User Manual

EMpro - multi-functional energy measuring devices for front panel installation
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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 ad-
dresses are in the range from 1-247.
The default setting for the devices is address 1. Address 0 is reserved for broadcast mes-
sages 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 infor-
mation. 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, be-
cause of the addressing:
– 1 x master 247 x slaves
Figure 11-1 Topology