byte stream order 9871

[TheSnark]

Hello All,

I am implementing an example from NI for Modbus on the cRIo using a 9871 for comm. I am trying to find out what is the byte stream order when data is transmitted out of the 9871 ports. Is it from MSB to LSB, or LSB to MSB? Since it is not specified in the Modbus protocol, I want to make sure I am sending the request in a way the slave can understand. Thanks! 

[charris]

Hello Gaussy!

I believe that the order is determined by the value you pass to the module. On the FPGA, we'll pass an 8-bit integer to the port on the module, so we control the order by definition.

If you're looking for a good read/write example to illustrate this, I would definitely recommend the 987x Serial Loopback project in the example finder. In LabVIEW, you can open it up by going to "Help --> Find Examples." Once there, you can browse to "Hardware Input and Output --> CompactRIO --> Module Specific --> Serial."

Let me know if you have any questions. Good luck!