User's Manual

Chapter One Introduction

Man_455U Rev 1.0 Page 9

1.3 Acknowledged Mode

“Acknowledged mode” provides “point-to-

point” communications similar to telephone

modems. In Acknowledged mode, the flow

of data is Acknowledged by the 455U units.

Each 455U 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 source module does not receive a ACK, it will re-transmit the data. The source module

will attempt to re-transmit the data, until an acknowledgment (ACK) is received or the

configured number of re-tries has expired. If an acknowledgment is still not received, then

the DCD signal on the RS232 port will be reset and an alarm message can be sent to the host

via the serial port.

An example of an application using Acknowledged 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 455U modules.

In Acknowledged mode, the destination address may be pre-set, or set on-line by the host

device using “Hayes” commands. 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.

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