Homebrew Computer Club
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Homebrew Computer Club Robert Reiling,Editor □ Post Office Box 626, Mountain View, CA 94042Joel □ Miller, Staff Writer Typesetting, graphics and editorial services donatedLaurel by Publications,17235 Laurel Rd., Los Gatos, CA 95030 (408) 353-3609 ver, Canada V6J 2G9, telephone (604) 738-9341 for RANDOM DATA By Robert Reiling information on the location. Did you know that Mini Micro Mart publishes a newsletter? They do, and you will probably find it in teresting. Write for a copy to Maury Goldberg, Mini You will want to be at the next Club meeting Nov.Micro Mart, 1618 James Street, Syracuse, N.Y. 13203. 12, 1976. A representitive of Anderson Jacobson, Inc.Tell him you learned about the newsletter in the will be on hand to discuss interfacing of Selectric ter Homebrew Computer Club Newsletter. minals. This should really be interesting for hard copy. The U.S. Postal System managed to lose some of And...at the same meeting Marty Spergel, M&R the items scheduled for the Bulletin Board this month. Enterprises will turn on the Astral 2000. You will see, I sent several items to Joel at Laurel Publications and, according to Marty, a demonstration of Astral 2000 this time, did not keep a copy. Recovery is therefore BASIC, the most powerful package ever implemented impossible for me. Please send in your items again for in a microcomputer. A unique feature of Astral 2000next month’s Newsletter. □ BASIC is user selectable precision in the floating point package. You BASIC fans should look this sys tem over. ^ THE COMPLETE Microcomputer user and hardware buffs will want AND UTTER IDIOT’S GUIDE to attend the S-100 System symposium, 1 to 4 p.m., TO COMPUTER PROGRAMMING Nov. 20, 1976 at Diablo Valley Jr. College. The S-100 oy ¡\ewettm w I is, of course, the Altair, IMSAI, etc. bus system. The bus, its viability for commercial applications, improve The First Thing About Computers ments and products employing the bus arrangement will be discussed by prominent authorities including The first thing you probably would like to know Harry Garland, vice Chairman Stanford Electrical Engi about computers is, “How do you get information into neering Department.; George Morrow, consultant exand out of them?” The answer is, generally, eight bits perienced in S-100 design and Lee Felsenstein, moder at a time. There are many diffferent devices that have ator of Homebrew Computer Club Meeting and an exbeen either specifically designed to be used with a perienced hardware designer. Jim Warren, Editor Dr.of computer or they have been adapted to either talk to, Dobbs Journal, will participate along with manufac or listen to a computer. There are two common ways turers of S-100 systems in this symposium. A fee ofto talk to a computer: serial and parallel. If you think $5.00 will be charged for admission. about the information as only going in one direction Micro Computer News, a digest translated from for purposes of this discussion it sorts out as follows: Japanese, describes the activities of Japan’s first micro Serial transfer requires a two wire circuit that is computer club. Nine symposiums are scheduled duringeither connected or disconnected (make or break/mark October and November, each one covering a differentor space/on or off). It requires that both the transmit manufacturer’s microcomputer. Both Japanese andting and the receiving device can agree on how fast the American products are featured. Visitors to Japan maytransmission is taking place (i.e. the Baud rate). Baud contact the club at this address: rate is ten times the number of bytes being transferred Japan Microcomputer Club per second, and is simply a timing convention so every The 1st Ohkura Building, 4F thing is talking to everything else at the same rate. 2-1 Nihonbashi, Chuo-ku Serial transfer generally uses a big chip called a Tokyo,Japan “UART” (Universal Asynchronous Receiver/ Transmit The first hobbyist club in British Columbia, Canada ter) which means that once synchronized with the data is in the process of forming. At the first meeting Sept.stream it can either receive or transmit. Tho UART 8, 1976 a total of 19 people attended. Future meetingstakes the agreed upon Baud rate and divides it by some will be held the first Wednesday of each month. Connumber (say 16) and sets its own counter. It looks at tact Karl Brachhaus, 203-1625 W. 13th Ave., Vancou the incoming line and takes 16 looks at the line per bit, 1 HCCNewsletter/Vol. 2, Issue 