Belden HIRSCHMANN OZD Modbus Plus G12 Manuel page 34

1 Introduction
Fiber-optic technology
The implementation of fiber-optic technology permits
very long transmission ranges and provides optimum
protection against EMI effects both along the trans -
mission link and (owing to the electrical isolation) at the
repeaters themselves.
Transmission rate
The Fiber-Optic Repeater OZD Modbus Plus G12 ...
functions at a transmission rate of 1 MBit/s.
Signal regeneration
The Fiber-Optic Repeater OZD Modbus Plus G12 ...
regenerates the signal shape and amplitude of the
received data.
This function permits as many repeaters as required to
be cascaded via optical links.
Redundancy
Redundant signal transmission ensures a very high
degree of transmission reliability.
Redundant operating voltage supply can increase
operational reliability even further.
Modbus Plus protocol
In a network topology as shown in Fig. 2, 3 and 4 (Ch. 2), a
response time must be taken into consideration at the
master and in the data terminal equipment.
The data which is sent out, is returned to each optical
port by the neighboring device. This status signal is used
to monitor the output and the rings.
The response time is composed of the transfer time in
the optical fiber (5 ns/m), the transfer time through a
repeater (< 1µs) and a device-specific pause time of 5 µs:
t = (2 l [m]
•
response
= (10 µs/km
where
l = maximum length occuring between two adjacent
OZD Modbus Plus G12 ... .
Example:
At a maximum distance between two adjacent OZD
Modbus Plus G12 ... of 2.3 km, the response time is as
follows:
t = (2 2300 m
•
response
= (10 µs/km
= 29 µs
32
5 ns/m) + 1 µs + 5 µs
•
l [km]) + 1 µs + 5 µs
•
5 ns/m) + 1 µs + 5 µs
•
2.3 km) + 1 µs + 5 µs
•
Modnet, Modbus and Modbus Plus -
the differences
With Modnet a complete communication system for
automation engineering was offered by the former AEG.
This is not a single bus system but a communication
system with three performance classes. Due to the
different requirements, these performance classes range
from usage in proximity to the process, to superordinate
backbone communication.
Performance class 1:
Communication in
proximity to object
Performance class 2:
System communication
Performance class 3:
Backbone communication
Here the following applies:
Modnet1/M+: Low cost bus, Modbus Plus
Modnet1/P: System field bus in accordance
with Profibus standard
Modnet1/IS: System and sensor-actuator bus in
accordance with Interbus standard
Modnet1/SFB: System field bus in accordance
with Bitbus standard
Modnet3/MMSE: Communication system for super-
ordinate levels in accordance with
IEEE 802.3 and MAP (MMS on
Ethernet)
Modbus is a widely used, defined message structure for
master-slave communication. A Modbus message, sent
from the master to the slave, contains the address of the
slave, the command, the data, and an error checksum.
Modbus RTU and Modbus ASCII are different data codes.
As only the data format is defined, any medium can be
used (RS232, RS422, RS485 copper cable, optical fiber,
radio, ...). Suitable optical fiber converters are OZDV 24...,
OZDV 114 and OZDV 485... .
Modbus Plus is a complete protocol and network
definition. Modbus Plus uses the Modbus command
structure, but it transmits the commands together with a
token that is passed rapidly from one network user to the
next. Modbus Plus defines how the token is forwarded,
how commands are repeated, how the data is checked
for errors, and how these errors are then indicated, and
of course the complete design of the physical interface,
this also relates to the cable and the network infrastruc-
ture (tabs, bridges, terminating resistors, ...).
The objective here is a real "plug and play" field bus
system.
Modnet1/M+
Modnet1/P, Modnet1/IS,
Modnet1/SFB
Modnet3/MMSE
Version 04 07/2014