Master and slave

Hi all siemens gurus

I have some confusion in understanding concept of master and intelligient slave in s7 300/400. I tried it by reading help. I could do it, but at the end I have some confusing doubts
1. What is objective? When should use such configuration.
2. As per my knowlwdge we can share only I and Q memory. Why shoud one waste I/O memory area. If some other Data Type (i.e. M area) want to share, what to do?

Thanks in advance

Hello ggchavhan;

Why shoud one waste I/O memory area

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Well, Profibus-DP means "Process Field Bus, Distributed Periphery"; translated to everyday English, this means it is designed for remote I/O, over more extended distances. You should take it as an extension of the CPU I/O capacity; that is why the prefered area is in I and Q addresses in the slaves.

understanding concept of master and intelligient slave in s7 300/400.

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Slaves in Profibus-DP can be dedicated, as in a ET-200 M station, or intelligent, as in a drive or a CPU. "Intelligent" here means a unit that has its own processor, and can act on its own; it is configured to transfer some data to a master, often data collected from its own I/O modules but modified/manipulated by the local processor, generally.
The only way to configure communications between a master and an intelligent slave is to create an "exchange table" between the two. That means, for the slave, setting up an I/O area independant of it's own local I/O modules, which will be used eclusively for the transfer of data from/to the master. The slave's CPU will read and write from that memory area when it checks messages from/to the master. The master has to build its own area reserved to exchange with each independant slave.
Some exchange tables are inbuilt in the intelligent slaves, such as drives or instruments. For others, especially cPUs you need to build it for each project, because project requirements change.
Hope this helps,
Daniel Chartier

Thanks Daniel, Very much.
How I can exchange DBs or M memory area?