AutomationTechBrian
Lifetime Supporting Member
Lately, I've been using Modbus for reading sensor values in process programming. I haven't used Modbus much, so I spent some time looking at examples of doing Modbus read/write instructions in different PLCs. One thing I've noticed is that there are different ideas about instructions *before* the Modbus instruction. I've seen:
Nothing prior to instruction... read happens every scan.
*Not* Read Error, or 1 second after a Read Error
First scan, or successful completion of read/write, or read/write error
Timed read/write... 500ms.
So I'm wondering about "best practices" for Modbus read/write instructions. Is there such a thing? Is there a reason to slow down the read/write instruction? Does it really affect scan times that much? Why would you put anything at all? Any thoughts on this? What do you do? Motion control has different considerations than process, but does it warrant different practices? This would be a concept I'd like to resolve... it's sort of a speed-bump for me as I try to think it through.
I attached a screen shot of my latest Modbus lab in CCW for an example. This is Modbus RTU on a temperature controller.
Nothing prior to instruction... read happens every scan.
*Not* Read Error, or 1 second after a Read Error
First scan, or successful completion of read/write, or read/write error
Timed read/write... 500ms.
So I'm wondering about "best practices" for Modbus read/write instructions. Is there such a thing? Is there a reason to slow down the read/write instruction? Does it really affect scan times that much? Why would you put anything at all? Any thoughts on this? What do you do? Motion control has different considerations than process, but does it warrant different practices? This would be a concept I'd like to resolve... it's sort of a speed-bump for me as I try to think it through.
I attached a screen shot of my latest Modbus lab in CCW for an example. This is Modbus RTU on a temperature controller.
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