Siemens CS 275 Mode of operation, Transfer control, data protection and transfer, Data protection

This catalog is out of date, see note on page 2

Mode of operation

Transfer control, data protection and transfer

Transfer control

Sequential data transmission systems require a transfer control unit for organization of the data traffic on the line. This unit orga- nizes and coordinates the data traffic so that only one sub- scriber transmits data on the bus at a time. This transfer control function is decentralized in the CS 275 bus system. The avail- ability of the data transmission system is therefore mainly deter- mined by the line length.

Data transfer control in the CS 275 bus system operates accord- ing to the token principle, i. e. each participant (local bus inter- face module) accepts the transfer control function (master func- tion) from time to time.

In a system with n participants, there is only one participant in the master condition at any one time, all other (n--1) participants are in the slave condition. The change of the master function from one participant to another is called master transfer. Since every participant can assume the master function, this function has n--times redundancy in a token system.

The ”bus protocol” also specifies how a participant becomes master and how this status is transferred within the system. The protocol complementary methods for the assignment of the master function to a participant are:

Time-out-controlled master transfer

Request-controlled master transfer Instruction-controlled master 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 the data protection:

1-bit, 2-bit and 3-bit errors,

Hamming distance d = 4

Odd bit errors, 1-, 3-, 5-, 7- and 9-fold errors, independent of location

Error bursts < 11 bit long

Error pattern (with exception of rectangular format).

The bit error rate of p =10-5satisfies the requirements of IEC- SC-65-WG6 for process buses. According to this, an unde- tected transmission error 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 and a sporadic (acyclic) data transfer via the bus system:

Dedicated transmitter and receiver blocks handle all functions associated with data transfer. Thus, the data transfer can be established easily by configuring, just as the automation func- tions.

The user need only carry out the following to produce commu- nication:

-- The inputs of the transmitter block must be provided with the addresses of the data to be transmitted

-- The transmitter block must be inserted into the processing sequence

-- The outputs of the corresponding receiver block must be linked accordingly in another automation system

-- The receiver block must be inserted into the processing sequence

-- The connection to the transmitter must be established using a coupling instruction.

This presents the following advantages:

Easy configuring of communication (only the transmitter and receiver blocks must be configured, not the interface modules for 20-m local bus)

Messages to several subscribers are simply transmitted on the bus (several receiver blocks can access one transmitter block)

Symbolic addressing (block name and number)

Complete documentation of all connections by self-documen- tation of systems

The communication procedure can be configured in a still simpler manner in full-graphic mode on the screen by means of the configuring tool PROGRAF and can also be documen- ted in the same representation (see page 2/5).

Cyclic data transfer

Analog values are transmitted cyclically 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 in addition to the cyclic data transfer.

These functions include image updating in an OS operation and monitoring system. The updating time is < 2 s in the TELEPERM M process control system. In order to guarantee this value, the bus system provides a function ”Read parameters”. This en- ables up to 30 different parameters per message to be read from one AS system. The functional sequence is as follows:

The OS system sends the request to the bus system to read the corresponding parameters from several AS systems for image updating. The addresses of these parameters have already been converted to on-line-accessible parameters from the sym- bolic addresses of the configuration during the system start-up. Thus, the bus interface modules can read the desired informa- tion out of the automation system memory by DMA, generate corresponding reply messages and return these to the corre- sponding OS systems.

Fig. 2/1 Cyclic data transfer, example OS/AS and AS/AS

2/2 Siemens PLT 130 . 1994