Bosch DCN Next Generation Instrukcja Użytkownika Strona 25

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DCN Next Generation Open Interface Release 2.4 General
en | 25
Bosch Communications Systems | 2007 February | SRS_INF | Du010933
The ‘simple’ feature protocol has also limitations, which are:
There is no communication check. A link-failure cannot be detected by the CCU
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.
There is no acknowledgement of the packets received (i.e. no ACK-packets).
The CCU does no retransmission of messages. The NAK-messages are ignored.
The header structure is not uniquely identified.
When the CCU receives a packet with an incorrect checksum, the CCU transmits a NAK-
packet back to the remote controller (The NAK-packet is defined as a message type, which is
described in §4.1.3.4). The remote controller then knows that his packet is received with
errors and can transmit the packet again.
Note that the CCU keeps no history about the packets sent to the remote controller. This
implies that the CCU cannot handle any NAK-packets sent by the remote controller and will
therefore ignore the received NAK-packets.
4.1.3.1 Timing values
This section presents the different value and time limits needed for handling the ‘Simple’
protocol.
Description Value
The maximum delay between two consecutive bytes of a message 50 ms
The maximum message data length 5000 bytes
Typical remote function execution time (exclusive the time needed for
transmission of the data).
< 0,5 sec.
4.1.3.2 Header format
The header format has the following layout:
ESC ‘$’ CR
The header has a fixed length and forms a leader to recognize the start-point of a message.
Note that all fields are defined as single bytes.
4.1.3.3 Checksum calculation
The packet is terminated with a checksum byte, which holds the checksum over all data
inside the header and message. The Checksum is calculated using the following sequence:
Sum all bytes over the header and the message.
Take modulo 256 of the calculated checksum and do a bitwise invert of the checksum.
This calculated checksum is sent along with the packet. The receiver executes the same
calculation and verifies the calculated checksum with the received checksum.
4.1.3.4 NAK Message definition
The ‘simple’ feature protocol has no special packets for sending the negative acknowledge.
Therefore the NAK is implemented as a special message type. The total packet format
follows the layout as described in §4.1.1.
The type present in the message defines the negative acknowledge and has the value
MDSC_NAK. The data-bytes are not used for the NAK message, which implies that the
length is set to zero.
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This implies that the CCU remains in the remote controlled state when the communication fails.
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