Bid an �,SCII y o r enc der Here is what you need to couple the key­ board you built in February to a computer, teletype, or teaching machine. by DON LANCASTER THE LOW-COST KEYBOARD, LIKE THE code is not desirable, the same en­ turning carriages, clearing, line feeds, majority of other typewriter style key­ coder may be used, through suitable bell ringing, and other control func­ boards, provides only a single "make" rewiring, to generate ESDIC, SELECTRIC, tions. A final 32-code sequences are contact for each key depressed (see BAUDOT or MORSE codes. Parity and reserved for lower case alphabet and Radio-Electronics, February 1973). the lOO-wpm (words-per-minute) serial some little used punctuation.This last Computer terminals, teaching ma­ converter are easily added to the basic group is very seldom used as most chines. etc., cannot directly use a encoder. computer communications can be han­ single-contact operation, and a device dled with only capital letters, numer­ called an encoder must be placed be­ What is the ASCII code? als, control commands, and common tween the keyboard and the computer. Many years ago, the American punctuation. The encoder converts the single con­ Standards Association decided to The complete code appears in tact closure into a seven- or eight-bit adopt a standard code that computers Table II. It is arranged in a matrix IC logic compatible parallel code. usu­ could use to talk to each other, to form to make it compact and easy to ally following the ASCII encoding their input/output devices, and to al­ read. For instance, the transparent scheme. and allowing for shift and low standardized connections between command "Carriage Return", or "CR" control key operations. After parallel different brands of computer ma­ has a code of 000-110 1, starting wi th encoding. there may follow a parity chinery. The resultant industry wide bit 7 on the left and going to bit I generator' for error detection, and a code is called ASCII, short for A meri­ (the least significant) on the right. A 100-word-per-minute parallel to serial can Standard Code for Information numeric "6" has the code Oil-OlIO. converter that allows the signals to be Interchange. This code is a sequence Note the right half of this code is the sent down a single wire or phone line. of six. seven, or eight bits (ones or same as a binary or a binary-coded­ The keyboard encoder described zeros).It may be sent either in serial decimal six. A capital L has the code here costs only a tiny fraction of com­ (bit by bit, least significant bit first) 100-1100, while the lower case L is a mercial equivalents. It uses three "dol­ form, or in parallel (all bits at once, 110-1100. lar" integrated circuits and a small on 6, 7, or 8 lines) form.Usually, par­ There are several ways to use the handful of surplus computer diodes. allel words are used inside machines, code, depending on how much you While this encoder was designed as a while serial words are used between want the code to do. If we are only companion to the low-cost keyboard. machines. Serial words are obviously interested in upper-case alphabet, nu­ it may be used with any keyboard. slower, but they take far less wire and merics, and punctuation, we can use provided the make contacts are less interconnections. the middle of the code and get by than 2000 ohms when ON and pro­ The basic code consists of seven with a six-bit code, sometimes called vided that the keys do not have a bits. If we look at all possible combi­ ASCII-6. This is useful in character common ground terminal. The en­ nations of seven ones and zeros from generators and displays that do not coder generates all the codes shown in 000-0000, 000-0001 ... through to need transparent commands or lower Table I. This includes all the capital ...111-1111, we'd find a total of 128 case alphabets. Many MOS integrated letters. all the often used punctuation. different sequences. Each of these may circuits are now available that convert all numerals, and all of the trans­ be used to represent something dis­ the six-bit subset into a recognizable parent or control functions. Often tinct. 