[editor : Carl T. Helmers Jr.] [publisher : Virginia Londoner, Gordon R Williamson] [art : Holly Carmen LaBossiere, Deborah Porter] #Magazine
#Abstract
On this month's cover, Robert Tinney has created a visual fantasy on a communications theme. Imagine a network of personal computers where each person's computer is a node. Each node can display some information about the network. The fantasy cover painting shows several such personal computers in a matrix of translucent network connections. A few message packets are in transit down gossamer conduits, and each computer shows a view of the network from that node's vantage point. As noted in this month's editorial, the real-world equivalent of this fantasy is the telephone network with low-speed modem equipment. While 300 bps is not the data communications equivalent of the bandwidth of a light beam, it is a good start which exists today. The nodes we know about via modems and telephones consist of our personalized directories of public access and private computer systems.
[author : Carl Helmers] #Edito
Extract : « [...] Communications is the theme of this issue of BYTE and our emphasis is on extensions to the scale of this technology. We are talking about telephone networks with personal computers attached. The medium is the telephone network, and computers are the tools. We offer a number of articles this month covering areas as diverse as the technology of modems to their use in new forms of electronic-publication services for personal-computer users. In future months, readers will see more articles on communications applications of small computers. [...] »
TABLE OF CONTENTS
Construction from precalibrated modules that eliminate the need for complicated adjustments makes this modem a practical project for the homebrewing hobbyist.
[author : Ronald G Parsons] #Electronic #Networks #Listing #Assembly
Extract : « [...] To transmit data at reasonable speeds over a telephone line, a modem is used to convert digital signals to an analog form for transmission over the telephone network. "Modem" is a hybrid of the words modulator and demodulator. A modem must be used because the telephone network was designed for analog voice transmission and not for digital data. The telephone network has an audio bandwidth of approximately 3000 Hz, so the modem must condition the signals to fit within this bandwidth. [...] »
Along with a discussion of the theory of serial I/O ports, here's a design for an economical RS-232C interface that is compatible with standard TRS-80 software.
[author : Steve Ciarcia] #Electronic #Interface #Printer
Extract : « [...] The presentation this month of a serial interface for the TRS-80 required a little more thought. Parallel ports are strictly hardware devices which in their simplest form only require execution of a single assembly-language or BASIC instruction to function efficiently. A serial interface, on the other hand, needs a software program to direct its operation. The many registers and buffers involved in the serial communication process must be synchronized by the execution of a serial-driver routine stored in memory. Any design for a serial port has to take into account the capabilities and memory location of this routine. Even the most splendid hardware circuit would be a failure if the software driver interfered with other computer functions. [...] »
Using predefined variable names, you can generate proper Z80 machine-language code.
[author : William T Powers] #Method #Programming #Assembly
Extract : « If you have a Z80-based machine and an 8080 assembler, you are at a crossroad. You can do one of three things: dispose of your old assembler and purchase a full Z80 assembler; restrict your coding to the subset of the Z80 machine language that is equivalent to the 8080 machine language; or hand-assemble the non-8080 instructions within your Z80 source code. This article details a method I have devised that allows me to assemble all the Z80 instructions using an 8080 assembler without resorting to hand-assembling.
[...]
The Z80 microprocessor has a number of powerful instructions and instruction modes that are not on the 8080. I devised the method presented in this article to enable me to use these instructions without having to buy a Z80 assembler. I hope you have found this approach as useful as I have. »
With proper transmission lines, extra terminals can make your personal computer flexible and easy to access from many locations.
[author : Mark R Tichener] #Electronic #Networks
Extract : « With the prices of microcomputer components becoming ever more attractive, the temptation to sprinkle terminals and peripherals throughout the house is becoming more difficult to resist. Since a computer is more flexible than a telephone, it's not unreasonable to have a bedside terminal (that wakes you in the morning and reminds you of your appointment with the dentist), a terminal in the study for serious work, another remote terminal in the den for the kids to play with safely, and the main system residing in the basement workshop.
The simultaneous and independent transmission of signals in opposing directions through a single line, as discussed here, has been done for years in communications systems (such as telephone links). I have not seen it applied to remote terminals or processors, so I present the idea along with some obvious applications. [...] »
This minimal hardware/software system shows that running multiple users on microcomputers is a simpler task to implement than most think.
[author : Don Kinzer] #Electronic #Algorithm #Listing #Assembly #OperatingSystem
Extract : « Now that the personal computer has become firmly established, many users are developing an appetite for more complex and sophisticated systems. Disk-based systems, quite rare among users several years ago, are now commonplace.
Among the concepts being investigated and implemented by advanced experimenters are: real-time operating systems; multiprocessor systems (eg: resource sharing); advanced disk-operating systems; multitasking systems; parallel processing; and time-sharing/multi-user systems. Indeed, almost every feature of large computer systems is being considered for implementation on microcomputers. This article will explain some of the problems, techniques, advantages, and disadvantages of time-sharing/multi-user microprocessor systems. For the most part, the techniques are applicable to all currently popular microprocessors (eg: 6800, 8080, 6502). However, the exact implementation and circuitry required may differ depending upon the microprocessor.
