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Theory of Operation- 4g4Ll4g4ApService,Vot. l

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U1055	cEN ON F/F U1041A

GEN ON

R/O

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GENERATOR RUNN!NG u1041

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Figure7-18. Characteron/of timlng.

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Charactercounters synchronizethe horizontaland verticalscan with the Z-Axis signalfrom the character g€neratorlC to draw the character. These counters, U2022,U2018,U2026,and U2014,divide by g for the columnswithina character(columnsA, B, Ci, divideby

8 for the rows within a chaiacterlrows A, e, C1,OiuiO. by 40 for the characterswithina iine lcharactersA, B, C, D, E, F), and divide by 16 lor the lines within a display. The countersare enabledonly when the gen- eratorhas controlof the crt beam(GEil RUNNING-lineasserted)and INCR {increment)ls htgh (whenINCR ls low,the crt beamis stoppedto writeJOoi on the crt).

The SKIP line from V2OS2permits software control of the allowablestatesof line counterU2014. By plac- ing a one in this bit of a character,the line count€ris allowedto count up to the next state.This will continue until a characteris €ncounteredwith ths skip bit set to zero. This allowsthe additionof a third line to the nor- mal two-linereadoutfor statusmessages,by operating with the circuit normailyin the 16-linemode(allbut the bottom and top lines start with a readout characterof

40hex,which has the SKIPline set high). Thus,all but th€ bottomand top linesare skipped. Whenlargemes- sagesare to be displayed,the SKlp line is set low for all charactersand16 linesaredisplayed.

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