Siemens step7 - why you need SLD 3 before loading word into AR1 as a pointer?

[plcnoob69]

I have a word in some DB which I want to load to AR1 and use as a pointer. In order to do this I need to write

L DBxy.DBW xy
SLD 3
LAR1

I understand what SLD 3 does, but can someone explain a bit more why this is necessary? And in general, when else can this SLD command be useful?

 

[Lare]

AR is pointer.
Pointer is pointing to byte address + 8 bit fields.
SLD 3 shift these 8 bits. (three first bits)


If you don't shift 3 then you point to bit field instead of full byte area.

 

[plcnoob69]

I'm not sure I understand, could you please give an example?

 

[Lare]

Bit level (No SLD 3 command)
OPN DB100
L DW 10 // (let assume that value of DB100.DB10 = 5
LAR 1 // [FONT=&quot]/Load AR1 with pointer in ACCU1 (0.5)[/FONT]
A I [AR1, P#0.0] //Check input I0.5 indirectly



Byte level
OPN DB100
L DW 10 // (let assume that value of DB100.DB10 = 5
SLD 3 // SLD 3 == multiple by 8.
LAR 1 // [FONT=&quot]/Load AR1 with pointer value (5.0)[/FONT]
A I [AR1, P#0.0] //Check input I5.0 indirectly