Bosch DCN Next Generation Instrukcja Użytkownika Pobierz pdf (Strona 21)

DCN Next Generation Open Interface Release 2.4 General

en | 21

Bosch Communications Systems | 2007 February | SRS_INF | Du010933

4.1.2.3 Escape Byte handling

To identify the header uniquely within a stream of bytes is difficult, because the data may also

hold the same sequence as the header. Therefore an escape byte is used to make the

header unique. The escape byte holds the value 0xC8. Because the data to be transmitted

could also contain the escape value, this implies that the data must be scanned to translate

the escape value into an escape sequence.

The ‘full’ feature protocol uses the 0xC8 byte value as escape sequence. This escape

sequence is always followed by another character-byte, which informs the actual function of

the bytes. The following character-bytes (header type) are defined:

0xC8 The 0xC8 byte is defined. This sequence is mostly used within the

message if the message contains the 0xC8-byte (converted during

message preparation).

‘$’ The escape sequence defines the header of a new data packet. The

header will be followed by a message and a checksum section.

‘@’ The escape sequence defines a acknowledge packet

‘#’ The escape sequence defines a negative acknowledge packet

‘?’ The escape sequence defines a heartbeat packet

others.... Invalid escape sequence. The packet is assumed to have errors and

therefore is ignored (after transmission of a NAK-message).

Note: The byte extension used for the escape-byte is not used for the checksum calculation.

The escape sequences should be first detected, solved and then the checksum should be

calculated.

4.1.2.4 Protocol handling

The communication between the CCU and the remote controller is done using a RS-232

interface. The data is checked for errors using the checksum received and an acknowledge

message is sent according to the result.

Figure 3 shows the state transition diagram of the ‘Full’ feature protocol. Note that in the

figure the heartbeat mechanism is not present. The heartbeat time-out can be seen as an exit

from the state transition diagram.

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Bosch DCN Next Generation Instrukcja Użytkownika Strona 21