User manual

805U Radio Modem Module User Manual
Page 22 © April 2011
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 805U unit, it will return an acknowledgment message. Both units will
activate their DCD LED, and also activate their DCD output signal. If the master unit does
not receive the acknowledgment, the DCD output will reset. When the connection is made
(DCD set), the 805U units can transmit data to each other.
Once the communications channel has been established, the 805U unit will accept input data
and send radio messages with data. When a 805U 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 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.
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
•
If the source unit does not receive an ACK message, it will re-transmit the same message. It
will attempt to transmit the message up to five times. If the unit still does not receive an ACK
message after five attempts, it will reset the DCD LED, and reset the DCD output.
During normal operation, if there has been no radio activity for a period (called the “link
check” period), the “master” unit will transmit the “connect” message to check the radio path.
The link check period is a time configured by the user.