HP 64000 Reference Manual page 89

MODEL 64000
SYSTEM OVERVIEW MANUAL
1. Acknowledgement (ACK) which signals that the preceding command was successfully
executed.
2. Negative acknowiedgement (NAK) which signals that the preceding command was not
successfully executed.
Consider, for example, the following sequence of events: If a command is received on
the serial port and is successfully executed, then an operator at the development
station tries unsuccessfully to execute a Monitor command, then an ENQ is received
over the serial port - the response will be NAK. Remember, the response of ACK or
NAK is to the last command. In the case of the above, the last command was the
unsuccessful try at the console. Thus it is clear that when using the serial liD port to
execute Monitor commands and query the system, the individual using the serial port
is responsible for determining whether the response of the system to an ENQ is valid
or not.
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