10/October 29, 1976 decides what it probably is and stores what it thinks it This is necessary because there may be a previous probably was. Since it has to know when the bytecharacter that the UART is working on and may not begins and ends, it requires some kind of signal thatbe done with. Then: there is going to be a byte coming. The bit devoted to telling it to start is called the “start bit” and it is LDA CHAR GET THE CHARACTER IN always a disconnect/break/off so that when the line REGISTER A circuit breaks it begins to take its 16 samples of each OUT DATA PUT IT OUT TO THE DEVICE of the incoming bits, making up its mind whether the bit was a zero or a one. Now counting the start bit we The UART begins to transmit by breaking the line have: and counting to 16. Then it either makes or breaks the line, depending on whether the bit was zero or one. /ST ART/D0/D1 /D2/D3/D4/D5/D6/D7/ Then it counts to 16. Then it gets the next bit and so on until it has completed 16 counts for D7. Then it And only one more thing is required: a stop bit, whichholds the line in a connect/make/mark condition for is always a connect/make/on circuit condition. Differ16 times the predetermined value of the stop bit. ent devices require different lengths of time for the Now as you can see, we have created a line condi stop bit. Some take 1, someIV2 and some take 2. So tion that is virtually identical to the incoming message. the serial line is divided into the following stream: Parallel Data Transmission /START/ D0/D1 /D2/D3/D4/D5/D6/D7/STOP/ Since Samuel F. B. Morse and others who devel Once the UART decides that the stop bit is there, oped the first data transmission devices were terribly it indicates that it has the whole byte by signallingshort of wire and bytes hadn’t been invented yet, they “input is ready.” This flag is variously called “DAV”never thought of sending eight bits at a time. Then, on (data available), “RDA”(receiver data available) or Aug. 19, 1917, Octavius Parallel invented the line using something like it. This signal indicates the status of theeight wires. It was many years later that the technique UART and is always available to any one who asks thefound practical use with the addition of two more UART what it is doing. Since a UART can be used as wires. Their names are: either input or output, other status is also reported when requested, and we’ll get to that in a minute. 1. DO Data Bit Zero Now the incoming circuit has sent the data stream 2. D1 Data Bit One 3. D2 Data Bit Two to the UART, and it has decided on what the eight bits 4. D3 Data Bit Three were in the manner just described. Now the eight bits 5. D4 Data Bit Four received are sitting in a special register in the UART 6. D5 Data Bit Five and the DAV/RDA flag is set. Now if and only if you 7. D6 Data Bit Six 8. D7 Data Bit Seven ask for the UART’s status it will tell you that it is 9. TYG There You Go ready with the input. In code it looks like: 10. IGI I Got It CHARIN IN STATUS GET UART STATUS ANI DAV ERASE ALL BUT DAV BIT Wires 9 and 10 are called “handshake” wires and JZ CHARIN LOOP UNTIL IT COMES IN T Y G signals the receiving device that all 8 bits are now ready to read in parallel. When the receiving device has The computer wants to have the data presented topulled the 8 bits in, it signals back IGI. So parallel its data lines eight bits at a time, and the UART is transfer occurs as fast as the two devices can get the 8 capable of emptying its special register out onto thebits and signal each other that the other has done its data line eight bits at a time so the next bunch of codething. looks like: The code is almost the same, but it looks like: GETCHR IN DATA GET CHARACTER FROM UART RET RETURN WITH CHARACTER PAROUT OUT CHAN1 PUT CONTENTS OF A ON PARALLEL IN REGISTER A OUTPUT PORT MVI A.TYG GET THE FLAG Serial Transmission OUT CHAN2 PUT TYG ON HANDSHAKE LINE GOTIT IN CHAN2 GET THE HANDSHAKE LINE Now that we know what the byte looks like ANI IGI ERASE ALL OTHER BITS JZ GOTIT LOOP UNTIL HE GETS IT coming in, it is easy to figure out what the UART XRA A CLEAR THE FLAG does when it has to put a character out to some device. OUT CHAN2 The computer asks for the UART status and waits until the UART is ready to take the byte. The code is: Please note that these examples of code are not exact examples of how to set up your own input and CHAROUTIN STATUS GET THE UART STATUS output, but are instead only general outlines of the ANI TBE ERASE ALL BUT THE TRANS MITTER BUFFER EMPTY BIT activities necessary.