Can I substitute 2 (4 x 12bits) analog output cards for 1 (8 x 12 bits) card?

Hi, I'm new to PLC's, Can anyone please tell me whether I can substitute one siemens (8 x 12bits) analog output cards with two siemens (4 x 12bits) analog output cards and if possible how do I get by the slot addressing issue. Thanks.

The short answer is yes. The implementation will depend upon what you have, what you want to do, and what you are willing to do.

This explanation assumes you are working with an S7-300 or 400 (I am not sure about the S7-200s), and you have the STEP 7 Hardware Configuration software, and that you are willing to change the configuration.

Since we do not know your hardware configuration, let us assume you have the following. Also assume you let the system select the addresses, so you probably have the addresses as indicated.
Slot 4 - SM 332 Analog Output 8 channel (Addresses 256 - 271)
Slot 5 - SM 321 Digital Input 16 channel (Addresses 4 - 5)
Slot 6 - SM 322 Digital Output 16 channel (Addresses 8 - 9)

So, you replace the 8 channel analog card in slot 4 with a 4 channel analog output card in the same slot and put a second 4 channel analog output card in slot 7. The system may select the following addresses automatically.
Slot 4 - SM 332 Analog Output 4 channel (Addresses 256 - 263)
Slot 5 - SM 321 Digital Input 16 channel (Addresses 4 - 5)
Slot 6 - SM 322 Digital Output 16 channel (Addresses 8 - 9)
Slot 7 - SM 332 Analog Output 4 channel (Addresses 304 - 311)

But, the addresses for the second analog card do not match the addresses you had for the last four channels in the original card. You can change the addresses for the new second analog card in the Hardware Configuration utility. Open the properties for the second card and select the Addresses tab. Deselect (uncheck) the System Selection box and then enter a new Start address. In this case, select 264. Now your addresses for your two 4 channel analog cards are the same as those on the original 8 channel analog card.

As an alternative to changing the system addresses, if your addresses have symbolic names and those symbolic names are used in your program, you could readdress your symbolic names in the symbol table.

Also, in this example, we could move the digital cards from slots 5 and 6 to slots 6 and 7 and then put the second analog card in slot 5. The two analog card are next to each other and rewiring may be easier.

There are a number of ways of doing this.

If this is not what you had in mind, then maybe you could explain what you want with more details.

If you are using a S7-300 or 400, download the Configuring Hardware and Communication Connections STEP 7 V5.3 Manual from this link if you do not have it. (It should have installed on your PC with the STEP 7 installation. From the Windows Start Menu -> Programs -> Simatic -> Documentation -> your language -> Step 7 Configuring Hardware) Chapters 1 and 2 explain rack and module configuration.

Oops, that link did not work. Enter "Configuring Hardware" as search words and you should get a link to the proper manual.