Westermo RM-455U-D - Manual (Page 10)

Chapter One Introduction
Man_455U-D Rev 3.04 Page 11
UNACKNOWLEDGED MODE
ADDRESSED TO ALL UNITS USING
WILDCARD ADDRESS 0
DATA
DATA
DATA
DATA
Data received at the serial port is transmitted out of the radio port, addressed to the configured
destination 455U-D module, or to all modules by using address 0. Data received from the
radio with the correct addressing is transmitted out of one of the serial ports (RS232 or
RS485).
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.
If error checking is not configured at the receiving unit, data will start to be output
immediately after the first byte of data has been received. If error checking is configured, data
will be output approx 1mSec 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 23 msec after the
data is input, if there is no error checking, or 31 msec after the data is input if error checking
is configured - this assumes the minimum lead-in time of 20 msec is configured.
1.3 Acknowledged Mode
In Acknowledged mode, data is transferred between the RS-232 ports of two modules (that is,
a point to point link). One of the modules is configured as a “master” (or initiator) unit and
the other as a “slave” (or
responder) unit. There can be
many slave units in the system,
however the master unit will only
link to one slave at any one time.
Note that Acknowledged
mode only applies to the
RS-232 port. The RS-485
port always operates in
Unacknowledged mode.
To establish a link, the master unit transmits a special “connect” message. This initial message
does not include any data. If the addressed slave unit receives the connect message, and is not
already connected to another 455U-D unit, it will return an acknowledgment message. Both
units will activate their DCD LED, and also activate their DCD output signal (if configured).
ACKNOWLEDGED MODE
DATA
DATA
NO
DATA
DATA MESSAGE
ACK