805U Radio Modem Module
User Manual
Page
20
©
November 2004
addresses can communicate via a repeater. The group address is used for repeater operation in
transparent mode. The group address is not used in controlled mode.
A 805U can also be configured with a “unit” address - this gives the module a unique
identification. The unit address feature is not used in transparent mode, but it is used in
controlled mode.
3.4
Transparent Mode
In transparent mode, radio messages are transmitted without unit addressing. Units do not
provide handshaking functions to control the flow of data. Every unit which receives the
radio message, and has the correct system and group addresses, will output the data.
Transparent mode operation is effectively a broadcast system.
Data received at the serial port is transmitted out of the radio port. Data received from the
radio is transmitted out of the serial port. Prior to transmitting, units will listen to the radio
channel to ensure that it is clear - units will hold off from transmitting until the radio
channel is clear. At the RS232 port, the CTS pin can be configured to go high while there is
space in the input data buffer - otherwise it is always high.
Host devices should provide a suitable protocol to ensure that error checking, handshaking
and implementation of an appropriate re-transmission scheme is provided. This mode of
operation is particularly suited to devices designed to operate over a multidrop network, such
as PLC systems designed for operation over a RS-485 network.
The time taken to transmit a message is :-
lead-in
system/group addr
data
error check (if configured)
|---------------|-----------------|------------------------------------------|--------------|
5 msec
2 bytes
0.5ms per byte
2 bytes
1 ms @19.2Kb
@ 19.2Kb
1ms @ 19.2Kb
If error checking is not configured at the receiving unit, data will start to be output
approximately 1 msec after the system address has been received. If error checking is
configured, data will be output approx 2msec after the end of the message. For example, a
message with 20 bytes of data transmitted at 19200 bits/sec will begin to be output approx
12msec after the data is input, if there is no error checking, or 24msec after the data is input
if error checking is configured.
3.5
Transparent Mode Repeaters
A module can be configured as a transparent mode repeater. In this mode, the module will
operate as a normal transparent mode module, plus it will also re-transmit any radio messages
that it receives. However it will change the group address in the re-transmitted message. The
transparent mode repeater is configured with two group addresses. When it receives a radio
message with one of these group addresses, it will re-transmit the message using the other
group address (assuming that the system address is correct).
One of the group addresses will be its normal or “first” group address. If a transparent mode
repeater receives a message on its first group address, it will output the data on its serial port,
and will also re-transmit the message using its second group address. That is, the re-
