Dynamic Shift Registers - Bitsavers - Trailing-Edge

fo....ca ...CDCCDC!J...CD...Co)...caca~(.)s~Co)caec.c.c:(enE! en>enCD~I-o~IZc:(which is under development. If we examine theother possible fonts, these same two monolithicelements will satisfy the requirements if they were64 x a x 5 and 64 x 6 x a respectively. Therefore,the added memory storage is being incorporatedinto the MM5240 and MM5241. In some of thesecases the font is scanned in the horizontal dimensionwhile in others the font is scanned in thevertical dimension. You find both the a x 5 and6 x a elements capabie of satisfying the fontmatrix requirement. Since all the ROMs listed arestatic by design, there are no special clocking hard·ships induced with the solution of any of theselarger fonts. This is not true for all dynamic ROMsolutions.As mentioned before and shown in the table, thesame ROM element is used in both raster scan orvertical scan applications. If we recall the designsolutions showing the refresh memory and charactergenerator for a 5 x 7 display, the first thingwhich is apparent is that the sequencing of thecharacter generator is different in each of the twobasic techniques. In one case the character genera·tor is sequenced at the character rate (raster scan)while in the other case the generator element issequenced at the column rate (vertical scan) of thefont.Since a display utilizing the vertical scan techniqueshas input address changes at some multipleof the display character rate, a clocking system fora dynamic ROM character generator must besupplied. This requires the addition of a frequencydivider and clock generator which results in ahigher system cost when dynamic ROMs are used.A second consideration which should not be overlookedin systems cost is the compatibility ofROMs in mUlti-package character fonts. OptimumROM usage and organization will result in lowersystems cost. ROMs will also find applications inmicro-programming and code conversion wheresynchronous operation is preferred.The a x 10 font is much better and 12 x 16 isalmost optimum for legibility. Small, lower casecharacters can be sharply defined, too, and theyalmost appear to be drawn with continuousstrokes.System designers considering these fonts for lowcostdisplays run, at present, into CRT cost problems.The least expensive displays are televisiontypeCRTs with limited video bandwidth. Bandwidthalso limits the number of characters that canbe displayed simultaneously. Not counting thetimes required for beam retrace and functionsother than character generation, which reduce thetime available in a refresh cycle for dot handling,the necessary bandwidth is roughly:BW = (dots and spacing bits per character)X (characters per display row or page)X (refresh rate)TV·type CRTs have a maximum bandwidth ofabout 4 MHz, of which only about 2.5 MHz isgenerally useful. If one uses a 5 x 7 font with onespacing bit (6 x 7 total) at a 60-Hz refresh rate,each displayed character needs 2.52 kHz of bandwidth,so the limit is about 1,000 characters. Incontrast, the new ROMs take as little as 700 nano·seconds to generate a dot line, or about 5 fJ.S percharacter. That's fast enough to generate 200,000characters a second, or a display of more than3,000 characters at the 60-Hz refresh rate. Theactual dot rate in the serial bit stream to the CRTcan approach 10 MHz. And if larger fonts aregenerated in some multiplexed addressing mode,the required bandwidth can be much higher.Luckily, these problems are not insurmountableand there are alternatives to using oscilloscopequalityCRTs or storage tubes, which are fine forhigh performance applications but too rich forlow cost terminals.Obviously, the designer can drop the refresh rates.New CRTs with longer persistence phosphorsfacilitate this. Also, CRT manufacturers have beenresponding to the new terminal market by workingon bandwidth improvements, and they are appar·ently going to reach 10 MHz in moderately pricedvideo systems soon.Finally, the designer is not obligeq to display hischaracters digitally just because he uses a MaSROM. Don't forget that the ROM is really workingas a code converter, generating a 35-bit machinelanguage code from a communications code. Thelanguage translation can be whatever the situationrequires.All 'that need be done is update methods used inanalog displays, wh ich form characters withstrokes rather than dot lines or columns. TheROMs can be programmed such that the bit outputs,when integrated, control X and Y rampgenerators. The slopes of the ramp functions aredetermined by the number of bits in a sequenceand the lengths are determined by the locationschosen for turn-off bits. As in the vertical scantechnique, the ROM is addressed at the characterrate.Even though some characters can be formed withone or two strokes (I, L, etc.),.equal time shouldbe given to all characters in a page display to keepthe character rows aligned. A standard sized areaof the MOSFET array, such as 6 x a or 5 x ashould be used for each character. Most patternswould thus be a combination of stroke and nostrokeoutputs. The single-chip fonts have ana-stroke capacity for each of 64 characters whichis more legible than the standard segmented typeof instrument readout, since slant lines could begenerated wherever needed.238