User's Manual

905U-D Radio Modem Module User Manual
Page 18 © May 2000
3.2.3 Radio Data Rate
The data is transmitted by radio as direct modulated synchronous data at a rate between
19200 and 115200 bits per second. The user must configure the radio data rate at each
905U-D module. The configured radio data rates must be the same for each module in a
system.
The radio message also includes the following :-
• A 30 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 905U-D systems on the same radio channel. Each 905U-D unit in
the same system must be configured with the same system address - refer Section
4, Configuration. Although other 905U-D modules will hear the radio transmissions,
because they have a different system address, the radio transmission is ignored and
no serial data is output.
• A “frame flag” appears once at the beginning of each message, and once at the end of
each message. It indicates the start and end of a message packet.
• The RTS status of the source unit is included in the message. In controlled mode only,
this is used to set the CTS signal at the destination module.
• In controlled mode, unit addressing is included.
• An error-check (16 bit CRC) may be configured by the user.
Up to 520 bytes of data may be transmitted in a message - the maximum message size is
configurable between 10 and 520 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. After each
message is transmitted, a 905U-D unit will not transmit another message during the transmit
delay time. After a message is received, a message will not be transmitted during the
receive delay time. These parameters may be used to fine tune and give priority to different
905U-D units in a system.
The default time of the transmit delay (40 msec) is selected for polling applications. If the
host device sends more than one poll command, the second poll message will be delayed to
allow a response to be received for the first poll message. The user can reduce this transmit
delay time if it is not required.