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algorithm” has to be used. Every time the Message field contains a byte equal to the START or the
STOP, the same byte must be preceded by a STUFF byte and incremented of an OFFSET. Every
time that in the Message field there is a byte equal to the STUFF byte it must be preceded by another
STUFF byte.
Values of START, STOP, STUFF and OFFSET bytes are shown in Table 4.1.
Name Value
START 0xab
STOP 0xcd
OFFSET 0x01
STUFF 0xef
Table 4.1
4.3.2 Message protocol
The message protocol is used by the Host to configure and to manage the SWS Gateway. It is an
acknowledged protocol, so that every time the Host sends a command to the Control Interface of the
Gateway, it will acknowledge the command with an ACK (Figure ).
Therefore there are two types of messages:
• Command/Response messages
• ACK messages
Gateway
Host
Command
Ack
Response
Figure 10
If the Command do not respect the protocol or if its parameters are out of range, the Control Interface
of the Gateway will send only a NACK (acknowledge with status set to ERROR) and no response.
Gateway
Host
Command
Nack
Figure 11
ACK and messages are:
2 bytes 1 byte
START Opcode Status STOP
Status message for ACK is 0x01, status message for NACK is 0x02.