The 3964R procedure controls the data transfer of a point-to-point link between the CPU
and a communication partner. The procedure adds control characters to the message
data during data transfer. These control characters may be used by the communication
partner to verify the complete and error free receipt.
The procedure employs the following control characters:
n
STX:
S
tart of
T
e
x
t
n
DLE:
D
ata
L
ink
E
scape
n
ETX:
E
nd of
T
e
x
t
n
BCC:
B
lock
C
heck
C
haracter
n
NAK:
N
egative
A
c
k
nowledge
You may transfer a maximum of 255byte per message.
Procedure
When a DLE is transferred as part of the information it is repeated to dis-
tinguish between data characters and DLE control characters that are
used to establish and to terminate the connection (DLE duplication). The
DLE duplication is reversed in the receiving station.
The 3964R procedure requires that a lower priority is assigned to the
communication partner. When communication partners issue simulta-
neous send commands, the station with the lower priority will delay its
send command.
The USS protocol (
U
niverselle
s
erielle
S
chnittstelle = universal serial interface) is a serial
transfer protocol defined by Siemens for the drive and system components. This allows to
build-up a serial bus connection between a superordinated master and several slave sys-
tems. The USS protocol enables a time cyclic telegram traffic by presetting a fix telegram
length.
The following features characterize the USS protocol:
n
Multi point connection
n
Master slave access procedure
n
Single master system
n
Max. 32 participants
n
Simple and secure telegram frame
3964
USS
System 300S
+
Deployment PtP communication
Protocols and procedures
HB140 | CPU | 315-4PN23 | en | 18-02
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