Error Detection Protocols - VeriFone Omni 3200 Reference Manual

Error Detection Protocols

Debugging an Application Program
• <count> is the number of bytes that follow the command or
response packet. This lets the terminal know how long the packet
will be. The count includes the following:
Bytes
1
1
(variable)
1
1
• <command> is a one-byte character identifying the command.
• The <data> portion of the packet contains the variable
information, if any, related to the diagnostic command.
• The two CRC characters (<CRC1> <CRC2>) represent the
standard CRC16 accumulation for the foregoing packet; they are
used for error detection.
If the terminal receives a packet with the wrong CRC, it sends a
packet containing the NAK (negative acknowledgment) back to the
diagnostic computer. This indicates to the diagnostic computer that it
has to re-send the packet to the terminal.
The diagnostic computer can also send an identical NAK packet to
the terminal. This tells the terminal to re-send the last packet and can
be used by the diagnostic computer whenever the terminal sends back
a packet containing the incorrect CRC.
The format for the NAK data packet is:
<FFh> <55h> <count> <NAK> <CRC1> <CRC2>
Remote Diagnostics and Debugging
Item Counted
Count
Command
Number of bytes = length of data in bytes
1st CRC character
2nd CRC character
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