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System using start codeIn certain data transmission systems, every data message is preceded by astart code consisting of a character combination (in its normal or invertedform) which does not occur in the subsequent message. The start code cantherefore be used at the receiving end for ascertaining whether the modulationhas been carried out in phase position A or B. When position B is useda change-over switch in the data-handling receiver or a phase shifter in thedata transmission receiver will invert the message which follows a start codeand will thus eliminate ambiguity in reception.4. The Mechanical Construction of the SystemEquipment constructionThe design of the mechanical equipment for this data transmission systemfollows the new L M Ericsson construction principles for transmissionequipment, involving the use of miniature components in connection withthe adoption of transistor and printed-wiring techniques.The sender functions are distributed among 12 units and those of thereceiver among 17 units. Each unit contains one function, such as a filter,modulator or amplifier. To facilitate maintenance the most important unitshave been provided with test points for fault location at the front of theunit. The adjustment of bit speed, signal centre frequency and level iseffected by means of links at the front sides of the units.The power supply for the sender and the receiver is obtained from an individualor a common mains supply unit. The alarm equipment indicatesfailure of the power supply and at the receiving end also the occurrence offaulty signal levels.Shelf and bay constructionThe units are mounted in a shelf construction developed for 19" wide bays.Each shelf is designed for 12 units. A horizontal connecting strip on theshelf comprises connecting links, pads, maintenance measuring points andfuses. A panel on the right-hand side of the shelf is designed for the baycabling. The units are protected by dust covers. The horizontal connectingstrip, however, is directly accessible. The data transmission sender occupiesone shelf (fig. 7), whereas the data transmission receiver occupies 2 shelves(fig. 8).5. Technical DataMiscellaneousModulation speeds /,,Signal centreLinerequirementsfrequenciesbandwidth requiredCode requirementsfor synchronizing the clockpulse generator of the receiver1 000 bauds1 500 bauds1 100 c/s1 300 c/s1 500 c/s1 700 c/s1 900 c/sswitched or leased telephone lines withmaximum frequency drift ± 10 c/s1 600 c/sat least one 0-elementper 75 1-elements