Elpro Technologies 905U-D User Manual download pdf (Page 24)

905U-D Radio Modem User Manual

Man_905D_1.10.doc Page 24

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 905U-D unit, it will return an acknowledgment message. Both units will

activate their DCD LED, and also activate their DCD signal. If the master unit does not receive

the acknowledgment, the DCD output will reset. When the connection is made (DCD set), the

905U-D units can transmit data to each other.

Once the communications channel has been established, the 905U-D unit will accept input data

and send radio messages with data. When a 905U-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.

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.

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Elpro Technologies 905U-D User Manual Page 24

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