315-4PN33

with
STX - Start sign
STW - Control word
LGE - Telegram length
ZSW - State word
ADR - Address
HSW - Main set value
PKE - Parameter ID
HIW - Main effective value
IND - Index
BCC - Block Check Character
PWE - Parameter value
A request can be directed to a certain slave ore be send to all slaves
as broadcast message. For the identification of a broadcast message
you have to set bit 5 to 1 in the ADR byte. Here the slave addr. (bit
0 ... 4) is ignored. In opposite to a "normal" send command, the
broadcast does not require a telegram evaluation via FC/SFC 218
SER_RCV. Only write commands may be sent as broadcast.
n The Modbus protocol is a communication protocol that fixes a
hierarchic structure with one master and several slaves.
n Physically, Modbus works with a serial half-duplex connection.
There are no bus conflicts occurring, because the master can only
communicate with one slave at a time.
n After a request from the master, this waits for a preset delay time
for an answer of the slave. During the delay time, communication
with other slaves is not possible.
n After a send command, the acknowledgement telegram must be
read by a call of the FC/SFC 218 SER_RCV.
n The request telegrams send by the master and the respond tele-
grams of a slave have the following structure:
Telegram structure
Start sign Slave
address
Function
Code
Data Flow control End sign
n A request can be directed to a special slave or at all slaves as
broadcast message.
n To mark a broadcast message, the slave address 0 is used.
n In opposite to a "normal" send command, the broadcast does not
require a telegram evaluation via FC/SFC 218 SER_RCV.
n Only write commands may be sent as broadcast.
Broadcast with set bit 5
in ADR byte
Modbus
Broadcast with slave
address = 0
VIPA System 300SDeployment PtP communication
Protocols and procedures
HB140 | CPU | 315-4PN33 | GB | 14-43 98