Siemens SIMATIC S5 CP 523 Manual page 108

Communications Mode CP 523
7.1.1 3964 and 3964R Protocols (relevant in 3964(R) mode only)
Protocol data
The 3964 and 3964R protocols control the flow of data between your programmable controller
and the partner in the communications link.
The data to be transmitted must be entered in the CP 523's output buffer, and is then forwarded
to the partner in the link together with the 3964 or 3964R protocol. The line protocol retries the
transmission where necessary; fatal errors are flagged in the coordination byte.
Data coming from the parameter in the link is entered in input buffers. If the data is received
without error, it can be fetched by the CPU for post-processing.
The 3964 and 3964R protocols are asynchronous, bit-serial transmission procedures. All parameters
on the module and on the partner in the communications link, except for the priority, must be
identical.
Control information and useful data are transmitted over the connecting cables. In order to
enable the receiver to recognize each character and to be able to check for error-free trans-
mission, additional bits are prefixed or appended to each character transmitted. The character
frame is initialized in the parameter block.
Sample character frame:
SA 0 1 2 3 4 5 6 7 PA SO
Stop bit
Start bit Data bit no. Parity bit
Figure 7-1. Example of an 11-Bit Character Frame
When the 3964R protocol is used, a block check character (BCC) is transmitted at the end of each
data frame to increase transmission reliability and ensure data integrity. The BCC is the even
longitudinal parity (EXORing of all data bits) of the frame transmitted or received. Its generation
begins with the first useful data byte (1st byte in the frame) following connection buildup, and
ends on connection cleardown after the DLE ETX character.
Sending
In order to build up a connection, the CP transmits the STX control character (start of text). If the
partner in the link replies with DLE (data link escape) before time-out (QVZ)*, the protocol goes to
Send mode. If the partner responds with NAK (negative acknowledgement) or a random character
(other than DLE), or if no response comes before time-out, the connection cannot be established.
After a total of six unsuccessful attempts, the procedure is aborted and the cause of error
**
flagged in coordination byte CBS.
* Default value in 3964(R) mode: 2000 ms
** Default: other values can be entered in parameter block 7
7-2
EWA 4NEB 811 6044-02a