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4.3
SSI TRANSACTIONS
General Data Transactions
ACK/NACK Handshaking
If ACK/NACK handshaking is enabled, all packeted messages must have a
CMD_ACK or CMD_NACK response, unless the command description states
otherwise. This parameter is enabled by default. This handshaking should remain
enabled to provide feedback to the host. Raw decode data and WAKEUP do not use
ACK/NACK handshaking since they are not packeted data.
Following is an example of a problem that can occur when ACK/NACK handshaking
is disabled:
•
The host sends a PARAM_SEND message to the reader to change the
baud rate from 9600 to 19200.
•
The reader cannot interpret the message.
•
The reader does not implement the changes requested by the host.
•
The host assumes that the parameter changes have occurred and acts
accordingly.
•
Communications are lost because the change did not occur on both sides.
If the ACK/NACK handshaking is enabled, the following occurs:
•
The host sends a PARAM_SEND message.
•
The reader cannot interpret the message.
•
The reader CMD_NACKs the message.
•
The host resends the message.
•
The reader receives the message successfully, responds with CMD_ACK,
and implements parameter changes.
SIMPLE SERIAL INTERFACE
79
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