User's Manual

Chapter Three Operation
Man_905UD_2.0.doc Page 21
3.3 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 address, 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 is high while there is space in the input data buffer.
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.
One 905U-D unit may be configured as a repeater. This unit will not output data at its serial
ports. Any message received at the radio port, with the correct system address, will be re-
transmitted.
The time taken to transmit a message is :-
lead-in system addr data error check (if configured)
|---------------|-----------------|------------------------------------------|-------------------|
40msec 2 bytes 1.9ms per byte 2 bytes
4ms @ 4.8KB @ 4.8KB 4ms @ 4.8KB
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 4800 bits/sec will be output approx 45msec
after the data is input, if there is no error checking, or 82msec after the data is input if error
checking is configured.
The time between transmissions is set by the transmit and receive hold-off times configured
by the user.
3.4 Controlled Mode
In controlled mode, data is only transferred between two modules (that is, a point to point
link). One of the modules is configured as a “master” unit and the other as a “slave” unit.
There can also be up to five intermediate repeaters in the link. Each 905U-D unit is
configured with a unit address - only the unit with an address matching the destination
address of the radio message will process the message and output the serial data.
To establish a link, the master will transmit a special “connect” message. This initial
message will not include any data. If the “slave” unit receives the initial message, and is not
already connected to another 905U-D unit, it will return an acknowledgment message. Both
units will activate their DCD LED, and also activate their DCD output signal. If the master