64 of these code sequences are bunch of dots on a TV screen or a used control functions such as DELETE. used for alphanumeric capital letters, line printer; these are called ASCII SPACE, LINE FEED, ESCAPE ( ALT MODE) . numbers, a blank, and punctuation.32 Character Generators. CARRIAGE RETURN, etc. are brought more sequences are used for trans­ Or, we can use all seven bits. ei­ out to separate keys. The output is parent or control commands that never ther with or without the lower case RTL. TTL, DTL and MOS com­ appear on a screen or in print.These stuff, picking up both alphanumerics patible, and a single 10-volt, 25-mA commands tell the machinery on the and control commands. This is often power source is needed. If an ASCII other end what to do-things like re- called the ASCII-7 code.Finally, if we APRIL 1973 • RADIO-ELECTRONICS 55 TABLE I OUT.,uy CODES like, we can add an eighth bit and use it for error detecting. It is ��lJed. the The output codes below are shown appear for the SHIFT or CONTROL buttons In HEXADECIMAL notatiOn to conserve depressed separatel y. All other keys. parity bit. In an even parity system, the parity bit makes the total number of space. Thus "30" is an ASCII 011-1101. whether or not they are ueed with SHIFT ones in the word even. If there are I. or output -I = 1. at""O. a:t" ' . etc.... or CONTROl. produce a Key Depressed The "Key Depressed" output does NOT output. 3, 5, or 7 ones in the word before the parity bit is added. the parity bit is a NORMAL SHIFTED CONTROL one. If there are 0, 2, 4, or 6 ones, the KEY CODE COOE CODE parity bit is a zero. This way. there is always an evell number of ones sent. 40 40 00 (null) @ At the receiving end, parity is once A 41 41 01 (soh) again tested. If an odd number of B 42 42 02 (stx) ones shows up. a mistake has been C 43 43 03 (etx) made, and the receiver can substitute a "?" or ask for the information over o 44 44 04 (eot) again. We could also use an odd par­ E 45 45 05 (eng) ity system just as well. provided both F 4� 46 06 (_ck) ends are playing the game with the G 47 47 07 (bell) same rules. This is called the ASCII-8 code. H 48 48 (1:lS) oa Seven bits are rarely sent between I 49 49 09 (ht) machines. An eighth wire or bit space J 4A 4A OA (If) is usually added, so that if parity is K 4B 4B OB (vt) added later, it doesn't take a bunch of rework. Similarly. most paper tape l 4C 4C OC (FF) punches and magnetic tape is in an M 40 40 00 (cr) eight-track code; if parity isn't used. N 4E 4E OE (SO) the eighth bit is usually all ones. 4F 4F OF (6i) o You may wonder what all those transparent commands stand for. Ac­ 50 10 (die) P 50 tually, a lot of them are only used on 51 51 11 (DC1) Q very big and very complicated ma­ 52 52 12 (DC2) R chines, and thus aren't really very S 53 53 13 (OC3) common. The ones you'll probably use are few in number. For instance. CR 54 14 (0C4) T 54 is a carriage return that starts a new U 55 55 15 (NAK) line on a typewriter. LF is the line 56 56 16 (SYN) V feed, used to skip a line. BEL rings a 57 57 17 (ETB) W bell or signals an operator. BS is backspace. It can only be used In 18 (CAN) x 58 58 some systems. for many CRT termi­ 59 19 (EM) y 59 nals and teletypes cannot back up. Z 5A 5A 1A (SUB) The direct control commands are la­ beled DCI .. DC2, DC3 . and DC4. 30 20 (lipace) 10 (die) o These are usually yours to do any­ 1 31 21 (I) 11 (DC1) thing you want with, such as turning 2 32 22 (") 12 (OC2) on and off equipment, remote signall­ 3 33 23 (#) 13 (0C3) ing, etc. NUL is a do-nothing com­ mand that everything sits in while not 4 34 24 (a) 14 (0C4) 5 35 25 ( ..) 15 (NAK) 6 36 26 (I) . t6 (SYN) 7 37 27 (') 17 (ETB) ASCII ENCODER PARTS LIST 8 36 28 (0 18 (CAN) R1-390 ohms. '4 watt (sets operating force- 9 39 29 ) 19 (EM) 0 see text) R2, R3, R4-100,OOO ohms, 'I, watt 3A 2A (.) 1A (SUB) RS, R7, R10, R1S-4700 ohms, 'I, watt 3B 28 (+) 1B (ESC) RS, RS, R9, R11. R12, R13, R14-9100 ohms. Yo watt S% R1S-470 ohms, 'I, watt < 2C 3C (.) . OC (ft) RH, R1S, R19-100 ohms. Yo watt 20 3D (-) 00 (cr) R20-2200 ohms, Yo watt > 2E 3E (.) OE (80) R21, R22, R23-10,OOO ohms, Yo watt C1-0.1-JlF disc ceramic capacitor ? 2F 3F (/) OF (ai) C2, C3-100-"F 1OV electrolytic J 01 thru 02S-1N914 or similar silicon com- 5E 5E 1E (ra) puter diode SF 6F lF (US) 027-1N473S or similar S.S,V Zener diode IC1, IC2.