Siemens S7-1200PLC timer features - Database & Sql Blog Articles

RF cable can be customized for other specifications

The IEC format timer of the Siemens S7-1200plc is a function block. When inserting a timer instruction, it is required to create a 16-byte DB structure of the IEC_Timer data type (ie, an instance data block) to hold the relevant data. In the function block, you can create a static variable (multiple background) of the IEC_Timer data type in advance and assign it to the timer instruction.
The CPU does not allocate dedicated resources to any particular timer instruction. Each timer uses a DB structure and a continuously running internal CPU timer (which I understand is a hardware timer) to perform timing.
When the timer is started on the rising edge of the input IN of the timer instruction, the value of the continuously running internal CPU timer is copied to the element START (start value) of the DB structure allocated for the timer instruction.
This start value will remain unchanged while the timer continues to run and will be used each time the timer is updated. A timer update will be performed under the following conditions:
1) Execute a timer instruction (TP, TON, TOF or TONR);
2) The element ELAPSED (time elapsed) or the bit output Q of the timer structure is used as a parameter of other instructions, and the instruction is executed.
When the timer is updated, the above starting value is subtracted from the current value of the internal CPU timer to obtain the elapsed time ELAPSED. The ELAPSED is then compared to a preset value PT to determine the state of the bit output Q of the timer. The elements ELAPSED and Q of the DB structure of the timer are then updated. After the preset value PT is reached, the timer will not continue to accumulate the elapsed time ELAPSED.
The V11 version of STEP 7 Basic adds a timer coil command similar to the S7-300/400 compared to the V10.5 version.
It can be seen from the timing mechanism inside the timer that the timing accuracy is greatly related to the scan period of the CPU when the timer is used. The input and output parameters of the timer remain unchanged between the CPU's two update timers.
In order to verify the above conclusion, the timer instruction TP is called in FB1, and FB1 is called by using I0.1 as the calling condition in OB1. Use the watchdog table to monitor the output Q of the timer and the elapsed time ET. After starting the timer with the rising edge of the input IN, if I0.1 is 0, the FB1 is not called and the timer instruction is executed. The output Q and the timer are output. The time ET remains the same. The value of ET changes only when FB1 is called and the timer instruction is executed.