Siemens SIMATIC S5 CP 523 Manual page 112

Communications Mode
Initiation conflict
If one partner in the link replies to a line bid (STX character) from the other link partner with STX
instead of DLE or NAK, the result is an initiation conflict, i.e. both link partners want to transmit.
The low-priority partner retracts its line bid and sends DLE. The high-priority partner sends its data
as described above. Following connection cleardown, the low priority partner can send its data.
The Priority parameter is set when initializing parameter block 7 in 3964R mode. Make sure that
one link partner is assigned high priority and the other low priority.
Example for resolving an initiation conflict:
CP 523
3964(R) mode
Low priority
* BCC in 3964R mode only
7-6
STX
DLE
DLE
STX
1st character
Figure 7-4. Resolving an Initiation Conflict
Link partner
High priority
STX
1st character
.
.
nth character
DLE
ETX
BCC
*
DLE
:
:
:
CP 523
EWA 4NEB 811 6044-02a