Product manual

Protocol Level

Data Packet

Data packets are packets with bytes embedded in the middle between the STX and the

ETX. Data packets from the CAD are called Command Data Packets, while data packets

from the Model 26 are called Response Data Packets. All Data Packets take the following

form:

Byte # 1 2 3 - - - - - - - - n n+1 n+2

STX LL SEQ # DATA BYTES ETX CHKSUM

Hex 02 00 or FF ~ ~ ~ 03 00 to FF

Low Level ACK Packet (LLACK)

Any packet received by either entity which is not itself an LLACK packet is responded to

with an LLACK. The LLACK packet confirms that the data packet received is valid. The

LLSEQ# takes the value of 00 or FF hex.

BYTE ACK LL SEQ #

Hex 06 00 or FF

Null Packet (NULL)

The NULL packet is transmitted at power on as a means of setting the receiving entity’s

receive LLSEQ# to some known value. Both the CAD and the Model 26 must make their

first transmission after reset a null packet (one with no data bytes) as shown here:

BYTE STX LL SEQ # ETX CHKSUM

Hex 02 00 or FF 03 00 or 01

Command Data Packets (From CAD)

The three Command Data Packets are Unit State Control (00), Station Relay Control

(01), and Station Full Status Request (02).

Byte 1 2 3 4 5 to n n+1 n+2

STX LLSEQ# HLSEQ# CMD# DATA BYTES ETX CHKSUM

Hex 02 00 or FF 01 to FF

00,01,02

See Note 1 03 00 to FF

Note 1: See Text - A Special Case on page 68