WSN Manual
Page 27 of 34
Rev. G
– 4 March 2007
6.2.5
Timeouts in MODBUS Modes
When a transmission from a master station radio is made in Modbus mode the radio will
calculate a timeout for a reply, this calculation is based on many configuration parameters
including the radio baud rate, lead in delay, host inactivity time, maximum message length,
power save timing etc. If power saving is enabled and the baud rate is low this time can be large
(the calculation limits the result to a maximum of 4.25 minutes. To reduce the possibility of
“hung” radios the destination radio will send a link-closing message if the destination Modbus
slave does not reply. This link-closing message is only used by the radios to close the link, it is
not passed to the Modbus master.
If the Modbus master itself times out before the radio link does, it can send another poll, and
radios along the link will cancel the previous route and set up the new one. The exception to this
is the previous destination radio if it is still trying to wake up its Modbus slave, it will ignore the
new message and try to download its original message when the slave awakes, a conflict will
then arise if a reply is sent. To avoid this situation the Modbus master timeout time should allow
the maximum “HOST INACTIVITY TIME” to expire plus the time required to get a message and
its reply through the link.
6.3 RFT ROUTING PROTOCOL
6.3.1
Setting Up RFT Routing Operation
The WSN can be programmed to route non-specific protocol messages in single master
systems using “RFT ROUTING” mode. This mode supports relay messaging. This option is
selected as the “INTERFACE PROTOCOL” in the “EDIT MODEM/INTERFACE” menu.
In describing operation the address contained in the host system message will be referred to as
the “protocol address” and address programmed in the radio under the “RADIO ADDRESS”
field in the setup program will be referred to as the “radio address”.
RFT Routing mode is controlled at the master station by picking out an 8 bit protocol address
field in the message to be sent, this address is then looked up in the routing table stored in the
master station radio. The routing table can contain the radio address (as programmed in the
RADIO ADDRESS field in the setup program) of a single radio connected to the required
destination device or a list of relay radio addresses plus the destination radio address. The
message is then transmitted from the base station radio as a packet with the routing information
prefixed to it. The message is then relayed through any relay radios specified until it reaches the
destination radio where it is output from the serial port in its original form with the packet
information removed. During this process each radio considers itself to be part of an established
link. A reply is then expected, however the outstation radios are not programmed with routing
tables, a reply issued is assumed to be destined to the master station. The address in the
protocol message is therefore not checked and the reply is simply sent back down the
established link to the master station radio where it is output from the serial port. As the reply is
passed back the link members no longer consider themselves to be part of an established link
and return to idle.
Note that there is no differentiation in operating mode between a relay radio and an outstation
radio, if an outstation radio is specified as a relay in a link any device connected to the local
serial port will be unaware of relay communications taking place.
The packet used to transfer protocol messages specifies the route to be taken and also the
current stage in the route, it is therefore of no concern if radios further down a relay link “hear”
the message before they are expected to repeat it, they will ignore the message until specifically
requested to repeat it.
