Elpro Technologies 905U-D User Manual download pdf (Page 21)

Chapter Three Operation

Page 21 © January 2011

3.3 Addressing

A 905U-D network comprises modules with the same "system" address. Only modules with the

same system address will communicate with each other. This feature allows more than one

system to operate in the same area on the same radio channel. We recommend that you select a

random number for the system address.

In transparent mode, each module is also configured with a “group” address. A system may

comprise several groups or sub-systems. Only modules with the same system and group address

will communicate directly with each other - but modules with different group addresses can

communicate via a repeater. The group address is used for repeater operation in transparent

mode. The group address is not used in controlled mode.

A 905U-D can also be configured with a “unit” address - this gives the module a unique

identification. The unit address feature is not used in transparent mode, but it is used in

controlled mode.

The system address also determines the “hop-set” for the spread spectrum radio. The radio

continually changes frequency according to a pre-set pattern or hop-set. There are 8 hop-sets.

The last three bits of the 8-bit system address determines which hop-set will be used. In

USA/Canada, there is effective radio isolation between hop-sets 1 – 4 and hop-sets 5 – 8. This

means that if you have two systems in the same radio range, you can select system addresses

such that the two systems will not use the same radio channels at the same time. If you use

system addresses that are exactly different by 4, then you can be sure of this. Systems with

system addresses 127 and 131 will never use the same radio channels at the same time.

Also refer to section 3.9 covering radio interference topics.

3.4 Transparent Mode

In transparent mode, radio messages are transmitted without unit addressing. Units do not

provide handshaking functions to control the flow of data. Every unit which receives the radio

message, and has the correct system and group addresses, will output the data. Transparent

mode operation is effectively a broadcast system.

Data received at the serial port is transmitted out of the radio port. Data received from the radio

is transmitted out of the serial port. Prior to transmitting, units will listen to the radio channel to

ensure that it is clear - units will hold off from transmitting until the radio channel is clear. At

the RS232 port, the CTS pin can be configured to go high while there is space in the input data

buffer - otherwise it is always high.

Host devices should provide a suitable protocol to ensure that error checking, handshaking and

implementation of an appropriate re-transmission scheme is provided. This mode of operation is

particularly suited to devices designed to operate over a multidrop network, such as PLC systems

designed for operation over a RS-485 network.

The time taken to transmit a message is :-

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Elpro Technologies 905U-D User Manual Page 21