自动测试设备的预算-Automatic Test Equip

文章摘要：Other minor problems can crop up when using the PC parallel port. Because only a small amount of power can be drawn from the unused parallel-port output pins (no more than 10mA with the output voltage perhaps dropping as low as 3V), an external power supply may be needed. Yet a carefully designed mi

自动测试设备的预算-Automatic Test Equip,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

Other minor problems can crop up when using the PC parallel port. Because only a small amount of power can be drawn from the unused parallel-port output pins (no more than 10mA with the output voltage perhaps dropping as low as 3V), an external power supply may be needed. Yet a carefully designed micropower system can eliminate the need for this external supply. Another possible problem is that parallel-port logic levels can vary from one PC to the next. Because computer manufacturers use S, TTL, LSTTL, or CMOS output drivers within their computers, some drivers provide output levels close to 5V whereas others are nearer to 3V.

Low-budget test systems often share a computer that runs other applications, and this can lead to problems. For instance, when a computer includes a print driver, it may maintain control of the parallel port even when nothing is being printed. Because most PCs include only one parallel port, no communication with test equipment via the port is possible under these conditions. Another source of possible bus contention is software protection keys that plug into the parallel port.

Today's computers typically include enhancements that allow bidirectional communication via the parallel port. The relatively modern ECP and EPP standards allow the parallel port to automatically transfer blocks of data to and from the PC (that is, bidirectionally). Sometimes the system BIOS disables these enhancements, and sometimes computers that include these enhancements are incompatible with other computers.

When communication with the test system must be carried out with precise timing, the parallel port may not be the right choice. The periodic intervals when the main processor refreshes the PC's dynamic memory often cause the waveforms synthesized by the parallel port to be "jittery." Even worse, when using Windows, the program driving the parallel port can be interrupted periodically. Although all programmed events occur in the correct order, the exact timing of these events is not assured.

The PC Serial Port (RS-232)

The PC serial port (sometimes called the RS-232 port) provides another easy method for connecting a PC to a device under test. Like the parallel port, a serial port is available on most PCs; an interface card need not be installed. However, unlike the parallel port, which uses logic-level voltages, the serial port signals with voltages that swing both positively and negatively. The transmitter voltage levels required by the RS-232 specification are at minimum ±5V. In reality, though, voltage levels can vary from ±3V to ±30V. The logic-level variations that occur when using a parallel port don't apply to a serial port, because after receiving the RS-232 signals an RS-232 receiver provides a logic-level output close to the voltage of the supply powering the receiver (if the output is loaded lightly).

The serial port allows only one driver on each signal line, so it can connect the PC to just one device at a time. Some devices get past this restriction by using the hardware handshake lines to signal; however, this is an unorthodox technique whose description is beyond the scope of this article. Because PCs usually include only one or two serial ports and each instrument requires exclusive use of a port, a test system based on the serial port has limited expansion capability.

上一页  [1] [2] [3] [4] [5] [6] [7] [8] [9]  下一页