User Manual

Communication
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11.2.2 Modbus/RTU
In the case of Modbus/RTU, the Modbus application layer protocol is supplemented with
additional information and physically sent via an RS-485 interface. The start and end of a
Modbus message is coordinated by means of timings.
Modbus/RTU describes a master/slave protocol. Here, the EMpro acts as Modbus/RTU
slave. An exception here is the gateway mode.
The additional information consists of a byte for addressing the device. Valid device
addresses are in the range from 1-247.
The default setting for the devices is address 1. Address 0 is reserved for broadcast
messages in the network and cannot be assigned as an address to a device.
According to the specification, addresses 248-255 are reserved and cannot be assigned to
the device either.
A cyclic redundancy check (CRC) is attached to the Modbus message as additional
information. The CRC is used to detect transmission errors. The calculation and the
polynomial used are documented in the Modbus specification.
The Modbus/RTU protocol is supported by all EMpro versions with an RS-485 interface.
11.2.2.1 Topology
The smallest network via RS-485 (Modbus) that makes sense consists of two devices:
– 1 x master 1 x slave
The typical limit is 32 devices.
– 1 x master 31 x slaves
The largest network via RS-485 (Modbus) that makes sense consists of 248 devices,
because of the addressing:
– 1 x master 247 x slaves
Figure 11-5 Topology