Infrared links between offices by JIM GREGORY

M any organizations are discover- ing a need for their own dedi- cated local communications

links, on top of those provided by conventional telephone systems. Sur- prisingly, establishing high-quality communication links between two buildings in the same town can often prove to be more difficult than setting up a similar link between sites hun- dreds of miles apart.

Current short-range transmission systems based on radio, microwave, or laser, can suffer from being diffi- cult to install and potentially hazar- dous to operate. Such systems also usually require some sort of registra- tion or licencing, which almost inevit- ably involves long and complicated approval procedures with the very real chance that permission to use the system will eventually be denied.

Cable does not suffer from these drawbacks, but it has its own prob- lems. It involves acquiring a right-of- way, purchasing the cable, contract- ing for trenching and laying of cable

Abstract: A US computer supplier has installed an infrared system for data communications between its 40 offices in one city. Infrared has installation and operational advantages over radio, microwave, cable and laser for short-range transmission.

Keywords: data processing, data communications, infrared transmission.

Jim Gregory is in the communications product division of Datapoint (UK) Ltd.

and placing repeaters along the line at 1000 ft intervals. The whole process is often slow, expensive, inconvenient and difficult to implement.

The Datapoint Corporation en- countered these problems at its head- quarters in San Antonio, Texas. As the company expanded across the city - it now occupies 40 sites - the problems of maintaining a data trans- mission system for its own ArcNet computer system became intolerably complicated. None of the systems then available provided a suitable solution to the problems, so Data- point designed its own equipment to meet its communications needs. The resulting infrared data transmitter is called LightLink.

LightLink

LightLink is capable of sending and receiving digital information, through free space, using modulated nonco- herent infrared light, at rates of more than 2.5M bit/s. The system has a useful range of about one mile - depending on ambient atmospheric conditions - and is not subject to regulations.

The LightLink system consists of five integrated packages:

@ the mechanical package, which provides a rigid dry, dust-free en- vironment for the optics and elec- tronics,

l the optics package, which performs functions similar to the antennae in a microwave system,

l the electronics package, which con- sists of all the circuitry to interface the database to the infrared trans- ceiver and to provide sensing con- trol and display of operational parameters,

l the power package, l the microprocessor firmware pack-

age, which generates and maintains all LightLink control functions.

The LightLink transceiver head unit contains all the optical and eletronic equipment, plus a mounting point.

Installation

LightLink is quite simple to install. The head unit is simply lowered onto a prepositioned pedestal and then lined up with its partner in another building. The unit may then be powered-up and fine tuned using a built-in alignment facility which monitors the signal until peak recep- tion is obtained.

Once the correct alignment is achieved, the unit is linked to the interprocessor hub and a check is made to ensure that uncorrupted data can be transferred between sites.

As with all free space optical trans-

The infrared transmitter.

26 0011-684X/85/02002~02$03.00 @ 1985 Butterworth & Co (Publishers) Ltd. data processing

communications

7’he transmitters link Datapoint’s 40 sites across San Antonio.

mission systems, LightLink is suscep- tible to signal degradation in poor weather, such as fog, snow and haze. Ii conditions impair transmission, LightLink will shut down to prevent excessive error. As soon as the condi- tions improve the link will automatic-

ally restart. In systems where short burst errors

are tolerated, LightLink may be pro- grammed to operate within those parameters.

LightLink has been specifically de-

signed and engineered to provide an efficient, low-cost method for over- coming short-range communications problems. A pair of LightLinks can be acquired for much less than any comparable method of information transfer.

Unlike microwave systems, it does

not require registration nor does it require the heavy investment of in- stalling a cable network. Its effective- ness can be judged by the fact that Datapoint use 80 pairs upon which their office automation system in 40 buildings depends.

How it works

Overall, the electronics package:

l automatically focuses the optics,

~0127 no 2 march 1985

controls the unit’s internal temper- ature and humidity,

protects the unit’s optics from da- mage by direct sunlight, maintains the optimum signal level, transmits the unit’s status to its partner, monitors and displays its own and its partners status, arbitrates the main channel data to and from the unit,

compensates for changes in the optics resulting from changes in temperature.

The receiver preamp photodiode is an avalanche diode which produces a current that is proportional to the intensity of light incident upon it. The avalanche diode gain varies with temperature and bias voltage. Auto- matic temperature tracking, incor- porated into the power supply, pro- vides constant gain by varying the bias voltage with temperature.

Because the input (infrared light) and the output (analogue composite data) are both at low level, the re- ceiver preamp is shielded from radi- ated noise generated by the optics stepper motors, solenoids and heat- ers.

The preamp output passes through the main amplifier (data/clock detec- tor) to raise the signal to a level that ensures accurate data and clock re- covery. The received signal is condi- tioned by removing all high-fre- quency components (above 10.7 Mz). To eliminate high-frequency noise this band-limited signal is split into two signals: main channel data and

auxiliary channel data. Processor-con- trolled digital attenuators develop additional automatic gain control, providing up to 40 dB of attenuation and dynamic range. Detection of sig- nal strength provides primary auto- matic gain information to the proces-

sor. Loss of phase lock is sent to the

processor to inhibit the transmission of corrupted data to the user’s system. Initial phase lock requires a signal-to- noise ratio greater than 24 dB. To

retain the phase lock the ratio must exceed 16 dB.

The optical bench holes and aligns

the transmitter and receiver optics. This includes a focusing mechanism which adjusts the focus of the trans-

mitter optics as the focal point changes with variations in tempera- ture. A similar mechanism is fitted on the receiver optics. This temperature and automatic gain control system

ensures that transmission is made at maximum power and that the opti- mum levels are maintained at the detector. It also ensures maximum dynamic range and the best signal-to- noise ratio.

Both the transmitter and receiver diodes are protected from direct sun- light by a shutter which closes when the signal level from the diode indi- cates that its damage threshold is being approached. The shutter re- mains closed for a present interval, then opens to allow transmission to resume. If the unit should lose power, the shutter drops automatically. 0

Datapoint (UK) Ltd, Datapoint House, 400 North Circular Rd, Neasden NW10 OJG.

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