Berkeley Varitronics Systems Fox Manual page 39

Binary Data Sequence: Variable number of bytes of binary data dependent on the command type.
Checksum:
C - The exclusive-or of all bytes after the @@ and prior to the checksum.
Message Terminator:
<CR><LF> - carriage return line feed denoting end of the binary message.
Every ONCORE Receiver input command has a corresponding response message so you can verify that the input commands
have been accepted or rejected by the ONCORE Receiver. The message format descriptions detail the input command and
response message formats. Information contained in the data fields normally is numeric. The interface design assumes that
the operator display is under control of an extemal system data processor and that display format and text messages reside in
its memory. This approach gives you complete control of display format and language. The ONCORE Receiver reads the input
command string on the input buffer once per second. If a full command has been received, then it operates on that command
and performs the indicated function. The following logic relates to the i nput character string checks that are performed on the
input commands:
A binary message is considered to be received if:
(1) It began with @@ and is terminated with a carriage return and a line feed
(2) The message is the correct length for its type
(3) The checksum validates
You must take care in correctly formatting the input command. Pay particular attention to the number of parameters and their
valid range. An invalid message could be interpreted as a valid unintended message. A beginning @@, a valid checksum, a ter-
minating carriage return line feed, the correct message length and valid parameter ranges are the only indicators of a valid input
command to the ONCORE Receiver. For multiparameter input commands, the ONCORE Receiver will reject the entire command
if one of the input parameters is out of range. 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
1/0 message format descriptions. Polarity of the data (positive or negative) is described via two's complement presentation.
Once the input command is detected, the ONCORE Receiver validates the message by checking the checksurn byte in the mes-
sage. Input command messages can be stacked into the ONCORE Receiver input buffer, up to the depth of the message buffer
(2048 characters long). The ONCORE Receiver will operate on all full messages received during
the previous 1 second interval and will process them in the order they are received. Every input command has a corresponding
output response message. This enables you to verify that the ONCORE Receiver accepted the input com-mand. The ONCORE
Receiver response message to properly formatted commands with at least one out-of-range parameter is to return the original
nonchanged value of the parameter(s). Input commands may be of the type that change a particular configuration parameter of
the ONCORE Receiver. Examples of these input conunand
types include commands to change the initial position, the ONCORE Receiver internal time and date, satellite mask angle,
satellite almanac, etc. These input commands, when received by the ONCORE Receiver, change the indicated parameter and
result in a response message to show the new value of the particular parameter. If the new value shows no change, then the
input command was either formatted improperly, or the parameter Was Out of its valid range.
Input commands may be of the type that enable or disable the output of data or status messages. These output status messages
include those that the external controller will use for measuring position, velocity, time, pseudorange, and satellite ephemeris
data. Status messages are output at the selected update rate (typically, once per second) for those messages that contain posi-
tion, velocity, time, or range data, or can be commanded to output the data one time upon request. Those messages that include
slowly changing data, such as satellite ephemeris data, satellite visibility tables, xDOP tables, etc., are output once when the
ONCORE Receiver detects a change in the data from the previous output data. For example, if the user enables the ONCORE
Receiver to output ephemeris data, the ONCORE Receiver will output the ephemeris data once upon receipt of the input com-
mand, and then once upon detection of the change of the ephemeris (typically once per hour).
All of the Position/ Status/Da ta message types can be selected independently to be output in a continuous fashion (at the
selected update rate), or once each time the data is requested (polled). The rate at which the data is output in the continuous
output mode is dependent on the type of data in the message. The Data Message Output Rates table shows the rates at which
the data messages are output for each type of message, depending on the setting of the continuous/ one-time option that is part
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