COVER 1122660 - Caterpillar

zz is the number of bytes in the message after this byte. zz does not include the checksum. This value will change depending on the data that is associated with the IID.Checksum Calculation
The checksum is an important part of the M5X message. By using the checksum, the integrity of the message is determined. The second to last byte of every IID is the checksum, which is followed by an ASCII carriage return ($0D). The checksum is a 2's complement value of the summation of all the data bytes in the message. The message is a good message if all the bytes and the checksum add to zero (0).The following message is an example.5000240400580082AE<cr>The checksum for this message is AE. Although the message is sent in ASCII, the checksum must be calculated by using the hexadecimal value.
Table 3
Byte     ASCII Value     Hexadecimal Value    
50     $35 $30     $50    
00     $30 $30     $00    
24     $32 $34     $24    
04     $30 $34     $04    
00     $30 $30     $00    
58     $35 $38     $58    
00     $30 $30     $00    
82     $38 $32     $82    
Total         $152    
Truncated to LSB         $52    
2's Compement     $41 $45     $AE    
Total         $100    
Truncated to LSB         $00    CCM Heartbeat
You should check the connection to the CCM before you log in to the CCM. You should also check the connection (heartbeat) to the CCM during normal operation from time to time. The recommended PID is $F0 $12. Use PID $F0 $12 to read the current Security Level at a regular interval as a heartbeat. If the CCM does not answer to the read request, then there is a problem with the connection. This PID will also monitor the Security Level, which may have been changed.Security Levels
Every PID has an associated security level (0, 1, 2, or 3) within the CCM. A user within a particular security level may use PID's in that level or lower levels. PID's cannot be accessed by the user in levels higher than the password allows. For example, a user at security level 2 can access level 2, 1, and 0, but not level 3.The answering modem will be hung up if the password is not entered (sets the security level) within one minute. Specifically, if DCD is held low by the PC, and the security level is at 0 for more than one minute, DTR will be toggled by the CCM (answering modem hung up). Also, if the RS-232C cable is disconnected for more than five seconds, DTR will be toggled, and the security level will be set to 0.When power is removed from the CCM, the security level will be changed to 2, if the CCM was operating at security level 3.The following chart defines the PID's within each security level