hi khodkar,
I can give u some of Siemens PLC system info.
Siemens
1.S5 PLC , Programming software is STEP5(now it is obsolate,but stll many applications are running,& hope will continue for another decade before retrofiting)
2.S7 PLC, 3 basic series available
a. S7-200 b.S7-300 and S7-400
a.In s7-200 CPU 221,222,224,224xp,226..are availble with a common Programming Software MicroWIN v4.0 free, no licenece required.
PC-PLC communication cable is called PC/PPI cable,or S7-200 can also communicate PC via MPI adapter. For Profibus or Ethernet you need separate module.
b.In S7-300 CPU 312,313,313c,313-2dp...many more new, MMC-Micromemory Card is required which stores complete Program on it.Very good concept of storing total Project on MMC.
c.In S7-400 CPU 412,413-1....many versions are there...very big configuration support, It just like DCS .....many more high end technical features available on this.
a common software is used for all S7-300 and S7-400 PLC CPU called Simatic Manager STEP7.I found this software is very much userfriendly.WHat ever u need it has.
for S7300 and 400 common onboard communication port MPI is used.some CPU can have Profibus/Ethernet/Serial Port on it.
HMI- two softwares r available WinCC flexible and Protool CS, both are used to develop TouchPanel, Control Panel, Text Display and can also be used to develop SCADA.
SCADA-WinCC is the very nice SCADA software.though I don't have too much afair with it, but worked on many project and found good.
Hope these information will be helpfull to you.