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USING A BREAK OUT BOX TO TROUBLESHOOT RS-232A Break Out Box is a test tool that is plugged in to the "middle" of an RS-232 link and shows the signal status on each wire. You unplug the DTE from the DCE. You then plug the DTE in to one side of the break out box. You plug a second cable from the other side of the break out box into the port on the DCE. The break out box is then "in the middle" of the cable. When a signal is asserted on an RS-232 line, a corresponding LED indicator lights up on the break out box. You get a visual indication of the status of the link.
Here you see a breakout box that has been connected between a DTE and a DCE. The original connection was simply disconnected, and the breakout box installed between the two devices. A word of caution: if you're going to put a breakout box inline with an existing RS-232 cable, don't do it while users are sending data across the link. The moment you disconnect the cable, you will break any connections that are in place across the link. If you break the link right in the middle of the critical database update then you might find that your problems become much greater than RS-232 troubleshooting!
Let's examine our imaginary break out box, as shown in the picture to the left. You should be able to apply our discussion to your real breakout box when you troubleshoot RS-232 connections. We are going to use this break out box picture to present a number of troubleshooting examples so you should become familiar with the convention used to describe the picture. TD, RD, and CTS are inactive (deasserted, false). The horizontal connecting bars between the DCE and DTE side of the box indicate that the internal link between DCE and DTE is connected. The breakout box allows the links to be opened for diagnostic purposes; as will be explained. The false condition is indicated by the fact that the light is off - the circle is FILLED IN. "Filled In = FALSE" The true condition is indicated by the fact that the light is on - the circle is NOT FILLED IN. "Not filled in = TRUE" Examine the signals shown on the breakout box. Notice that no data is flowing -- TD and RD are not active. Is the DTR operational? Yes, because the DTR light is on. Is the DCE operational in response to DTR? Yes again, the DSR light is on. Does the DTE want to transmit data? Yes, the RTS light is on. Can the DTE transmit the data? No, the DCE has not asserted CTS -- the CTS light is off. Why isn’t data flowing on the circuit? The DCE is preventing the flow of data; the DTE appears to be working properly. This is the essence of using a breakout box. Effective use of the box requires an intimate knowledge of the pinouts and signal behaviors specified for RS-232. |
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