Siemens S7-300 - Linking to Other Systems with FETCH;WRITE

Configuring and commissioning S7 CPs for Industrial Ethernet
Configuration Manual, 09/2013, C79000-G8976-C182-13
237
Linking to other systems with FETCH/WRITE
C
The FETCH and WRITE modes supported on ISO transport connections, TCP, and ISOon-
TCP connections can be used with any other device to access the S7 system memory areas.
To be able to use this type of access, for example for PC applications, you need to know the
PDU structure of the jobs. The required S7 or S5 headers for request and response frames
are 16 bytes long and their structure is described in this chapter.
a) Structure of WRITE frames
The meaning and values of parameters shown without values in the following table can be
found in the section "Parameter values".
WRITE request frame
WRITE acknowledgment frame
0
System ID
="S"
0
System ID
="S"
1
="5"
1
="5"
2
Length of header in bytes
=16d.
2
Length of header
=16d.
3
ID OP code
=01
3
ID OP code
=01
4
Length OP code
=03
4
Length OP code
=03
5
OP code
=03
5
OP code
=04
6
ORG field
=03
6
Ack field
=0Fh
7
Length ORG field
=08
7
Length ack field
=03
8
ORG ID
8
Error field
=No
9
DBNR
9
Empty field
=FFh
A
Start address
High byte
A
Length empty field
=07
B
Low byte
B
C
Length
High byte
C
D
Low byte
D
free
I
Empty field
=FFh.
I
F
Length empty field
=02
F
Data field up to 64 K
a) Structure of FETCH frames
The meaning and values of parameters shown without values in the following table can be
found in the section "Parameter values".
FETCH request frame
FETCH response frame
0
System ID
="S"
0
System ID
="S"
1
="5"
1
="5"
2
Length of header
=0x10
2
Length of header
=0x10