Siemens Protocol Data Types

I've got some minimal C code that I wrote some time ago to talk to Siemens PLCs. I used it with a 1200. At the time I only hit I, M, and Q memory. I now need to go back and modify it to hit DB data block memory. I don't have the original documentation that I used to write it.

In one place I have codes for different data types.
I 0x81
M 0x83
Q 0x82

Two questions - the first is what is the code value for DB memory.
The second question - the two bytes in the packet for the DB number - do I send the low byte first or last?

Thanks to all.

Why dont you use snap7:
http://snap7.sourceforge.net/

It gives you a decent base to work with, its tested and tried by many, and its easy to work. Reading a DB will take you a couple minutes to implement.

Code for DB = 0x84


Can't answer the byte order for the DB number.



+1 for snap7 - I use it all the time.

The byte order depends on the processor, the S7-300/400 accumulator will swap the bytes, the newer 1200/1500 will not.

The byteorder of the S7 protocol is independent of the processors byteorder. The classic S7 protocol uses the network byte order for most parts, which is big endian. As you can get only access to "not optimized" datablocks with this simple old access method, even in the S7-1200/1500 the data of the payload is in big endian endian order, the same as in the 300/400 (compatibility mode).


See screenshot of a Wireshark capture of a read request reading DB101.DBX0.0 BYTE 462 (does only work on S7-400).

Thanks to all, especially Thomas.