Any manuals you may need for the specific process you have pertaining to Siemens 505 should be found
HERE
Basically what happens is similar to being on the Internet...each device on a network has a specific ID. They use a specific protocol, which is a specific format for transmitting and recieving data and the sending device can tell when its done sending and the recieving device can tell when its received the data.
Dont confuse protocol with the connection used, this may be RS232, RS422/485, etc, there are others.
SCADA means Supervisory Control and Data Acqusition, basically this means the program can collect data and control the plc(s) in the process. It can read and write to the plc(s) registers.
A very simple example would be a system that ran "batches" that called for different ingredients. The SCADA program could be setup to allow an operator to choose batch 1, 2, 3, etc and the process add the necessary ingredients to make a product. A SCADA system can also act as HMI and/or provid visual displays of the process.
To answer your specific question: The PLC and Computer with IFix are connected by cable using a protocol (profibus is common with Siemens) to communicate (send/recieve data). The computer can kind of be looked at as the BOSS that tells the WORKER (plc) what it wants it to do next. The BOSS also may want to KNOW what it has ALREADY done (data acquisition). Data acquisition could be a multitude of things..how much product was made, how much of a specific ingredient was used, how long machine ran production..etc etc.
Hope this helps, if you have more specific questions please ask. Others may offer even better answers.
