Siemens SIMATIC S5-95U - Page 53

S5-95U, Second Serial Interface Computer Link with 3964(R) Transmission Protocol
Example of an error-free send procedure:
Figure 3-11 Error-Free Communication During Sending
*
BCC only in the 3964R transmission protocol
S5-95U
3964(R) driver
Communication partner
STX
DLE
1st character
nth character
DLE
ETX
BCC
*
DLE
a a a
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.
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Receiving
In the idle state, the 3964(R) driver does not process any send request and the PLC waits for the
communication partner to establish a connection.
If the PLC receives any character (except STX) in the idle state, it waits for the character delay time
to elapse and then sends a NAK character.
If the PLC receives the STX character from the communication partner and if the receive buffer is
not full, the PLC responds with DLE. Incoming characters are then written in the receive buffer. If
two successive DLE characters are received, only one of them is transferred into the receive buffer.
After each receive character, the receiver waits for the next character during the specified character
delay time. If the character delay time elapses without a character being received, the NAK
character is sent to the communication partner.
EWA 4NEB 812 6095-02 3-21