User's Manual

Chapter Three Operation
Man_905UD_2.0.doc Page 23
If error-checking is selected, then the destination module will only return a ACK if the error
check is valid. If the error check is not valid, then a NACK message is returned. If the error-
checking function is not selected, then the destination module will return an ACK every time
it receives a message (a NACK is never sent). Each repeater will re-transmit the ACK or
NACK message until it is received by the source module.
The source module will calculate a waiting time for the acknowledgment - this time is based
on the radio baud rate, the message length (number of bytes) and the number of repeaters.
If the source module receives a NACK, or it does not receive an ACK within the waiting time,
it will transmit the same message again. The 905U-D will transmit the message up to five
times. If it does not receive an ACK after the 5
th
attempt, it will assume communications
failure and reset its DCD status. The unsuccessful message will not be re-transmitted and
will be lost.
Repeater Communications
Successful
Unit A
Source
• Transmit
message
Receive ACK
--->
<---
Unit B
Repeater
• Receive
message
• Re-transmit
message
• Receive ACK
• Re-transmit
ACK
--->
<---
Unit C
Repeater
• Receive
message
• Re-transmit
message
• Receive ACK
• Re-transmit
ACK
--->
<---
Unit D
Destination
• Receive
message
• Transmit
ACK
• Output data
Repeater units may also act as destination addresses for some transmissions and output
data to a host device.
3.5 What Operating Mode to Use ?
Transparent or Controlled Mode?
Transparent mode provides faster operation as the units do not acknowledge transmissions
received. However reliable operation in transparent mode will only occur if the host devices
check the messages and return acknowledgments. Generally, if a device is able to operate
on a RS485 multi-drop serial link, it is suitable for transparent mode.
Controlled mode is suitable for point-to-point links, rather than multi-point networks.
If it is not known whether a device is suitable for transparent mode, then controlled mode
should be used. If faster operation is desired, then transparent mode can be tried, and if the
system operates reliably, then transparent mode is suitable.