Timer and Counter Instructions
Chapter 5
517
Table 5.C
Counter Instructions
NOTE:
The Counter word address, XXX, is assigned to the counter Accumulated areas of the Data Table. To determine which addresses are valid
accumulated areas, the 3rd digit from the right in the word address must be even.
The word address displayed will be 3, 4, or 5 digits long depending on the Data Table Size. When entering the word address, use a leading zero if
necessary.
Keytop Symbol
Instruction Name
1770T3 Display
Description
-(CTU)-
UP COUNTER
XXX
-(CTU)-
PR YYY
AC ZZZ
Each time the rung goes TRUE, the Accumulated Value is
incremented one count. The counter will continue counting
after the Preset Value is reached. See Note.
The Accumulated Value can be reset by the CTR
instruction.
The Accumulated Value Overflow" bit is bit 14. See Note.
-(CTR)-
COUNTER RESET
XXX
-(CTR)-
PR YYY
AC ZZZ
XXX - Word address of the CTU it is resetting.
YYY - Preset Value automatically entered by the Industrial
Terminal.
ZZZ - Accumulated Value automatically entered by the
Industrial Terminal.
When the rung is TRUE, the CTU Accumulated Value and
status bits are reset to 000. See Note.
-(CTD)-
DOWN COUNTER
XXX
-(CTD)-
PR YYY
AC ZZZ
Each time the rung goes TRUE, the Accumulated Value is
decreased one count.
The Accumulated Value Underflow" bit is bit 14. The
Enable bit is bit 16. See Note.
-(SCT)-
SCAN COUNTER
XXX
-(SCT)-
PR YYY
AC ZZZ
When the rung is true, the Accumulated Value is increased
once each scan.
Execution time depends upon the type of instruction, the amount of data
operated upon and whether the instruction is true or false.
Scan time is the time required to monitor and update I/O and to execute
instructions requested by the program. The scan is performed serially. First
the I/O image tables are updated, then the user program is scanned.
Scan time can increase during scans where subroutines are executed and
decrease when jump instructions are used to skip over portions of the
program without scanning them.
Nominal scan time is 6 ms for 1K of memory. The scan time is increased
by approximately 4% or one millisecond, whichever is greater, when the
industrial terminal is connected to the processor and approximately 8% or
5.5
Scan Time and Instruction
Execution Times
5.5.1
Scan Time
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