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Navman 11 User Manual

Page 9

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MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.

Figure 3-1 Binary message header format

3.2 Binary message header

The binary message header format has been

modified slightly from the NavCore V format to

accommodate message logging requests. The

format of the new message header is shown in

Figure 3-1.

3.2.1 Message header word 1
Each input/output message starts with a

synchronisation word of the form 0x81FF

HEX

with

DEL (255 decimal) occupying the first eight bits

followed by the Start Of Header (SOH) (129

decimal) occupying the second eight bits of the

synchronisation word.

3.2.2 Message header word 2
Word 2 contains the numeric message ID. For

example, word 2 for Message ID 1000 would be:

or 0x03E8

HEX

.

3.2.3 Message header word 3
Word 3 contains the word count for the data

portion of the message. The word count does not

include the data checksum word. A zero data word

count indicates a ‘header-only’ message.

3.2.4 Message header word 4
The fourth word of the message header is a 16-bit

field allocated to protocol and message related

flags. These flag bits extend control over ACK/

NAK requests and implement message logging

requests. The zero’s represented in the word 4

field shown in Figure 3-1 are reserved bits and

should be set to zero within this word.

The ACK/NAK control mechanism gives the user

the ability to request either ACK or NAK, or both,

independently for each message request. The user

sets the request (R) bit and either the acknowledge

(A) bit or negative acknowledge (N) bit, or both, to

select the proper acknowledge behaviour. With this

approach, the user can configure requests only

to be NAKed, alerting the user when a problem

arises without incurring the overhead necessary to

continuously process ACKs.

The lower six bits of the flags word can be used

as an additional input identifier. This identifier

is not explicitly processed by the receiver; it is

echoed back, in the same location, as part of

the header in ACK/NAK responses. This feature

allows the user to uniquely distinguish which input

message an acknowledgement corresponds to

when multiple input messages with the same

message ID were processed during a particular

period of time. The flags word now supports

message logging requests. The connect (C) and

disconnect (D) bits are used to enable and disable,

respectively, message outputs, and can be used

either independently or in conjunction with the log

request bits.

A ‘header-only’ message, with a message ID and

the connect bit set, enables the specified message

with existing timing characteristics. Likewise, a

header-only message, with message ID and the

disconnect bit set, disables the specified message.

Figure 3-2 Standard log request message

format (data portion)

A message with both connect and disconnect

bits is ignored. Note that enabling and disabling a

message does not modify its timing characteristics

(trigger, interval, or offset). A log request with the

connect bit set will set up the message’s timing

characteristics and then enable the message.

Similarly, for a combined log and disable request,

the message will be disabled after the timing

characteristics are set. To disable all messages,

set the message ID to FFFF

HEX

(all bits set) and set

the disconnect (D) bit.

Setting the query (Q) request bit will output the

message specified by the message ID one time

High Byte

Low Byte

0000

MSB

0011

LSB

1110

MSB

1000

LSB

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