Product manual

Protocol Level

Sequence Numbers, Escapes, and Checksums

Low Level Sequence Numbers (LLSEQ#)

The low level sequence number is one byte which is either 00 or FF hex. The Low Level

Sequence numbers are alternated between 00 and FF for each new packet sent. If a packet

needs to be resent because of an error, the same low level sequence number is used.

When a packet is received correctly, a Low Level ACK is returned with the same

sequence number as the received packet. If an error is detected, a Low Level ACK is

returned with the sequence number of the last correctly received packed.

The low level sequence numbers for transmit and receive are independent. The receive

low level sequence number should change only when packets are received and the

transmit low level sequence number should change only when packets are sent.

High Level Sequence Numbers (HLSEQ#)

The High Level Sequence Number included in the data of each of the Command packets

is incremented by the CAD each time it sends a new packet. The Model 26 then returns

that sequence number as part of the resultant status change message and in the High

Level ACK/NAK. It starts at 01 and increments up to FF (255) before rolling back to 1.

The Model 26 responses which are not associated with a specific CAD command will

always have the high level sequence number 00. Any commands generated at the Model

26 control panel that create a status message to CAD will use the 00 sequence number.

Escapes (DLE)

Since the data bytes being sent are binary rather than ASCII, it is possible that data bytes

may have the same value as control characters. To account for this, in that part of the

packet between the framing STX and ETX, a DLE (10 hex) character is inserted before

any data byte, which is equal to STX (02), ETX (03), or DLE (10). These DLE characters

are included in the checksum.

Checksums (CKSUM)

This is a one byte checksum value calculated by taking the 2’s complement of the sum,

modulo 256, of all the Bytes between (but not including) the framing STX and ETX. This

usually includes all of the Bytes from the LLSEQ# through the last Data Byte, and

includes any escape (DLE) characters which have been inserted.