Topic: DMD0183
Instruction Flags
Do-more Designer uses the differently colored triangles in the upper left corner (called instruction flags) to draw attention to instructions that require special consideration when using them.
- Red (Fully Asynchronous) - designates an instruction that is
dependent on a shared device to complete the operation. When the instruction
is executed it will lock the device to gain exclusive use of that device
while the instruction is being processed across multiple scans. When the
instruction is completed it will unlock the device so that the device
can be used by the next instruction that needs it. These instructions
can be configured to "Jump to a Stage" when they are done, so
if configured in this way, these instructions must be placed in a Program
(not a Task).
The concern with these instructions is including them in a section of ladder logic (Program, Task, or Stage) that ends before the instruction has completed. This will leave the shared device in a locked state, making it unavailable for use by any other instruction that needs it.
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.
- Yellow (Multi-scan) - designates an instruction that will take
two or more PLC scans to complete. These instructions depend on the status
of the instruction from the previous scan to properly function during
the current scan.
There are some instructions with inputs that need to see an OFF-to-ON transition; for example the Count input of the Counter (CNT) instruction needs to see an OFF-to-ON transition to register as a legitimate count. Instructions with edge-triggered inputs will require the same minimum of two scans to operate as expected.
There are also instructions that are power-flow driven that require multiple scans to operate properly. A good example of this type of instruction is the TMR which only accumulates time when it is enabled.
- Dark Blue (Yielding) - designates an instruction that can temporarily
suspend its processing on the current scan and resume processing at that
point on the subsequent scan, for example YIELD, NEXT, WEND,
UNTIL,
GOTO
- No Designator - designates a simple
instruction that executes completely in-line, for example Contacts, Coils,
MATH Box.
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