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 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.
Broadcast with set bit 5
in ADR byte
Modbus
VIPA System 300S Deployment PtP communication
Protocols and procedures
HB140 | CPU-SC | 313-6CF13 | GB | 15-50 167
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