User's Manual

905U-D Radio Modem Module User Manual
Page 10 © May 2000
1.3 Controlled Mode
In “controlled” mode, the flow of data is controlled by the 905U-D units. Each 905U-D unit is
configured with an address by the user, and a destination address for the data to be
transmitted to. Data is transmitted addressed to the destination module, and only this
module will output the serial data. The source module will add an error-check (16 bit CRC) to
the data transmitted by radio. The
destination module will process the error-
check, and if correct, it will transmit an
acknowledgment message (ACK) back to
the source module. If the error-check is not
correct, then the destination module will
transmit a “fail” message (NACK) back to
the source module. If the source module
receives a NACK return, or does not
receive any return within 1 second, it will
re-transmit the data. The source module
will attempt to transmit the data up to five
times, until an acknowledgment (ACK) is
received. If an acknowledgment is still not received, then a “communications failure” output
will be activated, and the source module will not accept any more input data from its host
device.
An example of an application using controlled
mode would be a radio modem link between
an intelligent gas analyser and a monitoring
computer system. Intelligent transducers do
not normally provide addressing or error
checking functions - these would be provided
by the 905U-D modules.
In controlled mode, the destination address
may be set by the host device by initially
sending a “Hayes” command to the 905U-D
module, or by on-board miniature switches.
Hayes commands are a standard set of commands used with conventional telephone
modems. An example of an application that would use Hayes command to set destination
addresses would be a central computer polling data loggers for periodic information.