User's Manual

455U Radio Modem Module User Manual
Page 26 © November 2004
device is used to “wake-up” the 455U unit. The 455U unit will then operate normally until
the DTR signal is reset by the host device.
3.6.3 Single-Connect Acknowledged mode
Single-connect is similar to a dial-up modem. In this mode, the module will only make one
attempt to connect to a destination address. A host device will issue a connect (ATD)
command, and the module will send a connect message to the destination address. If the
destination module responds, then the link is connected and the DCD status is set (LINK LED
comes on, DCD signal and DIO signal turn on if configured). If the destination module does
not respond, then the link is not established and no data can be transferred. The module will
not attempt to reconnect again until it receives another connect command from its host.
The link is normally disconnected by the host issuing a disconnect command. In this mode, it
is normal for a host to connect and disconnect to several slave modules using AT commands.
A common example is a PC polling several analyzers or dataloggers. The PC connects to a
remote station one at a time, transfers data, disconnects and connects to the next station.
Up to five repeater addresses may be configured as part of the remote address. That is, up to
5 intermediate modules can act as repeaters between the master and slave modules.
The RS232 DTR signal can be configured to act as a disconnect signal. If so configured
(AT&D), the DTR signal will force a hang-up (DCD reset) if the host device resets the DTR
signal.
3.7 What Operating Mode to Use ?
3.7.1 Unacknowledged or Acknowledged mode?
Unacknowledged mode provides simpler operation as the units do not acknowledge
transmissions received. However confirmed operation in unacknowledged 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 unacknowledged
mode.
The RS-485 port only operates in Unacknowledged mode. It is possible for the RS-485 port to
be operating at the same time as the RS-232 port is sending data to another location, using either
Acknowledged or Unacknowledged mode.
Acknowledged mode is suitable for point-to-point RS-232 links. For multi-point networks, a
“master” host device must control connecting and disconnecting to remote units.
It is possible to configure different units in the same system with different operating modes,
however this requires care. A 455U unit configured in one mode will act as a repeater for
messages sent between two 455U units configured in the other mode, and it is always possible
to send Unacknowledged mode messages to the RS-485 port on a module regardless of the
configuration of the RS-232 port.
3.7.2 Error Check ?
Error-checking may be configured in both unacknowledged and Acknowledged mode. When
the error-check is configured, a 16-bit CRC (Cyclic Redundancy Check) attached to the end of