User's Manual

Chapter One Introduction
Man_455U-D Rev 3.01 Page 11
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).
If the master unit does not receive the acknowledgment, the DCD output will reset. When the
connection is made (DCD set), the 455U-D units can transmit data to each other.
A master can be configured to connect to a pre-configured slave address in two ways:
• on power up - the master will only connect to one fixed slave address and if the
connection link fails, the master will continuously try to make a new connection, or
• it can be “commanded” by its host device using AT commands - the host device can
control the master 455U-D to “dial” a slave address, connect, transfer data, and then
disconnect (or “hang up”), and connect to a different slave address.
Once the communications channel has been established, the 455U-D unit will accept input
data and send radio messages with data. When a 455U-D 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.
There can also be up to five intermediate repeaters in the link. Each 455U-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.
Establishing a Communications Link
Master Unit Slave Unit
• Listen to ensure channel is clear
•
If clear, transmit “connect” message
• Radio TX LED flashes
----------------->
• Receives message
• Radio RX LED flashes
• Check system and destination
address
• If OK, set DCD LED and output
• Radio RX LED flashes
<-----------------
• If message OK, transmit back an
ACK message.
Radio TX LED flashes
• Acknowledgment received okay
communication link established
• Set DCD LED and output