PCS 7 box question

Hello

-How many I/Os /digital and analog/ could I connect to a PCS 7 Box. Is there a limmitation?
-As I inderstood I can display 250 variables only on the Scada / it does not matter if they are a bit, byte, word or dword /. Is that right?
-When it is better to choose a PCS7 than a PLC system?

Regards

1. 16 kbyte I, 16 kbyte 0. But really less: 8 kbyte IO on DP, 2 kbyte 0 on MPI/DP.
2. Yes, but 250 is the quantity of external variables (in PLC memory) which are connected to SCADA. Quantity of internal variables is limited to 8k. If you want more, you must either use dword and row tags, or to buy the power pack.
3. PCS7 is more expensive. But all depends from IO quantity. PCS7 has own libraries - 1-st advantage; after creating object in Step7 it automatically is linked with WinCC - 2-nd advantage. If you have more than 500 IO, you can think about PCS7.

What internal variable does mean?

Internal means variable which isn't linked to PLC memory directly but is calculated in SCADA.

Thank you very much Gambrinus.

Hello Gromit:
This is from a PCS7 presentation document for version 6.0.


We've regularly more than 2500 tags in each PCS7 "boxes" in our last project.
PCS7 is an integrated plant-wide distributed control system; it is Siemens' "DCS on PLCs". It uses high-end S7-400 CPUs and hardware, Profibus-DP for field connectiosn, Industrial Ethernet for plant wide communications (between CPUs and PCs). It requires the use of special engineering languages CFC/SFC, not included in standard programming packages like Step 7 Basis or Professional. For these programming environments, there is a large library of functions, including digital and analog signal treatment, math, communications, PIDs... There are SCL-based techniques to create user-defined fuctions that can be included in the library.
The OS (operating stations) stations are based on Siemens' WinCC, but with extended functionalities, such as linkage with the program functions. Say you create a function for motors in your program; you will expect input(Start, stop, Speed reference) and output (Status, Alarms, Speed feedback) parameters; you will also associate the function with a graphic symbol from the image library in WinCC. When you compile the program, it will generate code for the PLC, as expected; but it will also generate a screen in WinCC, with the selected symbol and alarm displays, for example.
basically, treat PCS7 as a DCS. Use it where a DCS is required, for plant-wide operations, multiple local loops control, and distinct areas to be covered.
The last project we used PCS7 was for an aluminium plant, covering processes as dfferent as smelters, anode-forming and cooking, transport of ingots and anodes, silo monitoring and communications.
For a single (or a few number of) machine, it is a huge investment.
Hope this helps,
Daniel Chartier