Quick review
1.2 From schematic diagram to user program
Getting started with S7-1200
Getting Started, 11/2009, A5E02486791-01
17
Diode circuits: When diodes have been used in "old" circuit diagrams,
converting them into ladder diagram terms is not an altogether simple matter.
Since diodes represent connection lines in principle but only conduct current
in one direction, a similar solution is adopted here as with the bridge circuit.
For ease of comparison with the circuit diagram, the ladder diagram is
arranged vertically again.
For example, the following circuit provides three current paths: over switch
"d", over switch "e", or over switch "f".
*
GH
DEF
I
9
*
GH
DEF
I
EF F
The current running through the diodes can flow only from "b" to "d" or from "c" to "e". When
representing this circuit in LAD, the three current paths result in three framed sub-networks.
With switches "d", "e" and "f" on the same rail as output "G", these three sub-networks are
also linked to form one network.
Change-over switches: Changeover switches should likewise not cause you any problem
when you convert a circuit diagram into LAD. In principle, you can use a normally closed
contact and a normally open contact with the same input address to convert a changeover
switch. (For the following example, the current paths are highlighted.)
'
D
E
&
'
&
D
E
DE
Changeover switch b is
divided into a normally
closed contact that is
switched in series and
contributes to the effect at
output "C", or a normally
open contact that takes
effect in parallel with "a"
and switches "D".
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