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Snap7 1.0.0 - Reference manual
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Snap7 1.0.0 - Reference manual
Siemens communications overview
Snap7, by design, only handles Ethernet S7 Protocol communications.
Why only Ethernet ?
Having said that we are not talking about the fieldbus, but we are focusing on PC-PLC
communications, Ethernet has several advantages against Profibus/Mpi :
It’s faster, CP 1543-1 (for the newborn S71500) has a bandwidth of 1000 Mbps.
It's more simply to troubleshoot, in 50% of cases a "ping" is good enough to solve
your problems.
It’s cheaper, you don’t need a special adapter to communicate (which, moreover,
cannot not be used with any hardware and any virtual infrastructure as Snap7
does).
If you use Snap7Server, many more non-Siemens panels/scada can be connected
with your software.
Siemens PLCs, through their communication processors (CP) can communicate in
Ethernet via two protocols:
Open TCP/IP and S7 Protocol.
The first is a standard implementation of the TCP/IP protocol, it's provided mainly to
connect PLCs with non-Siemens hardware.
TCP/IP is a generic protocol, it only states how the packets must be transferred, and it
doesn't know anything about their content.
Finally, TCP/IP is stream oriented (though Siemens FC/FB needs to packetize the data
stream into blocks).
As said, it is a standard, so you don’t need of Snap7 to use it, your preferred socket
libraries are perfectly suitable.
S7 Protocol
S7 Protocol, is the backbone of the Siemens communications, its Ethernet
implementation relies on ISO TCP (RFC1006) which, by design, is block oriented.
Each block is named PDU (Protocol Data Unit), its maximum length depends on the
CP and is negotiated during the connection.
S7 Protocol is Function oriented or Command oriented, i.e. each transmission
contains a command or a reply to it.
If the size of a command doesn't fit in a PDU, then it’s split across more subsequent
PDU.
Each command consists of
A header.
A set of parameters.
A parameters data.
A data block.
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