VEGA VEGASCAN 850 Profibus DP signal output - Manual (Page 23)

Profibus DP signal output VEGASCAN 850 23
Supplement
2 Special information on
Profibus DP
Profibus DP is designed for quick data ex-
change on the sensor/actuator level. Here,
central control instruments (such as e.g.
PLC) communicate via a quick, serial con-
nection with decentralised input and output
instruments. The data exchange with these
decentralised instruments is mainly done on
a cyclical basis. The central control (Master)
reads the input information of the Slaves and
writes the output information to the Slaves.
Thereby the bus cycle time must be shorter
than the programming cycle time of the cen-
tral control, which in many applications is
approx. 10 ms.
The following diagram shows the transmis-
sion time of Profibus DP in relation to the
number of participants and the transmission
speed.
The considerable speed increase, compared
to the Profibus FMS, is particularly due to the
fact that the transmission of the input and
output data is carried out in a message cycle
by using the SRD service (Send and Receive
Data Service) of layer 2. Furthermore, the
transmission speed has been increased up
to 12 Mbit/s and minimum requirements on
the efficiency of the protocol implementation
have been set.
Diagnostic functions
The comprehensive diagnostic functions of
Profibus DP enable quick error localisation.
The diagnostic messages are transmitted via
the bus and evaluated in the Master. They
are divided into three levels:
Station-related diagnostic
Message about general operating condition
of a participant, e.g. excess temperature or
low voltage.
Module-related diagnostic
These messages indicate that within a certain
E/A subrange (e.g. 8 Bit output module) of a
participant, a diagnostic is due.
Channel-related diagnostic
Here, the reason for the error relating to an
individual input/output Bit (channel) is stated,
such as e.g. short circuit of output 7.
System configuration and instrument
types
With Profibus DP, mono or multimaster
systems are possible. A high degree of flex-
ibility for system configuration is thereby
ensured. Max. 126 instruments (Master or
Slaves) can be connected to one bus. The
bus structure enables non-reactive coupling
and decoupling of stations or step-by-step
setup of the system. Later extensions do not
influence the stations which are already in
operation.
On monomaster systems, only one Master
is active on the bus during the operating
phase of the bus system. In figure 5, the
system configuration of a monomaster sys-
tem is shown. The PLC control is the central
control component.
In multimaster operation, there are several
Masters on one bus. They either form inde-
pendent subsystems, each consisting of a
DPM1 and the respective DP slaves, or addi-
tional planning and diagnostic instruments.
Bus cycle time of a Profibus DP Mono-Master system
Frame conditions: Each Slave consists of 2 Byte input and 2
Byte output data. The min. Slave interval time is 200 micro
seconds. Tsdi = 37 bit times, Tsdr = 11 bit times
2
6
10
14
18
5 10 20 30
Bus cycle
time [ms]
500 kbit/s
1.5 Mbit/s
12 Mbit/s
DP-Slaves