Publication 1747-RM001G-EN-P - November 2008
7-16 Application Specific Instructions
TDF Instruction Operation
When the TDF is evaluated with a true rung state, the instruction calculates
the number of 10 µs ticks that have elapsed from the Start value to the Stop
value and places the result into the Destination location. The TDF instruction
with float addresses accurately computes the time difference between the Start
and Stop timestamps captured within 10.48575 seconds of each other
(1048575 10 µs ticks). The TDF with float addresses calculates an invalid result
if more than 10.48575 seconds have elapsed between the Start and Stop
timestamps. The TDF with integer addresses accurately computes the time
difference between the Start and Stop timestamps captured within 655.36 ms
of each other (65536 10 µs ticks). The TDF with integer address calculates an
invalid result if more than 655.36 ms have elapsed between the Start and Stop
timestamps. It is up to you to assure that the timestamps are captured within
the valid time difference range.
TIP
Measurements were calculated with both
communication channels active and no devices
connected to the processor. Worst case accuracy is
improved by shutting down an unused
communication channel.
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