Why Does the Execution Speed of the NI 9425 I/O Node Vary Depending on Which Digital Lines are Used?Hardware: CompactRIO>>Digital Input Modules>>NI 9425
Problem: When using a NI 9425 digital input module in a CompactRIO system, the update/transfer time seems to vary depending on which digital lines are being used. For example, when reading from digital lines 0 and 1, the update time is 1 microsecond, or 40 ticks. When reading from digital lines 0 and 31, the update time is 7 microseconds, or 280 ticks. Is this expected behavior? Solution: Although this is not listed in the NI 9425 Operating Instructions this is expected behavior. The NI 9425 returns bit information through a SPI protocol and the FPGA I/O Node will only stream back as many bits as it needs to. For example, if the IO Node is reading from digital lines 0,1 and 2, it will only stream back 3 bits and will execute faster than the specified max update rate. However, if it is reading from digital lines 0,1 & 31, it will have to stream back all 32 bits which will match the max update rate. This behavior is not listed in the operating instructions because for applications using less than 8 Channels, there are similar modules like the NI 9474 or NI 9472 that can perform much faster max update rates and are better suited for these applications.
Related Links: Product Manuals: NI 9425 Operating Instructions and Specifications Attachments:
Report Date: 08/28/2008 Last Updated: 08/28/2008 Document ID: 4ORC933S |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
