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Serial Port Complete: Programming and Circuits for Rs-232 and Rs-485 Links and Networks
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About Serial Port Complete: Programming And Circuits For
Product Description Explains how to design, program, and install RS-232 and RS-485 links and networks, covering topics including protocols, port architecture, and MSComm. Review Anyone who wants to build equipment and connect it to a computer will need the information covered here. It covers everything from the basic electrical and physical parameters, cables, and basic architecture to programming for PCs and microcomputers. -- Industrial & Control Systems, October 1998 Axelson's book is about as complete a reference as you'll find on serial communications. The code could save you some time; the reference material surely will. -- Embedded Systems Programming, January 1999 Despite the book's detailed technical content, the style is clear. Although it contains enough material to satisfy most hardware and software engineers who develop products containing communication ports, the book should also prove useful to serious hobbyists. Nevertheless, despite its broad appeal, Serial Port Complete is far from watered-down pablum. -- EDN, March 4, 1999 Serial Port Complete is an accurate title. This book covers everything you'd ever want to know about serial ports and network projects. And it's not all theory. This is a very practical book, full of detailed examples - including Stamps and Visual Basic. This book is a "must-have" for serious experimenters. -- Nuts & Volts, Sept '98 The part of the book I liked most comes in the section that describes how to connect serial devices. Axelson covers not only RS-232 communications but also RS-485, a serial standard that allows for party-line or multi-drop connections. You would spend a lot of time looking for all the information that the book collects in one place. If you plan to use a serial port in any sort of application, buy this book. It should go on your bookshelf alongside Axelson's earlier book, Parallel Port Complete. -- Test & Measurement World, October 1998 About the Author Jan Axelson writes about computer programming and electronic technology. Jan's books include USB Embedded Hosts, USB Complete, Serial Port Complete, and USB Mass Storage. Jan's articles have appeared in Circuit Cellar, EDN, Embedded Systems Programming, and Nuts & Volts. Excerpt. © Reprinted by permission. All rights reserved. From Chapter 6, Linking Two Devices with RS-232: Port-powered Circuits Some low-power circuits that connect to an RS-232 port don't need an external power supply. Instead, they draw all the power they need from the interface itself. The power comes from unused outputs. To meet the standard, an RS-232 driver's output must be at least 5V with a 3,000-ohm load. From this, we can use Ohm's law to deduce that each output can source at least 1.6 milliampere at 5V. In practice, most RS-232 outputs exceed the minimum, but staying within the specification will ensure that a circuit will work on any port. Using Signals as a Power Source Figure 6-7 shows ways of using RS-232 outputs as a power source. When in the On state, RTS and DTR are between +5 and +15V. To set the signals to On in Visual Basic, set MSComm's RTSEnable and DTREnable properties to True. Figure 6-7A shows a simple unregulated output. When an output is positive, it can serve as a positive voltage source. To double the output current, tie two lines together as shown, with a 1N5819 Schottky diode in each line. This prevents current from feeding back into the interface if the voltages differ. You can use any rectifier diodes, but Schottkys have a lower forward voltage than other silicon diodes. You can even use the TD line as a power source by setting the Break signal, but of course this prevents you from using the line for data. However, this technique might be useful if you're using the port as a synchronous interface, using handshaking lines for the clock and data. Regulating the Voltage Adding a high-efficiency regulator results in a steady output voltage with little wasted power. Figure 6-7B shows a regu
