User's Manual

455U Radio Modem Module User Manual
Page 8 © November 2004
number of bytes transmitted equals the maximum message length (user configurable - default
1024 bytes). If more than 1024 bytes is input, the 455U unit will transmit the first 1024
bytes, then the next 1024 bytes, and so on until all of the data has been transmitted.
Because the radio data rate could be less than the input serial data rate, an input memory
buffer of up to 8Kbytes is provided. The RS232 connection provides CTS control to prevent
the buffer overflowing. There are no data flow control signals for RS485.
A radio channel cannot provide as secure a data channel as a wired connection. The 455U
uses a radio band with a low level of natural or industrial noise, however there is a chance of
interference from other (unlicensed) users of the licensed radio channel. We recommend that
the flow of data over the radio channel is controlled by using error detection and
“handshaking” - that is, returning an acknowledgment transmission if a data packet is
received on the radio channel without error. This function can be performed by either the
host devices or the 455U modules. The modules may be configured by the user to operate in
one of two modes. In unacknowledged mode, it is assumed that the host devices control the
flow of data. In Acknowledged mode, the 455U units control the flow of data.
The modem supports encryption of data sent over the air to prevent eavesdropping. AES-128
encryption provides a high level of security and immunity to hacking.
1.2 Unacknowleged Mode
The default configuration of the 455U
modem is unacknowledged mode - the
modules are set in this mode at the factory.
In unacknowledged mode, the 455U
provides no control of the data
transmissions (no error correction). Input
data is simply transmitted by radio and
every other 455U unit in that system which
receives the transmission will output the
data. This mode relies on the host devices
to perform the “handshaking” function, and
re-transmitting serial data if the data is corrupted (no “handshake”). It also relies on the host
devices to include any addressing necessary in the data. In this mode, modules are not
configured with a unit address. Data is “broadcast” - every other 455U in the system will
receive the data and output the data to their individual host devices. The user may configure
the 455U modems to add error checking to each data packet transmitted - if error checking is
configured, data will not be output if it is received without a correct error-check. This feature
provides additional protection against corruption of the data during the radio transmission. If
error-checking is not configured, then the data received by radio will be output without
checking for errors.
Unacknowledged mode is “point-to-multipoint” communications, suitable for a host device
which is able to communicate on a multi-drop “bus” type network. An example of an
application is the use of radio modems to extend a PLC RS485 network. The serial messages
from the PLC’s already include PLC addressing and error detection/correction to control the
flow of data
HOST
455U
DATA
HOST
455U
DATA
HOST
455U
DATA