Siemens SIMATIC S5 Series Manual page 51

S5-95U, Second Serial Interface Computer Link with 3964(R) Transmission Protocol
Protocol data
The 3964 and 3964R transmission protocols control data flow between the programmable controller
and a communication partner.
The data to be sent must be written into the send buffer of the S5-95U. This data is sent together
with the 3964 or 3964R transmission protocol to the communication partner. If necessary, the send
procedure is repeated by the transmission protocol. Errors that cannot be eliminated are entered in a
coordination byte.
Data received from the communication partner connected are written in receive buffers. If the data
was received without error, it can be fetched by the CPU for further processing.
The 3964 and 3964R transmission protocols are asynchronous, bit-serial transmission procedures.
All parameter settings must be identical in both communication partners with the exception of
priority.
Control characters and net data are transmitted via the interface lines. To make sure that every
character can be recognized by the receiver and transmission can be monitored for errors, the
transmitted data is preceded/followed by additional bits. The character frame is parameterized in
DB1 by means of "DF" (data format).
Example of a character frame:
SA 0 1 2 3 4 5 6 7 PA SO
Stop bit
Start bit Information bit No. Parity bit
Figure 3-10 Example of an 11-Bit Character Frame
In the 3964R transmission protocol, a block check character (BCC) is sent at the end of each
data block. The block check character is the even longitudinal parity (XOR gating of all data bytes)
of a block that is sent or received. Parity generation starts with the first net data byte (1st byte of the
frame) after connection buildup and ends with the DLE ETX character after connection cleardown.
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EWA 4NEB 812 6095-02