SL869 SW User Guide
1VV0301002 Rev.6 – 2013-10-1
Reproduction forbidden without written authorization from Telit Communications S.p.A.- All Rights Reserved.
Page 9 of 25
Mod. 0806 2011-07 Rev.2
2.
Communication Interface
The serial communication interface between the SL869 receiver module and the Host
processor is based on the NMEA-0183 protocol standard specified by the National Marine
Electronics Association (NMEA). This is an ASCII-based standard that is widely used in the
GPS industry for serial communication with GPS receivers.
2.1.
UART
Serial communication with the SL869 is conducted over the UART port, which is assigned to
Pins 20 (Tx) and 21 (Rx). There is no hardware flow control. The default port settings are:
9600 Baud
Eight data bits
No parity bits
One stop bit
Note that manual user interaction with the SL869 can be achieved using a PC terminal
emulator.
2.2.
NMEA Characteristics
This subsection highlights characteristics of the NMEA-0183 protocol as they pertain to the
SL869 interface.
Start And Termination
An NMEA data packet is transmitted as an ASCII string beginning with a “$” character, but it
is terminated with <carriage return> <line feed> character sequence. The hexadecimal
representation of this sequence is 0x0D 0x0A.
Proprietary Packets
Proprietary data packets are allowed by the NMEA protocol standard. They begin with “$P”
followed by a Manufacturer’s Mnemonic Code that is assigned by the NMEA. The SL869
module is based on the STA8088 device from ST Microelectronics, who has been assigned
the code “STM.” Therefore, proprietary packets used by the SL869 begin with the character
sequence “$PSTM.”
Checksums
The NMEA standard specifies a two-character checksum field that follows a ‘*” delimiting
character placed at the end of the ASCII data string. The checksum is calculated as the 8-bit
exclusive-OR (XOR) of all characters in the string, excluding the “$” and “*’ delimiters.
The SL869 includes checksums on all output NMEA messages. With one exception,
checksums are not required on input messages and if present are ignored by the SL869.
