805U Radio Modem Module
User Manual
Page
8
©
November 2004
The operation of the 805U radio modem is relatively simple. As data is received at the serial
port, the data is transmitted on the radio channel. Up to 530 bytes of data can be transmitted
in one transmission. The radio transmission commences when the first data byte is received,
and ends when there are no more data bytes in the input buffer, or when the number of bytes
transmitted equals the maximum message length (user configurable - default 530 bytes). If
more than 530 bytes is input, the 805U unit will transmit the first 530 bytes, then the next
530 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 2Kbytes 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 805U
uses a radio band with a low level of natural or industrial noise, however there is a chance of
interference from other users of the unlicensed 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 805U
modules. The modules may be configured by the user to operate in one of two modes. In
transparent mode, it is assumed that the host devices control the flow of data. In controlled
mode, the 805U units control the flow of data.
1.2
Transparent Mode
The default configuration of the 805U
modem is transparent mode - the modules
are set in this mode at the factory. In
transparent mode, the 805U provides no
control of the data transmissions (no error
correction). Input data is simply transmitted
by radio and every other 805U 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 805U in the system will receive the data and
output the data to their individual host devices. The user may configure the 805U 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.
Transparent 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
805U
DATA
HOST
805U
DATA
HOST
805U
DATA
