Siemens has several options, 1 is SFB3 pulse timer and the other is SFB43 pulse generation.
If you have Simatic S7 then goto help and look up clock memory.
AB SLC:
Free Running Clock S:4
Status Only the first 8 bits (byte value) of this word are assessed by the processor. This value is zeroed at powerup in the REM Run mode. You can use any individual bit of this byte in your user program as a 50% duty cycle clock bit. Clock rates for S:4/0 to S:4/7 are: 20, 40, 80, 160, 320, 640, 1280, and 2560 ms.The application using the bit must be evaluated at a rate more than two times faster than the clock rate of the bit. This is illustrated in the example below for SLC 5/02 and higher processors.
Micrologix:
Free Running Clock S:4
All 16 bits of this word are assessed by the controller. The value of this word is zeroed upon power up in the REM Run mode or entry into the REM Run or REM Test mode. It is incremented every 10 ms thereafter.
[Application note: You can write any value to S:4. It will begin incrementing from this value. You can use any individual bit of this word in your user program as a 50% duty cycle clock bit. Clock rates for S:4/0 to S:4/15 are: 20, 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240, 20480, 40960, 81920, 163840, 327680, and 655360 ms The application using the bit must be evaluated at a rate more than two times faster than the clock rate of the bit.
RSLogix 500 - Copyright Rockwell Software 2000, 2001