Mode of operation
Siemens PLT130
.
1994
2/2
Transfer control, data protection and tr ansfer
Transfer control
Sequential data transmiss ion systems require a transfer control
unit for organization of t he data traffic on the line. This unit or ga-
nizes and coordinates the data traff ic so that only one sub-
scriber transmits data on the bus at a time. This t ransfer control
function is decentralized i n the CS 275 bus system. Theavail-
ability of thedata transmission system is thereforemainly deter-
mined by the line length.
Data transfer control in the CS 275 bus system operatesaccord-
ing to the token principle, i . e. each participant (local bus i nter-
face module) accepts the transfer control function (master func-
tion) fromtime to time.
In a systemwith npar ticipants, there is only one par ticipant in
the master condition at any one ti me, all other (n--1) participants
are in the slave condition. The change of the m aster function
from one participant to another i s called master transfer. Si nce
every participant can assume the master functi on, this function
has n--timesredundancy in a t oken system.
The ”bus protocol” also specifi es how a participant becomes
masterand how this status is transferredwithin the system. The
protocol complementary methods for the ass ignment of the
master function to a parti cipant are:
Time-out-controlled master transfer
Request-controlled master tr ansfer
Instruction-controlled mas ter transfer
Data protection
Data protection is achieved in the CS 275 bus system by gener-
ating line and column parities and is referred to as block parity.
The following errors are detected by t he data protection:
1-bit,2-bit and 3-bit errors,
Hamming distance d=4
Odd bit errors, 1-, 3-, 5-, 7- and 9-fo ld errors,
independent of location
Error bursts < 11 bit long
Error pattern (with except ion of rectangular format).
The bit error rate of p=10
--5
satisfies the requirements of IEC-
SC-65-WG6 for process buses. Accordi ng to this, an unde-
tected transmission er ror in a message may occur on average
once in every 1000 years (at 250 kbit/s).
Data transfer
Blocks in the TELEPERM M system components are available for a
cyclic anda sporadic (acyclic) data transfer via the bus system:
Dedicated transmitter and receiv er blocks handle all functions
associated with data transf er.Thus, the data transfer can be
established easily by conf iguring, just as the automati on func-
tions.
The user need only carry out t he following to produce commu-
nication:
-- The inputs of the transmit ter block must be provided with the
addressesof the data to be transmitted
-- The transmitter block mus t be inserted into the processi ng
sequence
-- The outputs of the corresponding receiver block must be
linked accordingly in another automat ion system
-- The receiver block must be inser ted into the processing
sequence
-- The connection to the transmitt er must be established using a
coupling instruction.
This presents the following advantages:
Easy configuring of communicati on (only the transmitter and
receiver blocks must be configured, not the inter face modules
for 20-m local bus)
Messages to several subscriber s aresi mply transmitted on
the bus (several receiver blocks can access one transmi tter
block)
Symbolic addressing (block name and number )
Complete documentation of all connections by self-documen-
tationof systems
The communication procedure can be configured in a still
simpler manner in full-gr aphic mode on the screen by means
of the configuring tool PROGRAF and can also be docum en-
ted in the same representation (see page 2/ 5).
Cyclicdata transfer
Analog values are transmitted cy clically with the coupling
blocks AKS/AKE, binary values with the BKS/BKE blocks and
characters with the ZKS/ZKE blocks .
There are also further functions i n addition to the cyclic data
transfer.
These functions include image updating i n an OS operation and
monitoringsystem. The updating time is < 2 s in the TELEPERM
M process control system. In orderto guarantee this value,the
bussystem provides a function ”Read parameters”. This en-
ables up to 30 different parameters per message to be read
fromone AS system. The functional sequence is as follows:
TheOS system sends the requestto the bus system to read the
corresponding parametersfrom several ASsystems for image
updating. The addresses of these paramet ers have already
been converted to on-line-access ible parameters from the sym -
bolic addresses of the configurat ion during the system start-up.
Thus, the bus interface modul es can read the desired informa-
tionout of the automationsystem memory by DMA, generate
corresponding reply messages and retur n these to the corre-
spondingOS systems.
Fig.2/1 Cyclic data transfer, example OS/AS and AS/AS
Created with "Chameleon CMYK PDF Creator" for Wolfgang Strozyk at Siemens AG
This catalog is out of date, see note on page 2