I am planning a configuration with a Dell server running Windows connected to a NI PXIe-1085 chassis via PXIe-PCIe8398. I have a need, in my configuration, for 89 RS-422 ports! Thus, I am planning on using the NI serial driver and the NI PXIe-8431/16 which has a FIFO size of 128 bytes.
Question: Is it correct that the message length of transmitted or received RS-422 messages will not be limited to 128 bytes? My guess is that the NI Serial Driver will break up messages longer than 128 bytes into pieces which the 8431/16 FIFO can handle, and that breaking apart of the message into smaller ones will be invisible to my code which is writing/reading the COM port.
I haven't found specific information to verify this, however I believe that the limitation that could present itself if working with large messages is in the event that too many bytes are written to the board in a way that it is not possible for it to unload the content on the board through the bus to the processor, which should generally be a faster process than the rate at which data is transmitted through the Serial Bus.
If we found ourselves in a case were the sample of sizes you are writing is greater than the spaces available in the FIFO and it is unable to empty it's space it would likely result in infomation being lost and a FIFO overflow. Again I haven't specific information to confirm this and I'm unsure how likely of an scenario it is, but it would be consistent with the behavior of some of the other NI drivers.
I agree that we probably devise a situation in which the FIFO is overrun. But, I believe it is the case that if the FIFO is empty and I write 1 and only 1 message to the driver which is 129 bytes long, all 129 bytes will be written to the serial line. That is to say just because a message is longer than 128 bytes does not necessarily imply a FIFO overrun with a the resulting message being only 128 bytes long.
Clear as mud, huh?