Dynamic Shift Registers - Bitsavers - Trailing-Edge

l>ZI~oCHARACTER {DATA'-------It~~~~~~~==~===L--Jumn select, Since the ROM stores five dot columnsfor each of 64 characters in a 5 x 7 font, 3 decodeline are necessary. Also, the ROMs are programmeddifferently for sawtooth or pedestalscanning. Because the output pins are committedfor all columns, external connections cannotsimply be used to reverse output bit order.Hard-copy printers can use the same fonts asvertical scan CRT displays. MaS registers may beused for data input buffering, but of course refreshregisters are not generally required. The charactergenerator output may be used to select some combinationof 35 hammers, needles or electrodes thatprint the 5 x 7 dot patterns on the paper. Onetechnique for handling the character generatoroutput is shown in Figure 12.In Figure 12, a TTL counter connected to divideby six (five columns and the blank column spacebetween characters) generates the column selectaddress. The ROM's outputs are accumulated inTTL latches (or held in TTL serial-in/parallel-outshift registers). When all dots for a character areready, they are printed. In tape printing applicationsin which a 7-transducer array sequentiallyprints or punches a column at a time as the papermoves under the transducers, the ROM outputscan be used as they are generated unless storage isrequired for some other purpose.Character generators are not needed for conventionalelectromechanical typewriters. But MaSROMs do have a role here-one version of theMM521, for example, is programmed to convertthe ASCII communications code into the Selectriccode used to control ball-type printers.PRINTING APPLICATIONSThe application of character generators in aprinting application is normally quite differentfrom that of the display system. Most printersrequire that a total character font be availablebefore the print is executed. An example of apractical method of accomplishing this (Figure 12)is to sequence the character generator elementthrough the fOllt sequence. Each of the characterFigure 12. Printer Application Block Diagramcolumns or rows is addressed. The character generatoroutput data at each of these address intervalsis transferred into bipolar memory. Thismemory not only satisfies the memory storage butalso the general power buffer which is requiredbetween the MaS character generator and theelectromechanical on thermo electric printer. Inthe printer application there may be a requirementto buffer the input data with data storage becauseof the relative differences in data and printer ratesbut generally there is no need to retain the printedcharacter intell igence.The data transfer from the character generator tothe bipolar memory in Figure 12 is accomplishedby sequencing the column address lines and enablingthe appropriate memory simultaneously.Each pair of DM8550s (SN7475s) then containsthe data for one of the five columns in a character.The DM8842 (SN7442)-one in 10 decoder providesthe decoding functions which are connectedto the enable line on the quad latches.LARGER, FASTER SYSTEMSMost low cost terminal designs have been based dnthe 5 x 7 font because of the high cost of diodematrixes and wideband video circuits. But it is byno means the most legible font. A 5 x 7 font isacceptable for applications in which the displaychanges slowly, but human engineering studiesindicate that it causes severe eyestrain when anoperator reads rapidly changing data.The greatest portion of the discussion has dealtwith a 5 x 7 font. A full 64 character display canbe coded into a single MaS package. Now that LSIhas entered the scene, we see a different trendtowards larger, more stylized font. The economyof MaS ROMs will provide the customer with amore legible character font at the present cost of"discrete" character generators. An analysis of themost practical solutions to various fonts are tabulatedin Table 2. The part types which have beenused to generate a 64 x 7 x 5 raster scan font arethe SKOOOl-3 ROM kit or the MM5240 which isunder development. The vertical scan font is satisfiedby the SK0002-3 ROM bit or the MM5241l>"C"C...oD)n~r+oo~D)...D)nr+CD...C)CD::lCD....D)r+o...U)237