Siemens Simatic S7-300 Service Manual download pdf (Page 257)

Programming Examples

B.3 Example: Timer Instructions

Clock Pulse Generator

You can use a clock pulse generator or flasher relay when you need to produce a

signal that repeats periodically. A clock pulse generator is common in a signalling

system that controls the flashing of indicator lamps.

When you use the S7-300, you can implement the clock pulse generator function by

using time-driven processing in special organization blocks. The example shown in

the following statement list, however, illustrates the use of timer functions to

generate a clock pulse. The sample program shows how to implement a

freewheeling clock pulse generator by using a timer.

Statement List to Generate a Clock Pulse (pulse duty factor 1:1)

STL Explanation

U T1 //If timer T 1 has expired,

L S5T#250ms //load the time value 250 ms into T 1 and

SV T1 //start T 1 as an extended-pulse timer.

NOT //Negate (invert) the result of logic operation.

BEB //If the timer is running, end the current block.

L MB100 //If the timer has expired, load the contents of memory byte MB100,

INC 1 //increment the contents by 1,

T MB100 //and transfer the result to memory byte MB100.

Signal Check

A signal check of timer T1 produces the following result of logic operation (RLO).

250 ms

As soon as the time runs out, the timer is restarted. Because of this, the signal check

made the statement AN T1 produces a signal state of 1 only briefly.

Statement List (STL) for S7-300 and S7-400 Programming

A5E00706960-01

B-7

1 2 ... 252 253 254 255 256 257 258 259 260 261 262 ... 271 272

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Siemens Simatic S7-300 Service Manual Page 257