In command
mode, the modem responds to received commands. In data mode, the modem
transmits and receives data over the phone lines. To switch from data mode to
command mode, a computer sends an escape sequence. The sequence is typi-
Formats and Protocols
25
cally ???+++??? sent with specific timing requirements to minimize the chance of
switching in error if transmitted data happens to contain the sequence. The
modem responds with ???OK???. Switching from command mode to data mode is
automatic after some commands, or a computer can send the ???ATO??? command
to request a switch.
Many global positioning system (GPS) devices have either an RS-232 port or a
USB port that functions as a virtual COM port. These devices typically communicate
with PCs using protocols defined in the NMEA 0183 Interface Standard
available from www.nmea.org.
The standard specifies parameters for serial communications and a format for
transmitted data. The communication parameters are typically 4800 bps,
8-N-1. (A high-speed addendum to the standard supports communications at
38,400 bps, and some devices support additional bit rates. Devices with USB
virtual COM ports don??™t use the serial parameters.)
The data is plain text, 8 bits per character. The devices transmit the data in
blocks called sentences. Standard sentences use this format:
In addition, vendors can define proprietary sentences, which begin with ???$P???
followed by a 3-letter manufacturer ID, vendor-specific data, and CR and LF
codes to end the command.
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