User manual

Chapter One Introduction

Page 9 © January 2011

Transparent mode is “point-to-multipoint” communications, suitable for a host device which is

able to communicate on a multi-drop “bus” type network. An example of an application is the

use of radio modems to extend a PLC RS485 network. The serial messages from the PLC’s

already include PLC addressing and error detection/correction to control the flow of data.

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 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

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 analyzer 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 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

PLC

905U-D

RS485

PLC

905U-D

PLC

HOST

905U-D

DATA

HOST

905U-D

NO DATA

HOST

905U-D

DATA