Siemens SIMATIC S5 CP 523 Manual page 110

Communications Mode CP 523
Receiving
When no Send request is pending in 3964(R) mode, the CP waits for the link partner to establish a
connection.
If the CP receives a character (other than STX) while in the idle state, it waits until the character
delay time has expired (ZVZ) and then sends NAK.
If the CP receives an STX from the partner and the input buffer is not full, it replies with DLE.
Incoming characters are now entered in the input buffer. If two DLE characters are received in
succession, only one is entered in the input buffer.
After each character, the CP waits until the character delay (ZVZ) has elapsed. If no character is
received during this time, the CP sends a NAK to the partner in the link.
How the CP terminates reception depends on whether it is set for 3964 or 3964R mode:
3964 mode:
If the CP detects a DLE ETX string, it terminates reception and sends DLE (if the message frame was
received without error) or NAK (if it was not) to the link partner.
3964R mode:
The CP terminated reception when it receives DLE ETX BCC. The CP then compares the block check
character with the longitudinal parity it has generated internally.
If the BCC is correct and no other receive error occured, the CP transmits DLE. If the BCC is in-
correct, the CP sends NAK and waits for a retry. If the frame cannot be received without error after
a total of six attempts or if the link partner does not initiate a retry with the block (frame) delay
*
time of 4 s , the CP aborts reception.
*
* Default value: Other values can be initialized in parameter block 7
7-4
EWA 4NEB 811 6044-02a