Computer Link with 3964(R) Transmission Protocol
S5-95U, Second Serial Interface
The PLC terminates the receive procedure in one of the following ways depending on whether it
uses the 3964 or 3964R transmission protocol:
•
3964 transmission protocol (i.e. without BCC):
If the PLC recognizes the character sequence DLE ETX, it stops receiving and sends DLE to the
communication partner for correctly receiving a block (or NAK in case of an error).
•
3964R transmission procedure (i.e. with BCC):
If the PLC recognizes the character sequence DLE ETX BCC, it stops receiving. It compares the
block check character (BCC) received with the longitudinal parity it has generated. If the block
check character is correct and no other receive error has occurred, the PLC sends a DLE. If the
BCC is invalid, NAK is sent to the communication partner. Then the communication partner will
expect a repetition of the procedure. If the block cannot be received without any error even after
a total of 6 attempts
*
or if the communication partner does not repeat the procedure within the
block waiting period of 4 s
*
, the PLC aborts the receive procedure.
If transmission errors occur during receiving (character lost, frame error, parity check error),
receiving is continued until connection cleardown and then NAK is sent to the communication
partner. The communication partner waits for a repetition according to the procedure described
above.
Example of an error-free receive procedure:
Figure 3-12 Error-Free Communication During Receiving
* BCC only in 3964R transmission protocol
S5-95U
3964(R) driver
Communication partner
STX
1st character
nth character
DLE
ETX
BCC *
DLE
DLE
a a a
a a a
a a a
a a a
a a a
a a a
a a a
a a a
a a a
a a a
a a a
.
.
.
* Default value in DB1, other values can also be parameterized
3-22
EWA 4NEB 812 6095-02