3.2. The ACK and NAK Messages and Their Purpose
A "NAK" message is a 16 bit header without a Control Item or parameters (Message length of 2) [02][00] . When the
NAK message block is returned by the Target, it indicates that the specified Control Item is not supported. This allows a
target to implement only the Control Items it actually needs. Any Host message requesting an unimplemented Control
Item will be returned the NAK message. The Host can then exclude this Control Item from its list of Items to control or
monitor.
As an example, suppose a Host requests the elevation setting from a rotor Target controller that only supports
azimuth readings. The Target controller would just return the NAK header.
Implementation on the Target side is easily done by simply decoding only the Control Item messages that it supports
and returning the NAK for all others.
On the Host side, one could initially poll the Target for all the Control Items it may use and then tag the ones that
return NAK for exclusion.
A Data Item "ACK" message is a 16 bit header with a Message Type = 011b with a single parameter byte specifying
the Data Item (0 to 3). The 16 bit header is a fixed value ( 0110 0000 0000 0011 = 0x6003 ). The parameter byte
following the header specifies which Data Item block that is being ACK'd.
For example if the Target received a block of Data Item 2 data correctly it could send the following back to the Host:
[03][60] [02]
The ACK response messages is to provide handshaking to data item transfers. If a data item message is received
correctly then an ACK response message could be sent back to the sender. This implementation is optional as one may
want to stream data without error checking or only ACK periodically the data stream.
Rev. 1.03 2011-11-01
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