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Commands
2014-
02
29
7.3
Communication via MODBUS TCP/IP
7.3.1
General Information on Data Communication via MODBUS/TCP
Data is exchanged between a MODBUS master (controller) and a MODBUS slave
(identification system) by reading and writing registers. The slave contains read and write
registers. Data exchange is always initiated by the master. The master initiates an identification
system function by transferring an identification command to the write register. The master can
then retrieve the response via the read registers. The functions
read holding registers
,
write
multiple registers
and
read/write multiple registers
are available for this purpose on the IC-
KP-B17-AIDA1 device. MODBUS communication occurs via port 502. The terms "input
register" and "output register" are defined from a PLC perspective.
7.3.2
Overview of the characteristics of the integrated MODBUS slave
■
Multimaster capability.
■
Data exchange using the commands
read holding registers
,
write multiple registers
and
read/write multiple registers
.
■
Each channel is assigned a separate register area so that different controllers can each
adopt a R/W head. Only one master has write permissions for each register area.
■
The output register data is stored temporarily in a FIFO memory.
■
A monitor master can also read the data from the identification system.
■
The same identification commands used with TCP/IP are also used here.
Multimaster capability
The device can communicate with several masters. An ident channel can be addressed by two
masters. The first master is the control master and addresses the device using device ID 1.
This master possesses both write and read permissions. Another master can be used to read
the data and is a protocolling master. This master then addresses the device using device ID 2.
The device can therefore be addressed on each channel once using device ID 1 and once
using device ID 2. If another master attempts to address the device on the same channel,
access to this channel is denied. A maximum of ten masters can communicate with the device
at any one time.
MODBUS commands
The device supports the commands
read holding registers
,
write multiple registers
and
read/write multiple registers
.