It is essential:
n
You may connect 1 master and max. 31 slaves at the bus
n
The single slaves are addressed by the master via an address sign in the telegram.
n
The communication happens exclusively in half-duplex operation.
n
After a send command, the acknowledgement telegram must be read by a call of the
FC/SFC 218 SER_RCV.
The telegrams for send and receive have the following structure:
Master slave telegram
STX
LGE
ADR
PKE
IND
PWE
STW
HSW
BCC
02h
H
L
H
L
H
L
H
L
H
L
Slave master telegram
STX
LGE
ADR
PKE
IND
PWE
ZSW
HIW
BCC
02h
H
L
H
L
H
L
H
L
H
L
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 mes-
sage. 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 con-
flicts 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.
Broadcast with set bit 5 in
ADR byte
Modbus
System 300S
+
Deployment PtP communication
Protocols and procedures
HB140 | CPU | 315-4PN23 | en | 18-02
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