BUILD Improved ASCII Encoder by DON LANCASTER Simplified circuit lets you convert virtually any surplus keyboard into the proper cod e to talk to a computer or the Radio-Electr onics TV Typewriter THE ORIGINAL RADIO-ELECTRONICS ASCII switches, contacts, or resistive elastomeric The complete ASC II code is shown in keyboard encoder pads. For low impedance contacts, you Table I. While there are 128 possible entries April. 1973) was designed(Radio-Electro to convertnics, the need a single +5-voIt supply. For resistive to the code, we are only interested in the single-make contacts of the Low Cost contacts up to 1000 ohms, you need a sec­ first six columns of these. Columns 0 and I Keyboard February, ond + 12-voIt supply at low current. are the transparent or machine commands. 1973) into the(Radio-Electronics, proper ACSII computer code In its present form, it will encode all the Things that don't end up in print or as part for talking to either a computer or the TV upper case alphabet, most punctuation, all of a message, but instead return carriages, Typewriter September the numbers, a spacebar, a carriage return, a start and stop equipment, indicate begin­ 73). Here's a(Radio-Electronics, greatly improved version of shift key to shift from numbers to punctua­ nings and endings, and so on. Column 2 is the same circuit. tion, and a control key for transparent or the most popular punctuation, while column It's much smaller, uses far fewer parts, machine commands. Additional keys are 3 is the numbers and more punctuation. In has true compatible outputs, provides easily added if needed. A slight modification this encoder, we get from column 2 to col­ an optionalTIL "there's two keys down" output of th e pre-encoder matrix on the umn 3 by using a key. Thus, a capital and is designed to exactly fit one end of the low-cost keyboard (FebruaryRadio­ "I" becomes a shi}iand so on. Finally Col­ currently popular "keypunch" type of sur­ Electronics1973) is needed to use the new unit. umns 4 and 5 are !,the alphabet and some les­ plus keyboards. Costs are about the same as ser used punctuation. the original but no kits are sold. It works More on We can get by with a basic assembly of 48 with any keyboard that has one isolated pair The standardASCII computer code in use today keys less a few if we're willing to do away of spst contacts that are normally open per is called ASCII, short for American Stan­ with [.1,].1' and __. With a Control or key. The contacts may be mechanical. reed dard Code for I nformation Interchange. CTRL key, we can shift any column 2 or 4 TABLE THE INDUSTRY STANDARD ASC111 COMPUTER CODE 0 1)7 • 1 1 b8 • 00 0 1 01 0 0 1 1 1 • �� 0 ()1 0 1 0 1 1 Sils b<I b3 1)2 bl Column 0 0 1 2 3 1 ..... Row 4 5 6 7 a 0 0 0 NUL DLE SP @ P P 0' I A 0 Q 1 1 SOH DC,. 1 Q a q 0 . 0 0 1 0 2 STX DC2 2 R r 8 b 0 1 1 3 ETX DC3 3 C S c • D # 0 1 () 0 4 EOT DC4 $ 4 0 T d 1 0 5 ENQ t 0 1 NAK % 5 E U e u ACK 0 1 , 0 v 6 SYN & 6 F V r 0 1 1 1 BEL ETB - w 7 W BS 7 G g 1 0 0 0 CAN ( H X 8 HT 8 h x , 0 0 1 9 EM ) I 9 V I 0 1 10 . 'I , () LF SUB : I z Z 1 1 11 J 1 0 ESC + r { VT ; K k 1 1 0 0 12 FF FS , < "- ---- �- L I I 1 1 0 1 13 CR M - = J m } -- -- GS - 1 1 1 0 14 SO RS > n --- N 1\ 1 1 1 15 Sf US - 0 DEL I / ? 0 59 NOTE: EXTERNAL DEBOUNCE & NKRO LOCKOUT EACH KEY SHORTS SHOULD BE ADDED IN MOST SYSTEMS. WIRES UNDER IT. "ROUND" KEYS ARE PC BOARD EXTRA AND OPTIONAL. '12V' NEEDED ONLY FOR l000u CONTACTS +5V +SV KP Al -:: A2 X a:: I- « (/) ::;; A3 .... IC2 :::J a Q, a: 7402 f- « :::J 0 0 '" A4 >- w ,. AS A6 A7 KRO (OPTIONAL) GND TQPVIEW +5V NOTE: 01-04 MAY OPTIONALLY BE PLACED ON KEYBOARD. 'Rl-R2 SAVING TWO MECH CONTACT -1 K (SVI INTERCONNECT ElAST. CONTACT-3.3K 02Vl LEADS. 