Deployment
Wieland Electric GmbH | BA001269 | 09/2021 (Rev. A)
37
Modbus/TCP
Modbus/TCP is a Modbus-RTU protocol, put upon TCP/IP. The Modbus protocol is a communication
protocol supporting a hierarchic structure with one master and several slaves. Modbus/TCP extends
Modbus to a client server communication where several clients may access a server. For the ad-
dressing happens by means of the IP addresses, the address integrated in the Modbus telegram ir-
relevant. Furthermore, the check sum is not required because the sequence insurance happens via
TCP/IP. After the request of a client, this awaits the answer of the server for a configurable time.
Modbus/TCP exclusively uses the RTU format. Every byte is transferred as one sign. This enables a
higher data pass-through than the Modbus-ASCII format. The RTU time supervision is omitted for
the header contains the size of the telegram length to be received. Data that are transferred via Mod-
bus/TCP may contain bit and word information. At bit chains, the highest bit is sent first, i.e. in a
word it is at the most left position. At words, the highest byte is sent first. The access to a Modbus
slave happens via function codes that are described in detail in this chapter further below.
6.2
Operating modes
Description
SF
red
MT
yellow
SB
green
Init
DHCP discovery
x
2 Hz
2 Hz
Update config
x
2 Hz
Run
Commissioning
1 Hz
Maintenance
x
Firmware update
1 Hz
1 Hz
Error
2 Hz
x: not relevant
off
Init
After PowerON, the Ethernet coupler is in
Init
state. Here, the Ethernet coupler performs a self-test
and the system is initialized. If successful, the Ethernet coupler automatically switches to either
DHCP discovery
or
Update config
, depending on the setting of the address switch.
DHCP discovery
The Ethernet coupler changes to this state as soon as an IP configuration is requested via DHCP. In
this state, the Modbus/TCP server is stopped and BASP (
B
efehls-
A
usgabe-
S
perre, i.e. command out-
put dis-able) becomes active and thus process data communication is stopped. After assigning the
IP address data, it automatically changes to the state
Update config
. If no IP address could be deter-
mined via DHCP, the Ethernet coupler automatically restarts and changes to the
Init
state.
Update config
In this state, the Ethernet coupler checks the module configuration and transfers the corresponding
parameters to the connected modules. If no errors occur, the Ethernet coupler automatically
switches to
Run
.
