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805U Radio Modem Module User Manual

Page 20 © April 2011

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 :-

lead-in system/group addr data error check (if configured)

|---------------|-----------------|------------------------------------------|--------------|

5 msec 2 bytes 0.5ms per byte 2 bytes

1 ms @19.2Kb @ 19.2Kb 1ms @ 19.2Kb

If error checking is not configured at the receiving unit, data will start to be output

approximately 1 msec after the system address has been received. If error checking is

configured, data will be output approx 2msec after the end of the message. For example, a

message with 20 bytes of data transmitted at 19200 bits/sec will begin to be output approx

12msec after the data is input, if there is no error checking, or 24msec after the data is input if

error checking is configured.

3.5 Transparent Mode Repeaters

A module can be configured as a transparent mode repeater. In this mode, the module will

operate as a normal transparent mode module, plus it will also re-transmit any radio messages

that it receives. However it will change the group address in the re-transmitted message. The

transparent mode repeater is configured with two group addresses. When it receives a radio

message with one of these group addresses, it will re-transmit the message using the other

group address (assuming that the system address is correct).

One of the group addresses will be its normal or “first” group address. If a transparent mode

repeater receives a message on its first group address, it will output the data on its serial port,

and will also re-transmit the message using its second group address. That is, the re-

transmitted message will now have the second group address. Only 805U modules configured

with the second group address will accept the re-transmitted message.

If a transparent mode repeater receives a message on its second group address, it will re-

transmit the message on its first group address, but it will not output the data on its serial

port.

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ELPRO 805U Bedienungsanleitung Seite 19