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Serial communication protocols
Modbus > Parameter data of Modbus
Broadcast with slave
address = 0
ASCII-, RTU Modus
5.5.2 Modbus at the CP from VIPA
Modbus Master
Modbus Slave short
Modbus Slave long
5.5.3 Parameter data of Modbus
Parameters
74
A request may be addressed to a certain slave or sent as broadcast telegram to all
slaves. For identifying a broadcast telegram, the slave address 0 is set. Only write com-
mands may be sent as broadcast.
Modbus supports two different transmission modes:
ASCII mode:
n
–
Every byte is transferred in 2-character ASCII code. A start and an end ID mark
the data. This enables high control at the transmission but needs time.
n
RTU mode:
–
Every byte is transferred as character. Thus enables a higher data throughput
than the ASCII mode. Instead of start and end ID, RTU uses a watchdog.
The mode selection is made at the parameterization.
The CP supports several Modbus operating modes that are described in the following:
In Modbus Master operation you control the communication via your PLC user application
in your host system. By means of the Modbus function codes you can access the Modbus
slaves with read write functions of the Modbus master. There is the possibility to transfer
up to 250byte user data with one telegram.
In Modbus Slave short operation the CP communicates with a Modbus Master.
Depending on the function code, the CP receives data from the Modbus Master or serves
for data for him. The data handling on slave side automatically takes place. This opera-
tion mode is especially convenient for the fast transfer of small volumes of data via
Modbus.
In Modbus Slave long operation only a changed data area, beginning with 0 is transferred
from the CP to the host system. If the Modbus master requests data, you have to serve
for the relevant data in the CP with an user program. Writing master accesses may not lie
outside of the receipt area!
Only after all data are present in the CP, the CP sends an answer tele-
gram to the master.
DS - Record set for access via CPU, PROFIBUS and PROFINET
IX - Index for access via CANopen
SX - Subindex for access via EtherCAT with Index 3100h + EtherCAT-Slot
More can be found in the according manual of your bus coupler.
VIPA System SLIO
HB300 | CP | 040-1CA00 | en | 18-28
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