Rice Lake Motorola DS457 Laser Scanner - Manual (Page 129)

SSI Interface 8 - 3
SSI Transactions
General Data Transactions
ACK/NAK Handshaking
If you enable ACK/NAK handshaking, all packeted messages must have a CMD_ACK or CMD_NAK response,
unless the command description states otherwise. This parameter is enabled by default. Motorola
recommends leaving this handshaking enabled to provide feedback to the host. Raw decode data and
WAKEUP do not use ACK/NAK handshaking since they are not packeted data.
Following is an example of a problem which can occur if you disable ACK/NAK handshaking:
•
The host sends a PARAM_SEND message to the imager to change the baud rate from 9600 to 19200.
•
The imager cannot interpret the message.
• The imager does not implement the change the host requested.
•
The host assumes that the parameter change occurred and acts accordingly.
• Communication is lost because the change did not occur on both sides.
If you enable ACK/NAK handshaking, the following occurs:
• The host sends a PARAM_SEND message.
• The imager cannot interpret the message.
• The imager CMD_NAKs the message.
• The host resends the message.
•
The imager receives the message successfully, responds with CMD_ACK, and implements parameter
changes.