15
Receive data at the RS interface is evaluated in accordance with the configured protocol, and the data field (data area (see chap-
ter 10.2.1)) is sent to the fieldbus Master. If more characters have been received than the fieldbus block length, the trailing bytes
are truncated and an Rx Overrun is indicated. If less have been received, padding with 0 occurs. If the "Length byte" is active, the
number of received useful data items is entered there. If the, "Trigger byte" is active, this is incremented by one after each complete
reception operation at the RS interface.
10.3 Protocol: 3964(R)
The 3964 protocol is used to transfer data between two serial devices. One partner must be a high-priority partner and the other
must be a low-priority partner in order to resolve initialisation conflicts.
10.3.1 Data structure 3964R
10.3.2 Protocol definitions
The telegram format is as follows:
•
The received net data is forwarded (transparently) in both directions unchanged.
•
Attention:
The DLE-doubling is excluded from it; that means one DLE (10H) on the bus-side is sent on the RS-side twice. A
double DLE on the RS-side is only sent once to the bus-master.
•
Data blocking is not scheduled.
•
The net data length is restricted to 236 bytes per telegram.
•
Communication always runs between high-priority and low-priority communication partners.
10.3.3 Data communication
10.3.3.1 Initiation of data communication by the low-priority user
If the low-priority user also receives an STX in response to a transmitted STX, it interrupts its transmit request, reverts to Receive
mode and acknowledges the received STX with DLE.
A DLE in the data string is duplicated and included in the checksum. The BCC is computed from XORing all characters.
10.3.3.2 Conflicts
10.3.3.3 Timeout times
The timeout times are preset by the definition of the 3964R protocol and cannot be overwritten !!!
tq = acknowledgement timeout time (2 s).
The acknowledgement timeout time is started after transmission of control character STX. If no positive acknowledgement arrives
within the acknowledgement timeout time, the job is repeated (max. 2 ×). If it has not been possible to complete the job positively
after two repetitions, the high-priority device nevertheless attempts to establish contact with the low-priority partner by transmitting
STX (cycle corresponds to tq).
tz = character timeout time (200 ms)
If the 3964 R driver receives data, it monitors arrival of the individual characters within period tz. If no character is received within
the timeout time, the protocol terminates transfer. No acknowledgement is sent to the coupling partner.
Triggerbyte
(optional)
FB-Lengthbyte
(optional)
STX
DLE
ETX
BCC
Data
Fieldbus
Data
RS
STX
DLE
ETX
BCC
Data
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