Chapter Three
Operation
Man_805U Rev 1.8
Page
19
3.2.3
Radio Data Rate
The data is transmitted by radio as direct modulated synchronous data at 19200, 38400 or
76800 bits/second. The user must configure the radio data rate at each 805U module. The
configured radio data rates must be the same for each module in a system.
At 19200 and 38400 bits/sec, the 805U adds forward error correction in the transmitted data.
This is added automatically and is different to the configurable CRC error-check.. The radio
range at 19200 baud is better than 38400 or 76800. The expected range at 38400 is 60% of
maximum and at 76800 is 30%.
The radio message includes the following :-
•
A 5 - 10 msec leading sequence of alternating 1’s and 0’s provides the receiving unit
with time to capture and lock onto the incoming signal.
•
A system address is superimposed on each message to provide discrimination between
different 805U systems on the same radio channel. Each 805U unit in the same system must
be configured with the same system address - refer Section 4, Configuration. Although
other 805U modules may hear the radio transmissions, because they have a different system
address, the radio transmission is ignored and no serial data is output.
•
In transparent mode, a group address is included, and in controlled mode, unit
addressing is included.
An error-check (16 bit CRC) may be configured by the user.
Up to 530 bytes of data may be transmitted in a message - the maximum message size is
configurable between 10 and 530 bytes. The data consists of a sequence of 8 bit bytes. Start,
stop and parity bits are not transmitted, but they are re-generated at the receiving unit (if
configured).
A “transmit delay” time and a “receive delay” time may also be configured. These parameters
may be used to fine tune and give priority to different 805U units in a system.
•
After each message is transmitted, a 805U unit will not transmit another message during the
transmit delay time. This could be used to allow a reply message to be received before the
next message is sent.
•
After a message is received, a message will not be transmitted during the receive delay
time. This could be used to delay a reply message until other messages have been sent.
3.3
Addressing
A 805U 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
