Siemens SIMATIC S5-115U, User Manual

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Communications Capabilities S5-115U Manual
Sending and Receiving Frames
• Each character whose value is 10
H
(DLE) is transmitted twice in succession so that the receiver
does not interpret it as the control character for connection buildup. The receiver enters only
one of the two characters in its Receive buffer.
• The receiver monitors the time that elapses between transmission of two consecutive
characters. If it exceeds the specified character delay time (ZVZ), the receiver sends a NAK and
waits the amount of time defined in word 6 of the parameter list for the frame to be
retransmitted. (Character delay time: Word 3 in the parameter list).
• The following occurs when the receiver's Receive buffer is full before the transmitter has
initiated a connection cleardown:
-
receiving continues until the connection has been cleared down
- the receiver subsequently transmits the NAK control character
- the error is flagged in the CBR.
• If the receiver sends a NAK character to the transmitter while transmission is in progress, the
transmitter aborts the data transfer and retransmits the entire frame, beginning with the first
character.
• If the receiver sends a character other than NAK while transmission is in progress, the
transmitter ignores it and continues its transmission
• The receiver reacts to a transmission error (character lost, bad frame, parity error) as follows:
-
Reception continues until the connection is cleared down
- NAK is then transmitted
- If an attempt to transmit is still possible (word 8 of the parameter list), the receiver waits
for the frame to be retried. How long the receiver waits depends on the frame delay time
(word 6 in the parameter list).
The receiver aborts transmission and reports an error in CBR
-
if the data block could not be received at the last send attempt
or
- if the sender does not start sending within the block waiting time.
• The "BREAK" signal causes the transmitter to
-
abort the current transmission
- send NAK
- flag an error in the CBS.
• If a message frame is not accepted (no positive acknowledgement) after the set number of
tries to erect or tries to send, the sender responds by sending a NAK.
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EWA 4NEB 811 6130-02b