Sunday, February 26, 2012

Digital Servo Module (DSM) Communications with the PLC

The DSM314 and DSM302 are intelligent alternative modules that work asynchronously with the CPU module of Series 90-30. Data is replaced between the DSM and a CPU automatically using memory of %AI, %AQ, %Q, and %I. A CPU PLC needs time to write and read the exchange data across the backplane of PLC with the module of DSM. The sweep list impact for a variety of possible configurations of DSM. For extra timing considerations that relate to the modules of DSM, according to the below manuals:
• GFK-1464, User’s Manual of Motion Mate DSM302 for Series 90-30 PLCs.
• GFK-1742, User’s Manual of Motion Mate DSM314 for Series 90-30 PLCs.

Standard Variations of Program Sweep
Additionally to the standard program sweep of normal execution, convinced variations can be forced or encountered. These variations, explained in the below paragraphs, can be showed and/or adjusted from the software programming.

Constant Mode of Sweep Time
Every sweep performs as fast as possible with a differing amount of time consumed each sweep in the standard program sweep. An option to this is mode of CONSTANT SWEEP TIME, where every sweep uses the same time amount. You can reach this by set up the Constant Sweep Configuration, which will then develop into the default mode of sweep, thus taking result every time PLC switches from mode of STOP to RUN. You might set a mode value of CONSTANT SWEEP TIME between 5 to 200 msecs for CPUs 311 to 341 or within 5 and 500 msecs for the 350 to 364 and 374 CPUs.

Because of variations in the time necessary for a variety of parts the PLC sweep, the stable sweep time should be set as a minimum 10 misecs higher than the sweep time that is showed on the line status when the PLC is in mode of NORMAL SWEEP. This avoids the happening of unrelated over sweep errors.

Use the mode of CONSTANT SWEEP TIME when register values or I/O points have to be polled at a stable frequency, for example in control algorithms. Another reason may be to make sure that a sure amount of time passes between the scan output and the next sweep’s scan input, allowing inputs to resolve after receiving output data from the program. The whole sweep, together with the communications windows, is finished if the stable sweep timer runs out before the sweep completes.

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