User`s guide

Chapter 3 - Receiver Descriptions
Binary Data Sequence:
A variable number of bytes of binary data dependent on the command type.
Checksum:
The Exclusive-Or of all bytes after the '@@', and prior to the
checksum.
Message Terminator:
'0x0D 0x0A' - Carriage Return/Line Feed pair denoting the end of the binary message.
Almost all receiver input commands have a corresponding response message so that you can
determine whether the input command(s) have been accepted or rejected by the receiver. The
message format descriptions in Chapter 5 detail the input command and response message
formats. Information contained in the data fields is normally numeric. The interface design
assumes that the operator display is under the control of an external system data processor and
that display and message formatting code reside in its memory. This approach gives you
complete control of the display format and language.
All M12+ receivers read command strings in the input buffer once per second. If a full command
has been received, the receiver operates on that command and performs the indicated function.
Input character string checks are performed on the input commands. A binary message is
considered to be valid if it began with the '@@' characters, the message is the correct length for
its type, the checksum validates, and the command is terminated with a CR/LF pair. Improperly
formatted messages are discarded.
You must take care in correctly formatting the input command. Pay particular attention to the
number of parameters and their valid ranges. An invalid message could be interpreted as a valid
unintended message. A beginning '@@', a valid checksum, a terminating carriage return/line
feed, the correct message length and valid parameter ranges are the only indicators of a valid
input command to the receiver. For multi-parameter input commands, the receiver will reject the
entire command if one of the input parameters is out of range. Once the input command is
detected, the receiver validates the message by checking the checksum byte in the message.
Input and output data fields contain binary data that can be interpreted as scaled floating point or
integer data. The field width and appropriate scale factors for each parameter are described in
the individual I/O message format descriptions. Polarity of floating point data (positive or
negative) is described via the two's complement presentation.
Input command messages can be stacked into the receiver input buffer up to the depth of the
message buffer (1200 characters long). The receiver will operate on all full messages received
during the previous one second interval and will process them in the order they are received.
Previously scheduled messages may be output before the responses to the new input
commands.
Almost all input commands have a corresponding output response message. Input commands
Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05
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