VEGA VEGAFLEX 81 Modbus and Levelmaster protocol Coax probe - Manual (Page 73)

73
11 Supplement
VEGAFLEX 81 • Modbus and Levelmaster protocol
42283-EN-130314
Electrical protective measures
Protection rating
IP 66/IP 67
Overvoltage category III
Protection class I
Approvals
Instruments with approvals can have different technical data depending on the version.
For that reason the associated approval documents of these instruments must be carefully noted.
They are part of the delivery or can be downloaded under www.vega.com and "VEGA Tools" as
well as under "Downloads" and "Approvals".
11.2 Basics Modbus
Bus description
The Modbus protocol is a communication protocol for the communication between instruments. It is
based on a Master/Slave or Client/Server architecture. By means of Modbus, a Master and several
Slaves can be connected. Each bus participant has an unambiguous address and can send mes-
sages to the bus. This is initiated by the Master, the addessed Slave answers. The versions Ethernet
and serial (EIA-232 and EIA-485) as well as the modes ASCII, TCP and RTU are available for the
data transfer. In the RTU and ASCII mode interesting here, data will be transmitted in binary form.
The telegram consists generally of the address, the function, the data as well as the transmission
check.
Bus architecture
In the version Modbus RTU, up to 32 participants can be connected to the bus. The length of the
twisted two-wire cable can be up to 1200 m. The bus must be terminated on both sides at the last
bus participant with a terminating resistor of 120 Ohm. The resistor is already integrated in the
VEGAFLEX 81 and is activated/deactivated via a slide switch.
4
1
3 3 3
2
+
+
power supply
MODBUS
D0 D1 IS GND
USB
+
+
power supply
MODBUS
D0 D1 IS GND
USB
+
+
power supply
MODBUS
D0 D1 IS GND
USB
1 3 4 5 2 off on ( )
(
)
(-)
(-) 1 3 4 5 2 off on ( )
(
)
(-)
(-) 1 3 4 5 2 off on ( )
(
)
(-)
(-)
Fig. 37: Bus architecture Modbus
1 RTU
2 Connection resistor
3 Bus participant
4 Voltage supply