Motorola M12 Oncore User's Guide download pdf (Page 39)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

EXCLUSIVE-OR CHECKSUM CREATION

This application note describes the procedure to calculate checksums used in the

serial messages of the Oncore GPS receivers. An example message is used to

illustrate the procedure.

Command name: Position/Status/Data Output Message (eight channel)

Command in Motorola binary format: @@EamC<CR><LF>

In this message, ‘m’ indicates the response message rate (ie. 1 = once per second, 2

= once every two seconds, etc.), and ‘C’ is the checksum. In calculating the

checksum, only ‘Eam’ are used. The exclusive-or operation yields a one if only one of

the bits is a one. Setting ‘m’ to ‘1’, we have the following:

Character

Hexadecimal Binary

E 45 01000101

a 61 01100001

Xor 24 00100100

1 01 00000001

Xor 25 00100101

The final checksum would then be ‘25’ in hexadecimal. The complete command

would then be as follows:

Message format @ @ E a m C <CR> <LF>

Hexadecimal 40 40 45 61 01 25 Od Oa

ASCII @ @ E a ^A % ^M ^J

To enter this command using the PC controller software type: @@Ea01

Within the PC controller software, the characters beyond the fourth character are

treated as hexadecimal numbers and the checksum is computed automatically. The

receiver will now report the eight channel position/status/data message every

second. Note that this is equivalent to entering the ‘ps8 1’ controller command.

The checksum for the ASCII Position Message (@@Eq) is computed in a similar

manner. The 8-bit checksum is converted to a decimal value between 0 and 255 and

sent.

NMEA checksums, which are optional, are also calculated in a similar manner. The 8-

bit representations of all the characters between but not including the starting ‘$’ and

the ending" are used. The high and low four bits of the eight bit checksum are

converted to and sent as ASCII characters.

Revision 5.0 08/30/02

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Motorola M12 Oncore User's Guide Page 39