Most packets both to and from the control panel use the following format:

NNMSD...OO CC (CR-LF)

• NN is the length of the packet including all characters except Length and CR-LF at the end of the packet. Legal values are ASCII hex 00 to FF. Permissible characters are ASCII 0-9 and upper case A-F.
• M Message/packet type ID. These are upper and lower case alpha characters. Upper case is used for responses from the control panel and lower case for commands to the control panel. Allowed values are a-z and A-Z
• S Sub-message/packet type. These are upper and lower case alpha characters. Upper case is used for responses from the control panel and lower case for commands to the control panel. Allowed values are 0-9, a-z and A-Z.
• D... 0 or more ASCII characters of data associated with the command/packet type. Any printable ASCII character is permitted.
• 00 Reserved for future development. The documentation indicates that "The only currently legal character is 0 (ASCII zero)" - and while this is mostly true, there are some cases where the reserved characters are used (i.e. ArmingStatusReport and UserCodeChangeReply and cases where it is not part of the packet at all (AlarmMemoryUpdate).
• CC 2-digit checksum. This is the hexadecimal two’s complement of the modulo-256 sum of the ASCII values of all characters in the message excluding the checksum itself and the CR-LF terminator at the end of the message. Permissible characters are ASCII 0-9 and upper case A-F. When all the characters are added to the Checksum, the value should equal 0.
• (CR-LF) Message terminator. Either CR or LF, or both CR-LF.

The M and S value combined are generally referred to as the "command".

see

Elk M1 RS232 Protocol Specification, Section 4

see

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