User's Manual

905U-D Radio Modem Module User Manual
Page 24 © May 2000
Unsuccessful Communications
Source Module Destination Module
• Listen to ensure channel is clear
• If clear, transmit message
• TX LED flashes
------------------->
• Receives message
• RX LED flashes
•
Check system and destination
address
•
If incorrect, transmit no
message and no serial output.
• Radio RX LED flashes
<-------------------
•
If addresses are correct,
check error-check.
•
If incorrect, transmit a NACK
message
• TX LED flashes
•
NACK message or no message
received
• Retry up to four times
------------------->
•
If no ACK message received
after five attempts
• “Comms fail” output activated
• DCD signal and DCD LED reset
The time taken to transmit a message is :-
|------------------HEADER--------------------|--------------------DATA-----------|
lead-in system unit repeater dest. zero data error check (if configured)
40msec address addr addrs's addr byte
No. of bytes 2 1 0 - 5 1 1 no. of data bytes 2
The time for each HEADER byte is 3.8mSec @4800 bits/sec, and the time for each DATA
byte is 1.9msec @ 4800 bits/sec.
If error checking is not configured at the receiving unit, data will start to be output approx
1msec after the "zero" byte 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 4800 bits/sec with no repeaters, will be output approx 88msec after the
data is input, assuming that error checking is configured.
The time between transmissions is set by the transmit and receive hold-off times configured
by the user.
Intermediate Repeaters
Where intermediate repeaters are configured, each repeater will re-transmit the message
onto the next address. When the destination address is reached, the destination module
will return an acknowledgment (ACK) with the reverse address structure as the received
message.