User manual

Chapter One Introduction

Man_805U Rev 1.9 Page 9

1.3 Controlled Mode

“Controlled mode” provides “point-to-

point” communications similar to telephone

modems. In controlled mode, the flow of

data is controlled by the 805U units. Each

805U 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 transmit the

data up to five times, until an acknowledgment (ACK) is received. If an acknowledgment is

still not received, then the DCD signal on the RS232 port will be reset and a alarm message

can be sent to the host via the serial port.

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

805U modules.

In controlled 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.

1.4 Repeater Units

A 805U unit may be used as a repeater to re-transmit radio messages. The purpose of a

repeater unit is to extend radio range.

PLC

805U

RS485

PLC

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PLC

HOST

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DATA

HOST

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NO DATA

HOST

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DATA