asynchronous serial 32 unit loads up to 256 devices with some hardware 4000 10M Data acquisition and control systems Ethernet serial 1024 1600 10G PC network communications IEEE-1394b FireWire 800 serial 64 300 3.2G Video, mass storage IEEE-488 GPIB parallel 15 60 8M Instrumentation I2C synchronous serial 40 18 3.4M Microcontroller communications Microwire synchronous serial 8 10 2M Microcontroller communications MIDI serial current loop 2 more with flow-through mode 50 31.5k Music, show control Parallel Printer Port parallel 2 8 with daisy-chain support 10-30 8M Printer SPI synchronous serial 8 10 2.1M Microcontroller communications USB asynchronous serial 127 16 up to 98 ft with 5 hubs 1.5M, 12M, 480M PC peripherals 3 Chapter 1 • A USB device accessed as a COM port doesn’t have to have an asynchronous serial interface. The device can have a parallel or other interface as needed to suit the application. • Wireless technologies enable transmitting serial data without cables. Limits No single interface is ideal for every purpose. Limits to asynchronous serial interfaces include these: • The computers at each end must convert between the transmitted serial data and the CPU’s parallel data bus. The conversion is usually handled automatically by hardware, however. • The specified maximum bit rate for RS-232 is 20 kbps. But many interface chips can exceed this rate, and RS-485 supports speeds of up to 10 Mbps. Communications between a PC and a USB Virtual COM ports aren’t limited by RS-232’s maximum bit rate. • Windows doesn’t promise real-time performance for serial communications. Sending or receiving data may need to wait as the operating system attends to other tasks. But the delays are normally short and are common to other interfaces on Windows systems. Embedded systems typically can control the scheduling of serial communications more precisely. System Components Communicating via serial ports requires three things: computers with serial ports, a cabl prom-electric.ru

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