User's Manual

Chapter Three Operation
Man_455U Rev 1.0 Page 23
the wildcard address, and to send repeated messages out their own serial port (AT&R). This
allows networks where messages are repeated several times throughout a complex network
of modems.
3.5.3 RS-485 Unacknowledged mode
The RS-485 serial port on the 455U always operates in unacknowledged mode. This mode is
best suited to broadcast-oriented protocols which use RS-485. Messages received at the
RS485 port of the modem are sent to the address configured in auto dial location 3 (AT&Z3).
Use the wildcard address (0 or 128) to send messages to multiple units in the system. The RS-
485 port becomes functional as soon as the module is powered up.
3.6 Acknowledged mode
In Acknowledged mode, data is only transferred between the RS-232 ports of two modules
(that is, a point to point link). One of the modules is configured as an initiator unit and the
other as a responder unit. There can also be up to five intermediate repeaters in the link.
Each 455U 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.
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 initiator transmits a special “connect” message. This initial message
does not include any data. If the responder unit receives the initial message, and is not
already connected to another 455U unit, it will return an acknowledgment message. Both
units will activate their DCD LED, and also activate their DCD output signal (If configured –
AT#O2). If the master unit does not receive the acknowledgment, the DCD output will reset.
When the connection is made (DCD set), the 455U units can transmit data to each other.
Once the communications channel has been established, the 455U unit will accept input data
and send radio messages with data. When a 455U unit receives a radio message, it will check
the system address and destination address, and also the error-check (optional). If these are
correct, it will return a ACK (acknowledgment) message to the source unit. If the system
address or destination address is not correct, or if the error-check is not correct, then no return
message is sent.