Dynamic Shift Registers - Bitsavers - Trailing-Edge

...... oca...U)CDc::CD~...CD...()...caca.t:.CJo ....t:.()cao...c.c.«U)ECD...U)>enCD.t:.~o~IZ«VERTICAL SCANNERS AND PRINTERSVertical scan character generators are generallyused in hard copy applications. Also, a verticalscan type of character generator can sometimes bemore suitable for CRT displays than raster scan.Displays or printouts of calculators and smallbusiness machines often show only numerals and alimited variety of symbols-not enough for a fullalphanumeric generator. Such fonts are easilyprogrammed into a small ROM such as the1024-bit MM522, which stores 128 8-bit words.There's room for 16-5 x 7 dot characters on thechip .These ROMs are also used in the SK0002 kit for a64-character ASCII-addressed font (Figures 2b and3b), which requires the storage of 3207-dotcolumns and a 7-bit address. Connected as shown,the DM8533 TTL binary counter will reset on thecount of 16. And with the gating and interconnectionsshown, the column select cycle is:Counter Outputs DCBDCB000001010011100101ROMs EnabledMK004MK005MK004MK005MK006reset (instantaneous)A CRT beam can be intensity modulated by theserialized output, as in the raster scan technique.However, the electron beam traces either a sawtoothor pedestal-type scan pattern on the screen(Figure 10). Every column of each character in thedisplay line is scanned in sequence, starting at theleft-hand side of the screen.The sawtooth scan is straightforward, but thepedestal scan requires that the bit order be reversedin the second and fourth columns. To dothis, the outputs of the M K005 ROM are simplyconnected to the output buses in the reverse order(i.e., output 1 to bus 7, output 2 to bus 6, etc.) .SAW TOOTH DISPLAYPEDESTAL DISPLAY~., I A • ----, .. ., t"' ,-----,f f:. ". • : I " : I,A)II.6l/ I • '.",: 1'llll'ill?j" I 1'1 I :.1:1/ 1;:. :: T I : I I.il II 1/ :. : : ,,:: :..... ,,' II', ~ .' I : I...L. : I..,., II II " ., I I I • I I• I , r • L .... L_J I~ ~Figure lOa. Two Techniques for Vertical ScanNEXTCHARACTERFigure lOb. Example of Character Generation UsingPedestal·type Scan.Long shift registers, operating at relatively slowrates can be used. The character rate-the registershift rate-is no more than 1/6 of the columnselectrate for a 5 x 7 font, since the beam tracesone complete character before going on the nextone. A dot counter loads spacing bits betweencharacters via the TTL shift register, a charactercounter triggers the sawtooth or pedestal scanningpatterns, and a row counter would control positioningof the beam in a page display system.I n the new single ROM (MM5241) version of thissystem, (Figure 11), a 9-bit address is needed,6 bits for the ASCII code and 3 bits for dot col-CHARACTERGENERATORZ AXISMOD.INPUTCHARACTERDATAPARALLelENABLEFigure 11. Vertical Scan Display System