Instrument Control (GPIB, Serial, VISA, IVI)

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Deterministic timing of RS422 Serial Communication

For a sensor network consisting of 50 nodes that all communicate over a dedicated RS422 interface, I am trying to synchronize the sampling of each node. As the sampling rate is rather low at 50-200Hz, my idea was to send each node a sampling command over the RS422 channel, after which all nodes will return a single sample.

 

As cards with lots of ports such as the NI PXIe-8431/16 are not equipped with FPGAs but use DMA and the host processor instead, I was wondering to what level of determinism I can send the sampling-commands to the sensors. The real-time target will be a PXI-8135RT so in terms of loop timing and determinism I think 10us of jitter is achievable. However, considering the 50 ports that are required, I don't really know if it is possible to write 50 sampling commands (in parallel?) to the transmit buffers and what kind of jitter the DMA FIFO's introduce.

 

Does anyone have experience with timing communication in RT systems with lots of serial ports?

 

And could anyone advise if these 16-port serial cards have integrated termination resistors for the RS422 receive channel? Are there common ways to add this to each channel?

 

Thanks in advance.

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Hi Rick00,

You can use the LabVIEW synchronization functions in RT. However, RT FIFOs were built for RT systems. Using the functions available in the Synchronization palette implies the inclusion of shared resources deliberately. We avoid synchronous behaviour in RT because synchronous operations require handshaking. Handshaking is always jittery since it uses an interrupt mechanism, and you can find yourself against a deadlock or a race condition if you are not careful of how you program synchronization.

The PCIe-8431/8 uses a 620 Ω bias resistor: https://www.ni.com/docs/en-US/bundle/371332f/resource/371332f.pdf

You can also add the channels using MAX: https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000x0VbCAI&l=en-US

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