U S I N G I N T E R N A L M E N U C O M M A N D S
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© RF Innovations Pty Ltd, 1998
Page: 29
223889, Revision 2.2
The protocol decoder for the packet protocols such as Modbus, DNP3 etc, allows a
system of extracting the destination RTU address from the protocol and then mapping
it to radio RX addresses. If a system contained a number of RTUs connected to a base
unit then the Master/Base unit could decode the RTU address of each slave and convert it to a remote
slave Rx address. RTU is an acronym for Remote Terminal Unit.
The Rx Address is the address of the RFI modem. Each modem must have an address so that it can
determine whether the RF data received (through its antenna) was intended for it or not. The RTU
address is the address of the device attached to the modem. This is analogous to dialing a telephone that
has a PC attached. The telephone has its own number, which must be dialed, yet the PC also has an IP
address. The RTU address is part of the protocol header sent from the Master Control to the RFI-9256
along the RS-232 link. It is up to the Routing Table to determine to which radios Rx Address the data
must be sent to in order for the correct RTU to receive the information. These ideas are demonstrated
in the following figure, using the Modbus protocol.
Default Route - This route is mostly used by Slaves and is probably a must have entry for any Slave in
a protocol network. This route only requires the Rx address of the destination radio. This route is used
when no other route can be matched in the routing table.
Single Route - This route explicitly maps an RTU address to a radios Rx address. It is the highest
privilege in a routing table.
Mapped Route - This route maps a relationship between RTU addresses and radios Rx addresses.
This is a very efficient method of describing a network providing the RTUs and radios can be
numbered sequentially and has some correlation. This route configuration requires an RTU address to
be assigned as the base for the route and an RTU address to define the range of addresses to be used by
the route. It also requires the slave Rx address associated with the RTU address used as the base. The
algorithm works as described in the next paragraph.
The decoder subtracts RTU base address from the newly decoded RTU address the result acts as an
offset that is added to the Rx (Radio) address that was assigned as base address. For example if a
following diagram was entered in the router then the base RTU address would be 20 and the range
Protocol
Decoding/Routing
Route Types
Figure 6-14 Modbus Protocol Routing Example
