Prodave MPI Windows - PLC S7 300 - String

[michelv]

Hello,

I'm trying to read/write String to S7 315 2 DP using the W95_S7.DLL

I've no problem with CHAR, REAL, INT, BOOL datatypes, but require information about the Siemens STRING type.

When you declare a STRING[10] in the PLC, and have a look to the hexa bytes, you find 12 bytes :
- 2 bytes : length and probably something else
- 10 bytes : then 10 characters, one character per byte.

Of course, to be able to write correctly a STRING in the Siemens structure, I need to know the structure of the 2 first bytes.

Can somebody give me that information ?

Thanks,

Michel.

[Ken M]

Michel,

This is the IEC1131 definition of a STRING data type. The first two bytes are always used to hold -

i) the maximum length of the STRING when it was declared
ii) the number of valid characters currently in the STRING

Regards,

Ken M

[michelv]

Hello,

According to your advice, I just try to understand some STRING[10] readed from the PLC and ... of course that's it.

Thanks a lot for your very very quick help,

Michel.

[nineball]

hi,
first byte is the reserved length (declaration in S7) and second is then length of the actual string.