User Manual

EMpro - multifunctional energy measuring devices for DIN rail mounting
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Broadcast messages in gateway mode
The Modbus specification does not describe the behavior of a gateway device when the unit
identifier in the MBAP header has a value of 0. In the Modbus/RTU network, this value
represents a broadcast message.
There are different interpretations of the behavior of a gateway in this case.
The following questions regarding the behavior of a gateway remain open:
– Forwarding the broadcast in the RTU network.
Since there is no broadcast in the case of Modbus/TCP, it is not defined whether or not
the MBAP address 0 is forwarded in the RTU network by the Modbus gateway.
– Response messages with the unit identifier 0.
Neither is there a definition on whether the gateway itself generates an answer to
messages with a unit identifier of 0.
By default upon delivery, devices of the EMpro product family behave as follows:
The EMpro does not forward messages with unit identifier 0 to the RTU network as
broadcast. Furthermore, the EMpro itself responds to the query as Modbus TCP server.
Timing in gateway mode
As gateway, the EMpro acts as Modbus master on the Modbus/RTU side and forwards
queries to the addressed slaves.
As Modbus/RTU master, the device requires a time-out for queries. If this time-out has
expired without a response from the Modbus/RTU slave, the gateway replies to the
Modbus/TCP client with an error message.
By default upon delivery, the duration for the time-out is 250 ms.
Unsuitable time-out settings both at the gateway and and at the querying Modbus/TCP
client can cause unwanted problems.
Problems can occur if the time-out times at the Modbus/TCP client or EMpro in gateway
mode are too short, or if the ratios between these two time-out times are unfavorable.
The time-out setting has to be individually configured based on the application.
Nevertheless, the following recommendation can be given; they should work in normal
cases.
– The default setting of the EMpro is adequate for most applications.
– If there are slaves with slow response times in the RTU network, the time-out setting of
the gateway should be increased.
– If very slow Baud rates are used on the RS-485, the time-out setting of the gateway
should be increased.
– The time-out duration of the querying Modbus/TCP client should always be longer than
that of the gateway.