Exercise 2: PROFIBUS - Universität Stuttgart

Universität Stuttgart
Institute of Industrial Automation and Software Engineering
Prof. Dr.-Ing. Dr. h.c. P. Göhner
Introduction:
Exercise 2: PROFIBUS
The PROFIBUS (PROcess FIeld BUS) was developed in the years 1987-1991 in a joint
project by several companies and research institutes. PROFIBUS was designed primarily for
fast data transmission on the actuator-sensor-level. But it also offers many powerful services,
which make it suitable for the connection of intelligent control devices in a decentralized
automation system.
Bus media access control:
The media access control (MAC) of the PROFIBUS shall fulfil two requirements: First, the
real-time data transmission between complex automation devices (active stations) and simple
decentralised process peripherals (passive stations) must be enabled. Second, for the
communication between equal automation devices which have their own intelligence, it must
be guaranteed that each station gets assigned enough time for its communication tasks.
PROFIBUS uses a deterministic MAC method. Decentralised active stations access the bus
medium according to the token-passing method, whereas a centralised master-slave method is
used subordinately for the communication between an active station and passive stations (cf.
Figure 1).
Figure 1: The PROFIBUS hierarchy
Master-Slave-Method:
In the master-slave method, an active station (the master) controls the media access by
explicitly giving a timely limited transmission right to the other stations (the slaves). This
means that the slaves cannot access the bus by themselves, but only if they receive a request
from the master. Thus, cyclical polling with a predictable response time can be easily
implemented.

Industrial Automation Systems Theory Exercise 2: Profibus 12
Token-Passing Method:
In order to achieve the coordination between the active stations (master devices), a tokenmessage
(special data frame) is passed on from one active station to the next (in order of the
ascending addresses). The active station, which possesses the token at the moment, has the
right to access the bus and to communicate with passive stations (slaves) or with other active
stations. When it has finished its communication tasks, it passes on the token to the next
station.
In order to guarantee that each station can send data in any case after a maximum period of
time, it is necessary to set a maximum period of time during which the token must have
completely passed once through the logical ring. For this purpose, the target rotation time T TR
is set during the configuration of the bus system. When a station receives the token, it
compares the target rotation time with the measured real rotation time T RR . The station is
allowed to send messages as long as the target rotation time is not exceeded. In any case the
station has the right to send one message with high priority. Figure 2 depicts the message
processing algorithm.
reception of
token
T RR = real rotation time
T TR
= target rotation time
yes
T RR
>T TR
?
no
yes
exists a message
with high priority ?
no
yes
exists a message
with high priority ?
no
process 1
message with
high priority
process
message with
high priority
yes
exists a message
with low priority ?
no
no
T RR
>T TR
?
yes
process
message with
low priority
no
T RR
>T TR
?
yes
pass on token
Structure of data frames and data services:
Figure 2: Message processing algorithm
In order to increase the security of the transmission, PROFIBUS uses the data representation
of the UART standard, i.e. one information byte is transmitted with 11 bit:
ST 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7
ST: start bit (always logical „0“)
P SP

Industrial Automation Systems Theory Exercise 2: Profibus 13
2 0 - 2 7 : information- bytes
P: parity bit
SP: stop bit (always logical „1“)
Data transmission of the PROFIBUS is based on a standardized data frame formats. They are
presented in the following:
Data frame without data field
SD1
DA
Data frame with fixed data field
. .
length
.
SD3 DA SA FC data field
. . .
FCS
(8 bytes)
Data frame with variable data field length
. . .
SD2 LE LEr SD2 DA SA FC data field FCS
. . .
(max. 246 bytes)
Token frame
SD4
DA
SA
SA
FC
FCS
ED
ED
ED
SD1...SD4: Start Delimiter
LE: Length
LEr: Length (repetition)
DA: Destination Address
SA: Source Address
FC: Frame Control
FCS: Frame Check Sequence
ED: End Delimiter
PROFIBUS uses two services to transmit the data between master and slave devices. With the
SDN service (Send Data with No acknowledge) the master can send data to the slave without
waiting for a response (cf. Figure 3a). The SRD service (Send and Request Data with reply) is
used to request data from a slave (cf. Figure 3b). The request frame can either contain data to
be send to the slave or a data frame without data field can be used if data should be polled
only.
master
slave
master
slave
data frame
request frame
response frame
immediate
reply
Figure 3a: SDN-service
Figure 3b: SRD-service
Bus length and baud rate:
The maximum value of the permissible bus length of the PROFIBUS depends on the chosen
baud rate:
baud rate [kbit/s] 9.6 – 93.75 500 1500 12000
max. bus length [m] 1200 400 200 100
(Bibliography: Klaus Bender (Hrsg.): „Profibus, Der Feldbus für die Automation“, Hanser
Verlag, 1990)