自动测试设备的预算-Automatic Test Equip

文章摘要：When considering board layout, it's important to examine the flow of the board's ground current. Board designers sometimes err by assuming that all ground-return paths stay at zero volts. Instead, current flow through the resistance of the ground traces creates unwanted voltage drops. Partic

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

When considering board layout, it's important to examine the flow of the board's ground current. Board designers sometimes err by assuming that all ground-return paths stay at zero volts. Instead, current flow through the resistance of the ground traces creates unwanted voltage drops. Particularly troublesome are the transient switching currents from digital circuits that flow through the ground-return system. Figure 7 illustrates the errors introduced not only by the unspecified trace resistance of the ground path but also by the trace resistance of the power path. When ground-return current flows, it elevates VSS in accordance with Ohm's law (this voltage equals the ground-return current multiplied by the trace resistance). In addition, the power-supply trace suffers a voltage drop due to the DUT's load current. The op amp within Figure 7 detects these voltage losses and adjusts the power supply to compensate for them. Note that the sense leads must connect directly at the VDD and VSS pins of the DUT. Connecting leads in this manner is commonly called a "Kelvin connection."


Figure 7. The Kelvin force-sense configuration ensures voltage accuracy and stability at the VCC and VSS terminals of the device under test.

Budget twice as much time for component placement as you think you need. Do a preliminary routing, perhaps with an autorouter, and look for congested areas, adjusting component placement to reduce the congestion. As mentioned previously, highlight all high-impedance signal lines, minimize their length, and keep them away from noisy signals. Prioritize the design constraints before starting the layout. Even the best layout designers cannot feed a 100mil trace into a 0.65mm fine-pitch lead.

Software Design

When controlling the instruments within a test setup, the commands that initiate this control need to be defined. Commands that control the parallel port send data in a "bit-banged" format. Both the RS-232 serial bus and the IEEE-488 bus transmit serial strings of character data.

The IEEE-488 standard governs the bus itself, not the kinds of messages sent over the bus. Despite this, there is some agreement as to what types of messages are sent. For example, modern DVMs use nearly identical command sets, saving customers development time. However, for older or nonstandard instruments, little or no agreement exists. The Standard Commands for Programmable Instruments (SCPI) is an attempt to increase the agreement within command sets. Look at the SCPI to see if any of its commands are suitable for your instrument, otherwise you will have to invent part or all of a command language unique to your instrument. Using SCPI is preferred, however, because it enables those who use the instrument to more quickly understand its operation.

It's best to develop software in small steps and in a logical progression. For example, when developing software for the signal-conditioner/pressure-sensor tester shown in Figure 5, its designer first developed the command processor, a collection of software routines that interprets text commands and controls the hardware accordingly. In its early stages, the command processor accepted commands only from the built-in RS-232 serial port of the tester's microprocessor. Once the designer of the tester built up the command processor using this port, he developed a number of low-level subroutines, using the command processor to test them. Gradually, he added device drivers for each of the subsystems (for example, the IEEE-488 controller, the A-to-D converter, and the digital I/O), always having known working code to fall back on if there were problems.

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