24
M2303-00-P00-805
March 2003
9
CCDI Reference
The CCDI data protocol is used to send and receive commands from a CCDI capable modem in a radio.
The A76 modem in the remote AVL radio communicates with the despatcher radio using CCDI commands. To
send and recieve messages using the over-air CCDI protocol, the despatched radio must have a CCDI interface
(e.g. Tait Orca, Tait Radio Modem or T201x with an A75 fitted).
The over-air CCDI protocol is not described as the CCDI interface at the despatcher radio described in this
manual is the only way to communicate with the A76 radio.
CCDI Message Format
All CCDI message packets take the general form:
[IDENT] [SIZE] [PARAMETERS] [CHECKSUM] <CR>
Restrictions
•
Where numeric values are represented in ASCII-hex notation (two characters per byte), characters A to F
are upper case.
•
The minimum length of a command packet is 5 characters. For example q002F is the QUERY command
where [SIZE] = 00 as there is no [PARAMETERS] field required.
•
The maximum length of the [PARAMETERS] field is 42 characters. The maximum length of the command
packet is therefore 47 ([SIZE] = 2F) characters.
Calculating the CCDI [CHECKSUM]
Note:
This is not required if an AVL PC application such as is used to send commands.
[CHECKSUM] is calculated by applying the following algorithm:
1. Take the modulo-2 sum of all message bytes preceding [CHECKSUM].
2. Retain bits 0 to 7, discarding any higher order bits resulting from the summation.
3. Form the two’s complement of the remainder.
4. Convert the binary number into two ASCII-hex digits, MSD first.
Field
Description
[IDENT]
The message identifier. Identifiers are single ASCII characters (lower-case alphabetical)
which categorise the message type.
[SIZE]
The number of characters which make up the [PARAMETERS] field. [SIZE] is an 8-bit
number expressed in ASCII-hex notation (two characters).
[PARAMETERS]
An optional field, depending upon the command. Parameter values are generally char-
acter strings unless explicitly stated otherwise. Parameter type is dependent upon the
command - there is no explicit type definition.
[CHECKSUM]
An 8-bit checksum of the [IDENT], [SIZE] and [PARAMETERS] fields. Expressed in two
character ASCII-hex notation.
<CR>
The carriage return packet terminator.
