User's Manual

455U Radio Modem Module User Manual
Page 20 © November 2004
3.2.3 Radio Data Rate
The data is transmitted by radio as direct modulated synchronous data at 1200, 2400 4800 or
9600 bits/second. The user must configure the radio data rate at each 455U module. The
configured radio data rates must be the same for each module in a system. Radio data rate is
set via the ATB command.
3.2.4 Radio Message
The radio message includes the following :-
• A 20 mSec leading sequence of alternating 1’s and 0’s provides the receiving unit
with time to capture and lock onto the incoming signal (Configurable for systems using talk-
through repeaters).
• A system address is superimposed on each message to provide discrimination between
different 455U systems on the same radio channel. Each 455U unit in the same system must
be configured with the same system address - refer Section 4, Configuration. Although other
455U modules may hear the radio transmissions, because they have a different system
address, the radio transmission is ignored and no serial data is output.
• Addressing for the sending unit, any repeater units, and the final destination unit
indicates where the message is to be sent and how to get there.
An error-check (16 bit CRC) and security encryption (AES128) may be configured by the
user. Error checking is configured with the AT&E command, Data Encryption is configured
with the AT#E and AT#S commands.
Up to 1024 bytes of data may be transmitted in a message - the maximum message size is
configurable between 4 and 1024 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).
The time taken to transmit a message is :-
Section Item Number Time (At 9600 Baud)
PEAMBLE Lead-in 20 mSec Default
(Configurable)
20 mSec
HEADER System Address 2 Bytes 2.08 mSec
Header Control 2 Bytes 2.08 mSec
Source Address 1 Byte 1.04 mSec
Destination Address 1 Byte 1.04 mSec
Intermediate
Addresses
1 Byte per address 1.04 mSec for each
intermediate address
Header Error Check 1 Byte 1.04 mSec
ENCRYPTION Initialisation Vector 16 Bytes (if data
Encryption is enabled)
16.64 mSec (If
configured)
DATA Message Data Data Bytes 1.04 mSec x no of
data bytes
CHECK CRC Error Check 2 bytes (If CRC
checking is configured)
2.08 mSec (If CRC
configured).