User's Manual

455U Radio Modem Module User Manual
Page 22 © November 2004
To improve reliability in this mode, Units may be configured to send each message multiple
times (S-Register S29) (The receiving unit will detect repeat messages and only send the data
out the serial port once).
In Unacknowledged mode, messages may be sent to a particular modem (Message is
addressed to that modem), or to all modems in the system (Messages is addressed to wildcard
address 0). Each modem is configured with a separate unit address. The wildcard address is
used to send a message to a group of modems.
Data received at the serial port is transmitted out of the radio port, addressed to the configured
destination module. 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
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
23msec after the data is input, if there is no error checking, or 47msec after the data is input if
error checking is configured.
3.5.1 Auto-Connect Unacknowledged mode
This operating mode is selected by selecting Unacknowledged mode (AT&M0) and leased
line mode (AT&L1) and by configuring a destination address into auto dial location 0
(AT&Z0). In this mode, the modem will go Online as soon as it is powered up. This will be
indicated by the LINK LED going green. All messages received at the RS-232 serial port are
sent to the destination address configured in the auto dial setting (&Z0). Use the wildcard
address (0 or 128) to send messages to multiple units in the system.
3.52 Single-Connect Unacknowledged mode.
This operating mode is selected by selecting Unacknowledged mode (AT&M0) and
deselecting leased line mode (AT&L0) In this mode, the modem will go to command mode as
soon as it is powered up. The RS232 port will print the message “OK” and wait for
commands. The host software chooses which remote module to configure by issuing a dial
command (ATD). As soon as the Dial command is issued, the modem will issue a “Connect”
message and go on-line. This will be indicated by the LINK LED going green. All messages
received at the RS-232 serial port are sent to the dialled address. Use the wildcard address (0
or 128) to send messages to multiple units in the system.
3.5.3 Unacknowledged mode Repeaters
Every 455U module automatically acts as a repeater by including its address in the address
list. To provide more flexibility, the modules may be configured to repeat messages sent to