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Serial Communication Drop in Performance with Long Cable or Breakout Box Connected

Primary Software: Driver Software>>NI-Serial
Primary Software Version: 1.7
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Serial

Problem: I'm writing to several serial ports, when I run the same LabVIEW VI with a breakout box or a long cable connected to my serial board, the VI runs slower than when I have nothing connected.

Solution: Interference due to the transmission results in cross-talk over the receive lines. If the cable is long, the cross-talk can be significant enough that the serial driver interrupts the processor thinking that it has received a signal. These interrupts make your VI run slower.

You need to connect the unused receive lines to ground (pin 5) or to a driven line with a known state (i.e. RTS), and the problem should go away.

If using the 16-port serial card with a breakout box, each of the unused ports can be connected for loop back communication and the slow transmission will be eliminated.

Related Links: Serial Communication General Concepts


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Report Date: 07/12/2002
Last Updated: 06/29/2004
Document ID: 2NBH82YY

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