线路驱动器与线路接收机不同特性的比较(英)
Line drivers, such as those for RS-232, RS-422, RS-485, and LVDS, constitute a class of integrated circuits that interface two systems or devices over some distance, including point-to point, multidrop, and bus and backplane applications. The need for such devices emerged in the early 1960s as the inherent limitations of sending high-speed signals over long distances using standard logic levels became apparent.
Today they are used not just for digital data transmission but also for control, synchronization and clock signals. In light of the advantages of connecting systems designed and built by different manufacturers, line drivers benefit greatly from the development of industry standards.
Today’s physical-layer devices offer low power consumption, low electromagnetic radiation and susceptibility, fast speed, longer-distance coverage and electrostatic discharge (ESD) robustness. This article will review the background of many commonly known standardized electrical-level interfaces and describe their relative application merits.
Logic circuits such as transistor-transistor logic (TTL) or CMOS can drive output signals on the order of 100 MHz over a few centimeters. One logic family, emitter-coupled logic (ECL)-including such derivatives as positive ECL, low-voltage PECL and negative ECL-is capable of gigahertz speeds and can cover distances on the order of meters. ECL devices are often used for line-driver-type applications but are still considered logic families.
Families of devices using logic family adaptations with ever narrower input-voltage thresholds, such as backplane-transistor logic, enhanced-transceiver logic and Gunning transceiver logic, were developed for backplane applications. These extensions offer increased speed, at a cost of increased susceptibility to noise. They are effective and efficient on some backplane applications in 19-inch rack systems.
Over time, the parallel bus schemes have required ever wider bit widths for ever increasing volumes of data. They are relatively power-hungry and require very careful channel-to-channel skew alignment. Most telecommunications switches today thus employ point-to-point mesh fabric, using such standards as Ethernet, RapidIO and PCI Express, for mass data transmission. These higher-level standards include protocol-handling schemes, reuse and reference line-driver standards. For example, PCI Express uses low-voltage differential signaling (LVDS), and Profibus uses RS-485 electrical levels.
The well-known RS-232 standard is a singled-ended standard with the advantages of low cost and simple implementation. The disadvantages are its susceptibility to noise and crosstalk, potential ground shifts, low data rates and relatively short drive distance. Also, the ground wire forms part of the system, so the receiver is prone to false triggering due to voltage transients that arise from the electromagnetic coupling of neighboring circuits or equipment such as electric motors.
