FIG. 2D) the transformer of FIG. 1. Sharpness of leading edge is apparent relative to a reference envelope RE of rectangular form, the y-axis of which is voltage offset, the x-axis being time. FIG. 3 shows party line conf iguration of a single module of the driver-receiver circuit D/R (which is replicated several times, at most three) .

FIG. 4 shows schematically the modular system's interconnection via a dual RJ-11 modular connector 12A/12E/14A and single R5-11 modular connector 12C linked via s single isolation transformer 30 to a plug 50 into a computer interface. A further modular connector male plug 14B could couple to 12B to continue a serial chain of computer products or link to a separate phone or PBX network. FIG. 5 illustrates a prototypical network and FIG. 6 is an iscmetric view of the RJ-11 connector.

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spi rit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

What is claimed is:

Claims

1. Local area computer network for coupling multiple computer equipment components, each having a bidirectional driver/receiver interface with pu lsing operation in the 5,000 to 10,000,000 Baud range, comprising: (a) means for interconnecting each such interface with a single pair balanced differential circuit of at least phone grade quality (e.g., any pair from a phone cable) , (b) isolation transformer means, within each such means for interconnecting constructed and arranged to establish an enhanced-detectability leading edge of each resultant signal pulse produced therefrom.

2. Network in accordance with claim 1 and further comprising suppression and isolation means enabling floating ground configuration of the interconnection means.

3. Network in accordance with claim 2, wherein telephone- line-standard connection configuration is employed.

4. Network with parallel T and star topology in accordance with claim 1 and further comprising means for limiting line imbalance and phase summing.

5. Network in accordance with claim 1, wherein the interwinding capacitance of the transformer is under 10 picefarads.