The impetus for time-sharing or multi-user systems is to allow for more efficient use of processor time and to allow several people to share the processor. A microprocessor can do only one thing at a time: the trick is to make it appear to be doing more than one thing simultaneously. In most home computer applications, the processor is input/output (I/O) bound — that is, the processor spends much of its time waiting for I/O. [...] »
Automatic telephone dialing can be done by two diverse methods.
[author : John Renbarger] #Electronic #Algorithm #Listing #Assembly #Telecom
Extract : « This article describes an application of computers to personal control of communication facilities. I have added some simple hardware to my KIM-1 microcomputer and have successfully dialed local and long-distance numbers on my home telephone. Although I made use of an expanded KIM to develop the programs listed here, the final program and data tables fit into the standard KIM memory.
In the form presented here, the system accepts a single telephone number from the KIM keypad, dials it, and stores it for redialing. In my system, only one number is stored in the computer at a time, but the routines could be used by a supervisory program to select and dial from a list of several stored numbers.
Two methods of dialing are available. One method uses pulses to control a solenoid that interrupts the telephone connection. The other method, which is faster, generates dual-tone, multiple-frequency signals that are acoustically coupled to the telephone receiver. [...] »
Peripherals that were once dedicated to a single computer can now be shared by using this communications scheme.
[author : Rod Hallen] #Listing #Assembly #Interface
Extract : « Business computers communicate with each other all of the time. This is true whether they are located in adjacent rooms or halfway around the world from each other. You may ask, "Why can't personal computers communicate in the same manner?" Well, they can, and an intercomputer communications scheme is not really that difficult to implement.
I have owned as many as five personal computers at one time, but presently I'm concentrating on my Cromemco Z-2 and Radio Shack TRS-80. My Z-2 is used for word-processing and assembly-language program development; I bought the TRS-80 because it is the most popular machine on the market, and I want to write about the hardware and software for a large number of readers.
The Z-2 supports two ThinkerToys DISCUS 8-inch floppy disks, a fast Malibu 160 line printer, and a lot of other S-100 hardware, using software oriented to the CP/M operating system. The TRS-80 is a 16 K Level II machine with only one peripheral. That peripheral device is the Z-2, and therein lies my story. [...] »
Some microcomputers can use the indexed-sequential access method, known as ISAM, instead of random access or sequential access.
[author : Reginald D Gates] #DataStructure #DataManagement
Extract : « More and more microcomputer systems are advertised as featuring ISAM files. The indexed-sequential access method (ISAM) permits rapid access to large amounts of data and is well suited to disk storage. However, ISAM does have some disadvantages. This article is intended to enable the personal-computer user to understand what ISAM is, how it works, and how to tell if the indexed-sequential access method is appropriate for a given application.
A brief look at two other access methods will be a helpful prelude to describing ISAM. Sequential access is the most common method for reading files, and it is easily understood. Records of a sequential file are accessed one after another in the order in which they are physically stored. The records are located adjacent to each other on the storage device.
In the random-access method, records are read or written via a unique key associated with each record. This key translates into a physical address — that is, the address in the storage device that contains both the specified key and its associated data. Here, the records are not necessarily located next to each other; they tend to be scattered over the storage area. Figure 1 shows the same file of three entries stored in both a random and a sequential manner.
The major problem with sequential files is speed. To obtain the one hundredth item from a sequential file, it is necessary to first read the preceding ninety-nine records. If the program makes a lot of unordered accesses to a sequential file, the response will be slow since the preceding records have to be read for each entry that is obtained. Events in the real world typically occur in an unordered manner.
This means that the slow response time of a sequential file often precludes its use in real-time systems. On the other hand, the advantage of the random-access file is speed. If the key of a record is known, we know exactly where to look for it. The programs can obtain any record in a random-access file with just one input/output (I/O) operation. [...] »
Decreasing costs and increasing availability of telecommunication facilities for microcomputers imply modes of communication vastly different from the ones we use today.
[author : James A Levin] #Telecom #SocialNetworks #Book
Extract : « We are in the midst of a major change in the ways that we communicate. This change will affect many areas of our lives — the ways we are informed, educated, and entertained; the ways we interact with friends, organizations, and the world. New communication media are arising from the grass roots as personal computers become widespread and are interconnected. These media allow new possibilities for interactive, personalized communication, so I will call them interpersonalized media.
Already there are small-scale efforts to interconnect personal computers via telephone lines. There are several national personal-computer networks and many local computer "bulletin board" systems, five in the San Diego area alone. These developments will lead to such radically modified institutions as personalized news, classroomless education, and interactive soap operas. In this article, I will focus on the influence of these new media on the interchange of information that constitutes news. [...] »
This is a Nim-like game in which players try to pick numbers that will add up to 15.
[author : John Rheinstein] #Listing #BASIC #GameBoard
Extract : « The game of Fifteen is a two player game. The players alternate picking numbers between 1 and 9, using each number only once. The object is to select numbers such that the sum of three of them is 15, and at the same time to prevent the opposing player from achieving a sum of 15 with three numbers. For example, assume that the two players are A and B. [...] »
#Book
A Race-Car Monitoring Program
Computing Time Between Dates