10 of 11 Instructions Manual
If the external device (Supervisor PC or PLC) requires data from a proprietary device, the answer is
obtained from the data locally available in the Gateway. For this reason a data request to a
proprietary device doesn’t generate any radio message, so there is a complete separation between
the local serial communications (between the Gateway and client) and the radio network traffic
(between Gateway and all proprietary devices).
10.2 Generic Modbus devices (attached to a Router-Bridge)
If the external device (Supervisor PC or PLC) requires data from a generic Modbus device the serial
message is converted in a radio message and it is sent to the Router-Bridge to which the generic
Modbus device is attached to.
As the Router-Bridge receives the radio message from the Gateway it arranges to send it through the
serial line.
The answer coming from the generic Modbus device arrives to the Router-Bridge, it is transformed to
a radio message and it is sent to the Gateway which, at last, arranges to turn it into a serial message
to be sent to the outside device.
Gateway and Router-Bridge are substantially responsible only for the transport of the message.
Unlike the first case there is not any separation between the local serial traffic (between Gateway and
outside client) and the radio traffic present in the network.
This involves that for a correct working of the network the local serial traffic must consider the intrinsic
limits of the radio network.
11 TRAFFIC LIMITATIONS
The requests turned to generic Modbus devices have the following limitations:
Message length
Both the requests and the answers must not exceed the length of 52 bytes; this involves the
prohibition to read more than 26 registers (InternalRegister or InputRegister) in a single message.
Request frequency
The time delay between the reception of an answer and the following request must not be less than
one second.
Frequency of the requests in the case of absence for answer
The time gap between two consecutive requests, the first of which has not received an answer, must
not be less than 3 second.
This is translated in a time-out of communication of at least 3 seconds.
12 ROUTER-BRIDGE DATA
It is possible to extract some information about the Router-Bridge’s operation or ID.
This information is accessible through the reading of 12 InputRegister addresses (40001 to 40011).
The reading request should be addressed to the same logical address of one of the Modbus devices
connected to the Router-Bridge.
12.1 ROUTER-BRIDGE INPUT REGISTERS
