Product manual

CAD Interface Installation
66 025-9158L
Note
In order for CAD to generate a checksum, it must add all of the
bytes after the STX up to but not including the ETX. The
resulting least significant eight bits is then complemented, one is
added and this checksum is placed at the end of the message.
This results in a CKSUM value, when added to the sum of the
Low Level Sequence number and all the data bytes between STX
and ETX, modulo 256, will equal zero.
Timing
A time-out period of 1 second should be used for receiving the Low Level ACK. The
response time for a High Level ACK/NAK is significantly longer since it depends on the
configuration of the Model 26, which command is sent, and if a response from the Model
6 is required. The high-level response time depends on the type of system and with
communication errors and retries, an RF system could take up to 15 seconds to send high
level responses on some commands. The CAD system must wait for the high level
response before sending the next command. If a time out does occur, the CAD system
should retransmit the same command.
Protocol Flow
Power on Initialization
When each side powers on and initializes, the following sequence will occur so that all of
the LLSEQ#s can be initialized. It does not matter which side initializes first because the
transmit and receive tasks should be independent and concurrent at the low level. It also
does not matter whether 00 or FF is used as the initial LLSEQ# since the receiving end
will synchronize on whatever is sent in the NULL message.
Time Transmitted from CAD to M26 Transmitted from M26 to CAD
t0 [Power On]
t1 NULL FF>
t2 [No Response for 2 seconds]
t3 NULL FF>
t4 [Power On]
t5 <NULL 00
t6 LLACK 00>
t7 NULL FF>
t8 <LLACK FF