User's Manual

Chapter Three Operation
Man_455U Rev 1.0 Page 25
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.
• No ACK received
• Retry up to four times
----------------->
• If no ACK message received
after five attempts
• “NO CARRIER” message sent
to host
• DCD signal and DCD LED reset
There are several configurations for Acknowledged mode:
3.6.1 Auto-Connect Acknowledged mode
Auto-connect mode is similar to “fixed line” modem operation. The master unit wants to
connect to only one slave address. Normally the slave address is preconfigured. On power
up, the master unit continuously tries to connect to the slave address. The slave unit, on
power up, does not try to connect but waits for a connect message. Once a connection is
made, the DCD status at both ends is set and data can be transferred in either direction. If
there is no data transfer within the preconfigured “link check” time, the master will send a
connect message to check the communications link.
The connection will be broken if a communication failure occurs, or if one of the hosts issues
a “disconnect” AT command (although this would be unusual for this mode). When the
master is disconnected, it will continuously try to connect to its auto-connect address.
Up to five repeater addresses may be configured as part of the auto-connect address. That is,
up to 5 intermediate modules can act as repeaters between the master and slave modules.
3.6.2 Low Power Auto-Connect Mode
For special installations, the 455U may be operated in a low power condition where it
switches off its receiver - power consumption is reduced to approx 30% of normal. The low
power condition will occur if the 455U is configured for auto-connect mode AND if the low
power mode feature is configured (AT&D). The DTR signal on the RS232 port will control
switching between low power and normal operation. When DTR is “low”, the module will
change to low power operation, reset any connected link and switch off its receiver. When
DTR goes “high”, the module will revert to normal operation and will auto-connect to its
slave address.
The use of this low power operation may be applicable in remote locations where there is a
limited power supply such as solar panels. In this situation, the DTR signal from the host