1PARTS LIST C1-O. H.lF disc ceramic, Mount flat. 01, D2. D3, D4-1N914 or equivalent silicon computer diode IC1-HDOI65 Encoder (Harris) IC2-7402 TTL Quad NQA gate IC3-MC789AP Hex Inverter , RTl. do not sub- stitute IC4, IC5-7400 TTL Quad NANO gate Ql, Q2-2N5139. silicon pnp Rl, R2-Varies with keyboard, 1000 ohms for mechanical contacts and +5 supply; 3300 ohms for elastomeric high resistance con­ tacts and + 12 supply. R3, R4-1000 ohms, 'I4-watt carbon MISC: PC Board, Solder; No.24 Soldereeze wire, 20 feet for keyboard wiring, sleeving, No.24 solid wire jumpers. NOTE: The following is available from Southwest Technical Pro ducts, 219 West Rhapsody, San Antonio, Texas, 78216 FIG. I-ASCII ENCODER CIRCUIT (top) is easy to build. FULL SIZE FOIL PATTER N (above) Is for the PC Board, etched and drilled: $5.75. circuit board, PARTS LAYOUT (right) shows where to mount the components on the circuit board. 60 code inJo '- column 0 code. and any column pressed, telling things on the other end that equals, greater than, and less than for 3-or 5 code into a column code.Thus, we something new is happening. We do shifted commands. This is clearly back­ need no new keys for theI control com­ delh'er a keypressed command for the NOTshift wards from the standard code. So if we are mands, unless we are really going to use that or control key, for they are always used in going to go along with the standard code command often. CARRIAGE RETURN is often conjunction with another key. And, in our (often we are forced to because of the used, so, it's handy to have a special key circuit, we get a free "there's two keys key tops on the keyboard we're going to that simullaneously gives us a CONTROL and pressed!" output that can be used to tell use), we have to arrange the shift key so that a M command. Similarly, we can get a whatever is on the other end that the typist it operates hackwards on these four keys. spacebar by simultaneously giving a SHIFT is running too fast or just made a mistake All this takes are two 211 Ie's, but this is a and a 0 command. Other special functions and please ignore what just arrived. One complex and painful little detail to resolve. DELETE. ESCAPE. ALT MODE. etc ....) are final. and slightly messy detail involves the The output of the code consists of seven easily( added in the same way. > = < and? keys. Normally, we like to bits in parallel, or all-at-once form. An To decide when a code is sent, a key­ type commas, dashes, periods, and slashes eighth parity bit can optionally be added for pressed command is given when a key is wilhoUI shifting, and save th e question, error detection, or the seventh bit can op­ tionally be dropped to get the 10-bit code that has only alphanumerics to run a charac­ ter generator. Should we want to talk to a computer or a phone line, we have to con­ vert this code to a serial form, easily done with either the circuit shown in the original article or with a new MOS terminal transmitter/receiver chip. Depending on the type of keyboard and the debouncing in the rest of the system, we may have to add a contact conditioning and debouncing system as well. About the new circuit The new circuit is shown in Fig. Except for (presently around $7.50), allI. the re­ mainingIe I parts are nickel and dime stuff, and there are only 19 components in all. Just like the code of Table we can split the problem into two parts, I,for the low er four bits TYPICAL KEYBOARD WITH ENCODER. The small encoder board Is mounted at the right end of the couldn't care less what the upper three are keyboard. doing, so long as everything ends up right. KP INPUT GOES TO 2-ADD THIS CIRCUIT for bounce-free Thus a lower four bits 1 101 code could be a GROUND IF KEY FIG.keyboard operation. can'iage return, a group seperator (a very PRESSED rare machine command), a dash or minus, an equals, a M, or a large unbracket. 1K singlehandedly takes care of the lower fourlei bits for us. has sixteen input lines and four lOOK SETS output lines.It you make any olle (only STROBE one!) input lineIf positive, it gives the binary 555 NKRO INPUT DELAY TIME equivalent to that code. Thus the third line GOES TO GROUND generates a 0011, the eighth line a 1000, and IF TWO KEYS _� so on. r- PRESSED \ ..... lOOK The inputs are RTL style and simply need +5 an impedance path to + 5 or + 12 to serve as an input command.Whatever else the input current flows through on the way to set up the upper three bits is of no concern to 2.2K OUTPUT - BRIEF so long as the current gets there whenIe it isI, PULSE 10 MSEC needed.