Siemens SIMATIC S5 CP 523 Manual page 109

CP 523
If the connection can be established, the useful data in the output buffer is transmitted to the
partner in the link at the specified baud rate. The partner in the link monitors the interval
between incoming characters, which may not exceed the character delay time (ZVZ)*.
Each DLE character found in the buffer is transmitted as two DLE characters (double DLE), i.e. 10
transmitted twice for each DLE character in the output buffer.
When the buffer is empty, the CP appends
DLE ETX in 3964 mode and
DLE ETX BCC in 3964R mode
as end-of-text identifier and waits for an acknowledgement. A DLE character from the partner
within the allotted time (QVZ) indicates that the data was received without error.
If the partner in the link responds with NAK, a random character, or not at all, the CP once again
transmits STX. After a total of six unsuccessful attempts
aborts the procedure, enters an error flag in the CBS, and transmits NAK to the partner in the data
link.
If the partner in the link sends a NAK character during an in-progress transmission, the CP aborts
the message frame and attempts a retry as described above. If the partner transmits another
character, the CP waits until the character delay time (ZVZ) has expired and then sends NAK to
bring the partner to the idle state before once again transmitting STX.
Example of error-free transmission:
CP 523
Communications mode 3964(R)
1) Only in 3964R mode
* Default value in 3964(R) mode: 220 ms
Value can be initialized in parameter block 7
** Default value: Other values can be initialized in parameter block 7
EWA 4NEB 811 6044-02a
STX
1st character
.
.
.
nth character
DLE
ETX
BCC
1)
Figure 7-2. Error-Free Data Interchange (Send)
Communications Mode
to transmit the message frame, the CP
**
Partner in the link
DLE
DLE
is
H
7-3