Host Engineering Forum
General Category => Do-more CPUs and Do-more Designer Software => Topic started by: pbw on February 26, 2020, 07:47:15 PM
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I have 12 devices that a BRX is pulling data from via HTTPCMD. I'm polling them once every second. I set it up like in the picture and it works.
Then I noticed that this command is fully asynchronous. Quoting the help file"Another consideration is that most instructions in this class have success and error status bits that are set to mark the completion of the instruction. To make sure that these instructions operate as designed, the ladder logic must stay enabled until these instructions have completed and set the state of their status bits."
Would there be anything to gain by removing the one shot? Just trying to be careful and avoid unforeseen headaches.
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"Active" here does not mean "has power flow". It means that the instruction itself must be scanned by the engine every scan until it has completed. What that means is that it can't be in a stage that you jump out of pre-maturely (because disabled stages are NOT scanned), or in a program code-block that has been EXITed pre-maturely (because EXITed programs are also not scanned).
The edge bit is for firing the instruction, but the instruction remains active as long as it takes to completely execute that instruction asynchronously across multiple scans, so the stage or code-block it is executing in must remain active.
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Thanks for clearing that up.