In this section, an oscilloscope is used to troubleshoot a fixed-function logic circuit when a device output is connected to several device inputs. Also, an example of signal tracing and waveform analysis methods is presented using a scope or logic analyzer for locating a fault in a combinational logic circuit.
After completing this section, you should be able to u Define a circuit node u Use an oscilloscope to find a faulty circuit node
u Use an oscilloscope to find an open input or output
u Use an oscilloscope to find a shorted input or output
u Discuss how to use an oscilloscope or a logic analyzer for signal tracing in a combinational logic circuit
In a combinational logic circuit, the output of a driving device may be connected to two or more load devices as shown in Figure 5–44.
The interconnecting paths share a common electrical point known as a node.
- 驱动设备的输出可能连接到两个或多个负载设备,如图5-44所示。这些相互连接的路径共享一个被称为节点的电气连接点。
- 驱动设备在图5-44中驱动节点,其他设备代表连接到节点的负载。一个驱动设备可以驱动多个负载设备的输入,直到其指定的扇出限制。在这种情况下可能会出现几种类型的故障,而且有些故障模式因为所有连接到节点的设备都受影响而难以定位到单个坏设备。
常见的故障类型包括:
- 驱动设备中的开路输出(Open output):这种故障会导致所有负载设备丢失信号。
- 负载设备中的短路输入(Open input):这种故障不会影响其他任何连接到该节点的设备,但会导致信号输出丢失。
- 驱动设备中的短路输出(Shorted output):这种故障可能会导致节点卡在低电平(短路至地)或高电平(短路至Vcc)。
- 负载设备中的短路输入(Shorted input):这种故障也可能会导致节点卡在低电平(短路至地)或高电平(短路至Vcc)。