1096 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



a particular switch is no longer a limiting factor in the size of the trunk 

 group. Once markers were invented it became possible to design systems 

 using markers to do the trunk testing and any type of switch to do the 

 connecting. When a trunk has been selected by the marker, the appro- 

 priate smtches can be operated to connect to the marked terminal. The 

 maximum size of trunk group need not be limited by the number of 

 terminals on one switch. With a primary-secondary switch array groups 

 much larger than those accessible on a single switch can be handled. 



The coordinate system was not developed for commercial use. The 

 first commercial marker system was PResident 2, a No. 1 crossbar office 

 cut into service in Brooklyn, New York, in February, 1938. Improved 

 crossbar systems have been developed since then including No. 5 cross- 

 bar and several types of toll crossbar systems 



There is an interesting sidelight on the development of crossbar sys- 

 tems. The crossbar switch was invented by J. N. Reynolds of the Western 

 Electric Company in 1913.* At that time proposed plans for using this 

 switch assumed that it would be used as a line switch. The arrangements 

 did not appear attractive and no serious attempt was made to develop a 

 commercial system using the switch either as a line switch or as a selector. 

 A number of years later an improved version of the crossbar switch was 

 developed by the Swedish telephone administration. Their plans con- 

 templated the use of the switch as a selector in a direct dial control 

 system. In 1930 W. H. Matthies of Bell Telephone Laboratories visited 

 Sweden and, impressed with the possibilities of the switch, ordered 

 samples from Sweden after his return to the United States. Work was 

 started to improve the switch and to develop a modern system around it. 

 The crossbar switch, as previously mentioned, was a small version of the 

 coordinate switch and the development of No. 1 crossbar was therefore 

 started on a plan which was based on principles used in the coordinate 

 system some of which had been successfully applied to the panel system 

 with the adoption of the decoder in 1927. 



TYPES OF COMMON CONTROL SYSTEMS 



Four basic variations have been used in systems with common con- 

 trols. These are (1) digit storage in common circuits on a decimal basis 

 and control of switches by the stored digits without translation ; (2) digit 

 storage in the common circuits on a decimal basis, fixed translation and 

 control of switches in a fixed pattern by the translated information; (3) a 

 modification of the preceding plan in which the translation can readily 



* U. S. Patent No. 1,131,734— J. N. Reynolds— issued March 16, 1915 and re- 

 issued December 26, 1916. 



