Timers - Siemens SIMATIC S7 System Manual

100BBasic instructions

7.2 Timers

All edge instructions use a memory bit (M_BIT) to store the previous state of the input signal
being monitored. An edge is detected by comparing the state of the input with the state of
the memory bit. If the states indicate a change of the input in the direction of interest, then an
edge is reported by writing the output TRUE. Otherwise, the output is written FALSE.
Note
Edge instructions evaluate the input and memory-bit values each time they are executed,
including the first execution. You must account for the initial states of the input and memory
bit in your program design either to allow or to avoid edge detection on the first scan.
Because the memory bit must be maintained from one execution to the next, you should use
a unique bit for each edge instruction, and you should not use this bit any other place in your
program. You should also avoid temporary memory and memory that can be affected by
other system functions, such as an I/O update. Use only M, global DB, or Static memory (in
an instance DB) for M_BIT memory assignments.
7.2 Timers
You use the timer instructions to create programmed time delays. The number of timers that
you can use in your user program is limited only by the amount of memory in the CPU. Each
timer uses a 16 byte IEC_Timer data type DB structure to store timer data that is specified at
the top of the box or coil instruction. STEP 7 automatically creates the DB when you insert
the instruction.
Table 7- 17 Timer instructions
LAD / FBD boxes LAD coils
170
SCL
"IEC_Timer_0_DB".TP(
IN:=_bool_in_,
PT:=_time_in_,
Q=>_bool_out_,
ET=>_time_out_);
"IEC_Timer_0_DB".TON (
IN:=_bool_in_,
PT:=_time_in_,
Q=>_bool_out_,
ET=>_time_out_);
"IEC_Timer_0_DB".TOF (
IN:=_bool_in_,
PT:=_time_in_,
Q=>_bool_out_,
ET=>_time_out_);
Description
The TP timer generates a pulse with a preset
width time.
The TON timer sets output Q to ON after a preset
time delay.
The TOF timer resets output Q to OFF after a
preset time delay.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05